what is definite assignment assertion

TypeScript 2.7

Constant-named properties.

TypeScript 2.7 adds support for declaring const-named properties on types including ECMAScript symbols.

This also applies to numeric and string literals.

unique symbol

To enable treating symbols as unique literals a new type unique symbol is available. unique symbol is a subtype of symbol , and are produced only from calling Symbol() or Symbol.for() , or from explicit type annotations. The new type is only allowed on const declarations and readonly static properties, and in order to reference a specific unique symbol, you’ll have to use the typeof operator. Each reference to a unique symbol implies a completely unique identity that’s tied to a given declaration.

Because each unique symbol has a completely separate identity, no two unique symbol types are assignable or comparable to each other.

Strict Class Initialization

TypeScript 2.7 introduces a new flag called strictPropertyInitialization . This flag performs checks to ensure that each instance property of a class gets initialized in the constructor body, or by a property initializer. For example

In the above, if we truly meant for baz to potentially be undefined , we should have declared it with the type boolean | undefined .

There are certain scenarios where properties can be initialized indirectly (perhaps by a helper method or dependency injection library), in which case you can use the new definite assignment assertion modifiers for your properties (discussed below).

Keep in mind that strictPropertyInitialization will be turned on along with other strict mode flags, which can impact your project. You can set the strictPropertyInitialization setting to false in your tsconfig.json ’s compilerOptions , or --strictPropertyInitialization false on the command line to turn off this checking.

Definite Assignment Assertions

The definite assignment assertion is a feature that allows a ! to be placed after instance property and variable declarations to relay to TypeScript that a variable is indeed assigned for all intents and purposes, even if TypeScript’s analyses cannot detect so.

With definite assignment assertions, we can assert that x is really assigned by appending an ! to its declaration:

In a sense, the definite assignment assertion operator is the dual of the non-null assertion operator (in which expressions are post-fixed with a ! ), which we could also have used in the example.

In our example, we knew that all uses of x would be initialized so it makes more sense to use definite assignment assertions than non-null assertions.

Fixed Length Tuples

In TypeScript 2.6 and earlier, [number, string, string] was considered a subtype of [number, string] . This was motivated by TypeScript’s structural nature; the first and second elements of a [number, string, string] are respectively subtypes of the first and second elements of [number, string] . However, after examining real world usage of tuples, we noticed that most situations in which this was permitted was typically undesirable.

In TypeScript 2.7, tuples of different arities are no longer assignable to each other. Thanks to a pull request from Kiara Grouwstra , tuple types now encode their arity into the type of their respective length property. This is accomplished by leveraging numeric literal types, which now allow tuples to be distinct from tuples of different arities.

Conceptually, you might consider the type [number, string] to be equivalent to the following declaration of NumStrTuple :

Note that this is a breaking change for some code. If you need to resort to the original behavior in which tuples only enforce a minimum length, you can use a similar declaration that does not explicitly define a length property, falling back to number .

Note that this does not imply tuples represent immutable arrays, but it is an implied convention.

Improved type inference for object literals

TypeScript 2.7 improves type inference for multiple object literals occurring in the same context. When multiple object literal types contribute to a union type, we now normalize the object literal types such that all properties are present in each constituent of the union type.

Previously type {} was inferred for obj and the second line subsequently caused an error because obj would appear to have no properties. That obviously wasn’t ideal.

Multiple object literal type inferences for the same type parameter are similarly collapsed into a single normalized union type:

Improved handling of structurally identical classes and instanceof expressions

TypeScript 2.7 improves the handling of structurally identical classes in union types and instanceof expressions:

• Structurally identical, but distinct, class types are now preserved in union types (instead of eliminating all but one).
• Union type subtype reduction only removes a class type if it is a subclass of and derives from another class type in the union.
• Type checking of the instanceof operator is now based on whether the type of the left operand derives from the type indicated by the right operand (as opposed to a structural subtype check).

This means that union types and instanceof properly distinguish between structurally identical classes.

Type guards inferred from in operator

The in operator now acts as a narrowing expression for types.

For a n in x expression, where n is a string literal or string literal type and x is a union type, the “true” branch narrows to types which have an optional or required property n , and the “false” branch narrows to types which have an optional or missing property n .

Support for import d from "cjs" from CommonJS modules with --esModuleInterop

TypeScript 2.7 updates CommonJS/AMD/UMD module emit to synthesize namespace records based on the presence of an __esModule indicator under esModuleInterop . The change brings the generated output from TypeScript closer to that generated by Babel.

Previously CommonJS/AMD/UMD modules were treated in the same way as ES6 modules, resulting in a couple of problems. Namely:

• TypeScript treats a namespace import (i.e. import * as foo from "foo" ) for a CommonJS/AMD/UMD module as equivalent to const foo = require("foo") .Things are simple here, but they don’t work out if the primary object being imported is a primitive or a class or a function. ECMAScript spec stipulates that a namespace record is a plain object, and that a namespace import ( foo in the example above) is not callable, though allowed by TypeScript
• Similarly a default import (i.e. import d from "foo" ) for a CommonJS/AMD/UMD module as equivalent to const d = require("foo").default .Most of the CommonJS/AMD/UMD modules available today do not have a default export, making this import pattern practically unusable to import non-ES modules (i.e. CommonJS/AMD/UMD). For instance import fs from "fs" or import express from "express" are not allowed.

Under the new esModuleInterop these two issues should be addressed:

• A namespace import (i.e. import * as foo from "foo" ) is now correctly flagged as uncallable. Calling it will result in an error.
• Default imports to CommonJS/AMD/UMD are now allowed (e.g. import fs from "fs" ), and should work as expected.

Note: The new behavior is added under a flag to avoid unwarranted breaks to existing code bases. We highly recommend applying it both to new and existing projects. For existing projects, namespace imports ( import * as express from "express"; express(); ) will need to be converted to default imports ( import express from "express"; express(); ).

With esModuleInterop two new helpers are generated __importStar and __importDefault for import * and import default respectively. For instance input like:

Will generate:

Numeric separators

TypeScript 2.7 brings support for ES Numeric Separators . Numeric literals can now be separated into segments using _ .

Cleaner output in --watch mode

TypeScript’s --watch mode now clears the screen after a re-compilation is requested.

Prettier --pretty output

TypeScript’s pretty flag can make error messages easier to read and manage. pretty now uses colors for file names, diagnostic codes, and line numbers. File names and positions are now also formatted to allow navigation in common terminals (e.g. Visual Studio Code terminal).

Nightly Builds

How to use a nightly build of TypeScript

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Definite Assignment Assertions (!)

The Definite Assignment Assertions or also called non-null assertion operator tells the TypeScript compiler that a value typed cannot be null or undefined which is a way to override the compiler's analysis and inform it that a variable will be assigned a value before it is used.

TypeScript definite assignment assertions

TypeScript never stops improving, although most changes over the past year have been “non syntactical” – i.e. there have been a huge swathe of improvements to how types are handled, and a large slice of improvements to make the tooling even better. It has a been a while, though, since we got a new character to decorate our code. The wait is over, thanks to the TypeScript Definite Assignment Assertion. Let’s take a look at it with a short example.

No definite assignment

The new feature is related to the following improved compile-time check. In the example below, I forgot to assign a value to the wordsPerMinute property. This can happen when you forget to add a default value, or when you forget to initialize it in the constructor, or (as below) when you forget to map a parameter to the property (remember, you don’t need to manually map constructor parameters !).

Whatever the reason, if you compile using the strict flag (I keep telling you to use it), you’ll get the following error, known as a definite assignment error because there is no definite assignment:

app.ts(2,13): error TS2564: Property ‘wordsPerMinute’ has no initializer and is not definitely assigned in the constructor.

Fixing, and definite assignment assertions

The correct fix is probably to assign this.wordsPerMinute = wordsPerMinute in the constructor – but in some cases, you may be doing something funky where the dependency will be resolved in a way the compiler is unable to determine.

When you need to allow a property with no definite assignment, you can use the definite assignment assertion . This is a very grand name for adding a bang (!) to the property name.

This will only work in TypeScript 2.7 and newer.

On the whole, unless you have an iron-clad reason to use it – you’ll probably want to avoid the definite assignment assertion. In most cases, the real value of this feature lies in the part that detects unassigned properties.

Warning Triangle, Public Domain. Wikipedia .

Steve Fenton is an Octonaut at Octopus Deploy and six-time Microsoft MVP for developer technologies. He’s a Software Punk and writer.

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blog-title#highlight" class="m-post-header__title"> Using The "Definite Assignment Assertion" To Define Required Input Bindings In Angular 7.1.1

In an Angular application, Directives represent a special type of Class. In a sense, they are a standard JavaScript Class with properties and methods. But, unlike a Class that gets created and consumed explicitly in your own logic, Directive classes are created, consumed, and managed implicitly by the Angular renderer. As such, properties that might otherwise be required as a constructor argument are, instead, provided through template input bindings. Which begs the question: how do you define a "required" property / input binding that is not actually available at the time of instantiation? As explained in the strictPropertyInitialization flag clean-up issue on GitHub , people on the Angular team have started to use TypeScript's "Definite Assignment Assertion" to denote required Class properties as "defined" even when they won't truly be defined until after the Directive's class constructor has been executed. I had never seen the "Definite Assignment Assertion" before, so I wanted to try it out for myself in a Component Directive in Angular 7.1.1.

Run this demo in my JavaScript Demos project on GitHub .

View this code in my JavaScript Demos project on GitHub .

To explore this concept, I created a "Badge" component that accepts a [user] input binding and renders the badge template with name, email, and avatar. The whole reason-to-be for this component is to render the passed-in User object. As such, this component makes no sense without the [user] input. This input binding is therefore required in order for this component to work. Without the user property, this Badge component would be in an "invalid state".

Before learning about the "Definite Assignment Assertion" notation in TypeScript, I might have tried to approach this Component definition by making the "user" property optional (meaning, for example, that it can also be set to null):

In this case, in the BadgeComponent constructor, I have to set the user property to null otherwise TypeScript complains that I didn't fully initialize the class property (thanks to the "strictPropertyInitialization" tsconfig setting). Of course, in the ngOnInit() and ngOnChanges() event-handlers, I am attempting to assert that the value must be defined (as a required input binding). But, when I go to compile this code, I get the following TypeScript error:

ERROR in app/badge.component.ts.BadgeComponent.html(4,10): : Object is possibly 'null'.

As you can see, TypeScript and Angular are complaining that I am attempting to use a potentially null value in my template expressions. Now, all of the logic for this component happens to be in the template; but, if my Class made additional references to the "user" property within its methods, similar errors would be thrown by the TypeScript compiler.

To fix this workflow, Angular internals have started to use the "Definite Assignment Assertion". This is a property name followed by an exclamation point. For example:

public user! : User;

This assertion tells TypeScript that the "user" property (aka, our required input binding) may look like it's not being fully initialized; but, that TypeScript should trust that the application is going to define this value before it is consumed by the given Class. In other words, we are telling TypeScript to treat this as a required property, but to not validate its initialization.

Of course, TypeScript can only make assertions about compile-time truths - it doesn't know anything about the runtime. As such, this won't prevent runtime errors if the "user" property is undefined; but, this assertion will better express the intended use of the Component Class.

Let's take a look at what the BadgeComponent looks like when we add the "Definite Assignment Assertion" notation:

As you can see, we are denoting the "user" property as "defined". Which allows the code to compile even though we've commented-out the constructor initialization. In this case, I'm still using the ngOnInit() and ngOnChanges() life-cycle methods to validate the input binding. But these are here to better report runtime errors and express the intent of the class. After all, if the input binding is missing, something is going to break.

To see this "Definite Assignment Assertion" in action, I've created an App component that attempts to consume the BadgeComponent both with and without a provided [user] input binding:

As you can see, the last of the "bn-badge" elements is omitting the [user] input binding. And, when we run this code in the browser, we get the following output:

Clearly, if the [user] input binding is required but omitted from the calling code, the page will still break. After all, the BadgeComponent is in an invalid state. But, the BadgeComponent itself expresses clear intent as to how the input binging is supposed to be used.

When you author an Angular Component, some input bindings are optional and some are required. With optional input bindings, you can provide default or fallback values in order to make the class properties easier to work with. But, when it comes to required input bindings, there is no default or fallback. As such, TypeScript will complain that the required properties aren't properly initialized. Thankfully, the "Definite Assignment Assertion" allows us to tell TypeScript to relax the property validation and trust that the Angular application will define these values before they are used.

Want to use code from this post? Check out the license .

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Reader Comments

Hello, Nice Article. I am actually trying to utilize the definite assertion in my angular project. I have a model class of my object, which is your interface by the way. But Im trying to model the db. So i have a class Rating{rating:number;} And an export class Review which has rating!:Rating; In my ts class, i did review.rating.rating = 0, but it is seeing review.rating as undefined.

Please can you help me

So, the Definite Assignment Assertion won't actually influence the runtime value of the property - it will only prevent the TypeScript compiler from complaining that the property isn't being initialized. The point of the Definite Assignment Assertion is handle cases in which a property is being set outside of the current context (such as with an input-binding), and we are assuring the compiler that this value will be set by the time it is consumed.

Based on what you are saying, it sounds like you are attempting to reference reviewer.rating before you actually set it.

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In later versions of TypeScript there’s the concept of a Strict Class Property Initializer, or Definite Assignment Assertion .

This feature all of a sudden produced type errors on previously working code, so let’s explore why it’s here and why you should be using it.

“Property … has no initializer and is not definitely assigned in the constructor.”

Sound familiar? Welcome to strict initialization and the requirement of definite assignment.

So, what it is? It’s a safer way to declare properties and how you expect the data to be made available in your application.

If you see this error, your property is simply being defined with no value. Or at least, no obvious value.

It’s most likely that you’ve seen this error in an Angular context, for example:

So how do we ‘fix it’? Well, there are a few ways. First, we can declare a ! after the property (using definite assignment):

What it means is: “TypeScript, I know there is no data here yet, but I’m confident it will be there soon, just trust me!”.

This is the preferred option, and I’d encourage you to stick with it.

Alternatively, you can supply a default value, and then it’s not needed (but this involves writing and importing code that doesn’t really do anything, I’m not a fan but it works):

Part of the error we see is “not defined in the constructor”, so by also doing that we can ‘fix’ it (but don’t do this):

Why don’t do this? Well, we’ve removed ngOnInit which is a great practice and you should be using it. Angular knows about ngOnInit , whereas JavaScript knows about the constructor . Angular has no control, therefore we should move to a lifecycle hook.

💥 Read my deep-dive on the difference between NgOnInit and the constructor .

You can (at your own peril) disable this feature in your tsconfig.json file as well, and possibly need to disable other strict TypeScript compiler options:

In a non-Angular context, it’s likely you’ll want to structure your data, properties and initializations better to avoid introducing bugs and other errors.

👋 Hello TypeScript dev! If you enjoyed this post, you’ll love my advanced TypeScript courses that I’ve built over years of experience and hard work.

Using the ! definite assignment in Angular is a safe and acceptable solution to writing components, services, directives and more - as demonstrated with OnInit , we can be sure we’re adopting the correct practice.

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Late binding and TypeScript definite assignment assertions

If you take a close look at the code for the previous UserSettings example component, you may notice a weird exclamation mark:

Now, what is it and why did we need it?

As the official TypeScript documentation ( https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-7.html#definite-assignment-assertions ) states, definite assignment assertions can be used to tell the compiler that variables will indeed be assigned, even if TypeScript can't determine this on its own .

The definite assignment assertion is used through the ! character, and is defined after a property or variable declaration.

If we consider ...

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Definite Assignment Assertion

Check out a free preview of the full Intermediate TypeScript, v2 course

The "Definite Assignment Assertion" Lesson is part of the full, Intermediate TypeScript, v2 course featured in this preview video. Here's what you'd learn in this lesson:

Mike discusses the definite assignment operator and explains how to use the exclamation mark (!) to assert that a variable will be assigned a value before it is used. A question about using the declare keyword instead of the exclamation mark is also covered in this segment.

Transcript from the "Definite Assignment Assertion" Lesson

[00:00:00] >> The next thing we're gonna talk about is definite assignment assertion. It also involves an exclamation mark and to do this part, this exercise, we're going to need to go into our tsconfig for this notes project and we're gonna turn on strictPropertyInitialization and set it to true. You should see some errors pop up on the left, and we do, perfect.

[00:00:26] So make sure you turn that on, we're gonna talk about what that is, and how we deal with it. So it's strictPropertyInitialization. So I have a class here and it's called ThingWithAsyncSetup and I've done something that's a little odd here, I have something async inside of a constructor and some might argue I'm already doing something a bit weird.

[00:00:53] But I might pushback against that and say, well look, I'm kinda just kicking off the promise. I'm doing nothing in the constructor that depends on the promise being resolved, and it looks like I have some state I can have in place where I can from the outside observe whether whatever this setup promise does I can observe that it is successfully, has successfully completed, right?

[00:01:21] The dot then will be called and then I'll see this flip. And I have here isSetup as a class field set to a boolean but I'm getting an error, and the error is that is set up has no initializer, and it's not definitely assigned in the constructor. So if I were to take this one line here and move it up into the constructor, the error goes away, and that's because TypeScript analyzes the constructor, it looks at all the class fields that don't have an initializer which would look like this, right?

[00:01:54] I mean, you might argue in this scenario, I should do this, but this is a good example here. The problem here is Typescript saying, I don't know, it doesn't seem like, at least in the general case isSetup, like by the time we return an instance, is that actually gonna be a Boolean?

[00:02:16] It seems like it might be undefined. And why do we think that it's saying that? Does anyone have any hypotheses? Any guesses? I mean, I'm setting it in here. >> Is that run after the constructor? >> Typescript sees a callback here, and it's like, callbacks. I don't know when promise invokes this callback, I don't know if, by the time this promise function, the constructor for promise, by the time it returns, how do I know that this callback has been completed?

[00:02:58] It doesn't know effectively that once we advance beyond line 49, whether isSetup will be set, and in the general case, that is a perfectly valid thing to be concerned about, right? You could get an instance of thing with AsyncSetup, and it claims to have a boolean value for setup, but it could be undefined, except this callback here, we would call this like the promise executor, right?

[00:03:29] It's the thing you use to, it's the function that represents sort of the work or the beginning of the work that could be async, right? This is invoked synchronously within the promise constructor. So I know that by the time we assign the promise to setupPromise, this function's already done, it has been completed.

[00:03:55] I know, definitely, that isSetup, this line of code on line 50, that will have been run, I know for a fact. Now, I also know that TypeScript, there's no way to articulate that in TypeScript and to say, this is a synchronous function, you should assume that this is completed before, like chill out TypeScript, this has been run, there's no way to say that.

[00:04:19] But I can say, look, I know about this particular callback, and I'm gonna say, isSetup is definitely going to have been assigned by the time the constructor completes. Now what I've given you here, it's actually a very, this is a true example, and it does actually guarantee that by the time you get an instance of ThingWithAsyncSetup, it's gonna have a boolean value there, that is true.

[00:04:53] Sometimes you might know something else, like you might know that look, this object is gonna be created, but no one's gonna be working with it or using it until this thing is done to it. Maybe you're initializing objects and you're collecting them, and then after they're already initialized, you set some stuff up on them, and you want downstream consumers to not have to deal with the possibility that a bunch of things could be undefined.

[00:05:23] And so you might blur the lines here and say, okay, technically TypeScript, you're correct, these fields might not have been initialized, but I'll take care of initializing them myself from the outside. And the thing I wanna be sure of is that whoever is working with isSetup, they don't have to deal with this type here, they don't have to deal with the truly honest state of where it is after instantiation.

[00:05:51] You might wanna say, look, right after I create it, I'm doing this thing to it, and I'm always doing that, and I'll take responsibility for making sure that that's true and that no one gets a hold of these things before. So an example of where you might see this is like framework level code, where objects are created, not by invoking the constructor, but by some other process like they're created as a side effect of doing something.

[00:06:17] And then you can be assured that, whenever we go through this pathway where these things are created. I'm grabbing that instance and I'm doing a bunch of stuff to it, and then when I hand it off, I want it to be represented as if the stuff was always there.

[00:06:35] You're taking on risk there, but sometimes it's a valid pattern. Sometimes it's preferable over modeling an interface, or a class that represents, the partially set up thing, and then you have another class that you have to deserialize and serialize into so you get the fully set up thing, it can get complicated if you try to stick very rigidly to the rules.

[00:07:00] Those are two use cases for definitely assignment operator. >> Is it a good practice to use the non-knowledge assertion operator in production code? >> Non-null assertion operator. >> The exclamation point. >> The exclamation point, so what I said before, and I'll stick to it here. I use this all the time in my tests.

[00:07:23] I try to not use it in applications, in libraries, in any code that's actually running and the reason is the mode of failure is usually unacceptable to me, it'll just throw your codes interrupted, and it'll fail however, it's gonna fail. And I usually like to, like I would say, if you want this equivalent thing in production, what you should really do is this.

[00:08:03] Do this. And then write something explicit, like how to handle, the case where it's not there, just use a type card. In tests, this will get very messy cuz I might, on every single line, I get a big JSON object like back that has tons of properties and things on it.

[00:08:27] My test would be three quarters, like by line count. Type guards and safely finding if things are there and throwing very specific errors, it's not just that the outer object wasn't there, but the inner object too and the inner inner inner object. And that's where using this non-null assertion operator, it's valuable cuz all I want is to throw the, as I try to probe into this object, drill into this object, and this is a very concise way of doing that, it's easier to maintain, your test remains a lot more readable, but I don't use this.

[00:09:10] I'm not gonna speak in absolutes here, it would have to be exceptional case for me to use this in production code, not even a production code, any code that's outside of the testing sphere. Cuz you can always do this and this is much more graceful error handling, this is okay when throwing is the desired outcome, that is an absolute statement I'll make because that's exactly what I'm near it'll throw >> Is there a difference between [COUGH] using the exclamation point on 53 and using the declare keyword?

[00:09:55] >> Like this? That's interesting. I wouldn't necessarily do this. I'm actually surprised that this works. I use declare mostly in ambient type information, or when I'm making a mod, like a type declaration, which we'll see later, where you can do that, you can augment types in other modules or in the global scope, but I generally don't use this.

[00:10:30] It does appear to work, it appears to work. I would be kind of, if I saw this in a poll request, I would comment on it and say, you're asking people to understand that you can do this and there's an equally effective way to do this and it's much more common and people will know it when they come across it.

[00:10:53] But that's interesting, you taught me something here. I guess we're saying what we're doing there with declare, if I had to guess, I don't know this for a fact, but I'll explain how my mental model is processing that. When we say declare, we might be stating that, this is a boolean, it's of type boolean, and it's sort of a different thing then checking for definite assignment, right?

[00:11:24] You're really, yeah, that's the best way I can articulate it. I'd recommend using this, I don't think I've ever seen the use of declare before field unless it's in a declaration file, like a DTS file. >> I've ended up having to use it like in a number of projects where I'm injecting, setting that to a bad service, using that decorator.

[00:11:50] And it's probably cause like my TS config wasn't set up right or something, but it would complain about the exclamation point there but it wouldn't complain about the declares. >> I see like a collision of syntax. Yeah, I would use declare there given that it works, but man, is that uncommon and please file a bug report because I think that's odd.

[00:12:13] It's odd to the point where I'm not sure it will continue to work that way, it's that unusual. It'll work in declaration files, all of definitelytyped like that's declared everywhere but in this space, I've never seen that, interesting. I believe you though, there are all sorts of typescript syntax things that sort of collide with other things like this, this is a when you mix this with JSX, it's quite interesting here, this is why we don't write arrays like, This is popular for a long time, but this makes it complicated in JSX, where the parsers get overwhelmed with the same kind of syntax, meaning different things in different spaces.

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Announcing TypeScript 2.7

Daniel Rosenwasser

January 31st, 2018 0 0

If you’re not familiar with TypeScript, it’s a language that brings optional static types to JavaScript by building on JavaScript itself. Running TypeScript code through its compiler emits clean readable JavaScript that runs on any browser, and can also make bleeding-edge ECMAScript features you write work on older browsers. That means that you can take advantage of the design time tooling and safety of types (like code completion & navigation), while still leveraging the familiarity, community, and ubiquity of JavaScript.

But if you already know what TypeScript is and want to start using 2.7, go ahead and get it on NuGet or download it over npm:

Visual Studio 2015 users (who have Update 3 ) can install TypeScript 2.7 from here , and Visual Studio 2017 users using version 15.2 or later will be able to get TypeScript by simply installing it from here . Sublime Text Plugin users can install the new version from Package Control .

TypeScript 2.7 will be available for Visual Studio Code very soon, but eager users can get it working pretty easily in the meantime .

We’ve got a lot in 2.7. While you can always take a look at the roadmap , we’ve put together a quick list for a bird’s eye view of this release:

Stricter class property checks

Definite assignment assertions, easier ecmascript module interoperability, unique symbol types and const-named properties, cleaner output in --watch mode, prettier --pretty output, numeric separators, fixed length tuples, in operator narrowing and accurate instanceof, smarter object literal inference.

We’ve also put together a breaking changes section towards the end of this post which existing users should be aware of.

So without further ado, let’s see what this release brings!

TypeScript 2.7 introduces a new strictness flag named --strictPropertyInitialization !

This flag makes sure that each instance property of a class gets set in the constructor body, or by a property initializer. In a sense, it brings some of the definite assignment checks from variables to instance properties in classes. For example:

In the above, baz never gets set, and TypeScript reports an error. If we truly meant for baz to potentially be undefined , we should have declared it with the type boolean | undefined .

There are certain scenarios where properties might be initialized indirectly (perhaps by a helper method or dependency injection library). In those cases, you can convince the type system you know better by using the definite assignment assertions for your properties.

Keep in mind that --strictPropertyInitialization will be turned on along with other --strict mode flags, which can impact your project. You can set the strictPropertyInitialization setting to false in your tsconfig.json ‘s compilerOptions , or --strictPropertyInitialization false on the command line to turn off this checking.

As expressive as we try to make the type system, we understand there are certain times you might know better than TypeScript.

As we mentioned above, definite assignment assertions are a new syntax you can use to convince TypeScript that a property will definitely get assigned. But in addition to working on class properties, TypeScript 2.7 also allows you to use this feature on variable declarations!

If we hadn’t added an exclamation point/bang ( ! ) after x , TypeScript would have reported that x was never initialized. This can be handy in deferred-initialization, or re-initialization scenarios.

Before ECMAScript modules were standardized in ES2015, the JavaScript ecosystem had several different module formats that worked in different ways. Once the standard passed, the community was left with a question of how to best interoperate with existing “legacy” module formats.

TypeScript and Babel took different approaches, and even now, there really isn’t a locked down standard. The short story is that if you’ve used Babel, Webpack, or React Native, and expected different import behaviors than you were used to, we have a new compiler option for you called --esModuleInterop .

Babel and Webpack allow users to import these CommonJS modules as default imports, but also provide each property on the namespace import (unless the module was marked with an __esModule flag).

Because TypeScript’s behavior differs, we added the --allowSyntheticDefaultImports flag in TypeScript 1.8 to allow users to get this behavior for type-checking (but not emit).

In general, TypeScript’s view of CommonJS (and AMD) modules is that namespace imports always correspond to the shape of a CommonJS module object, and that a default import just corresponds to a member on that module named default . Under this assumption, you can create a named import

However, ES namespace imports aren’t callable, so this approach doesn’t always make sense.

To give users the same runtime behavior as Babel or Webpack, TypeScript provides a new --esModuleInterop flag when emitting to

legacy module formats.

Under the new --esModuleInterop flag, these callable CommonJS modules must be imported as default imports like so:

We strongly suggest that Node.js users leverage this flag with a module target of commonjs for libraries like express, which export a callable/constructable module.

Webpack users may want to use this as well; however, your code should target esnext modules with a moduleResolution strategy of node . Using esnext modules with --esModuleInterop really only has the effect of turning on --allowSyntheticDefaultImports .

TypeScript 2.7 understands ECMAScript symbols more deeply, allowing you to use them more flexibly.

One highly-demanded use-case is being able to declare well-typed properties with symbols. For an example, take the following:

As you can see, TypeScript can keep track of the fact that x has properties declared using the symbols Foo and Bar since both Foo and Bar were declared as constants. TypeScript leverages this fact and gives both Foo and Bar a new kind of type: unique symbol s.

unique symbol s are subtype of symbol , and are produced only from calling Symbol() or Symbol.for() , or from explicit type annotations. They can only occur on const declarations and readonly static properties, and in order to reference an existing unique symbol type, you’ll have to use the typeof operator. Each reference to a unique symbol implies a completely unique identity that’s tied to a given declaration.

Because each unique symbol has a completely separate identity, no two unique symbol types are assignable or comparable to each other.

Other potential use-cases include using symbols for tagged unions.

TypeScript’s --watch mode now clears the screen after a re-compilation is requested. This can make it much easier to read messages from the current compilation. We’d like to thank both Philipp Kretzschmar and Joshua Goldberg for helping out on this feature!

TypeScript’s --pretty flag can make error messages easier to read and manage. We have two main improvements in this functionality. First, thanks to a pull request from Joshua Goldberg --pretty now uses colors for file names, diagnostic codes, and line numbers. Second, thanks to a separate pull request by Orta Therox , file names and positions are formatted in such a way that common terminals (including the one embedded in Visual Studio Code) can allow a Ctrl+Click, Cmd+Click, Alt+Click, etc. to jump to the appropriate location in your editor

TypeScript 2.7 introduces support for ECMAScript’s numeric separators proposal. This feature allows users to place underscores ( _ ) in between digits to visually distinguish groups of digits ( much like how commas and periods are often used to group numbers ).

These separators are also useful for binary and hexadecimal numbers.

Note that, perhaps counterintuitively, numbers in JavaScript are not well-suited to represent credit card and telephone numbers. Strings will act as better representations in such cases.

In TypeScript 2.6 and earlier, [number, string, string] was considered a subtype of [number, string] . This was motivated by TypeScript’s structural nature; the first and second elements of a [number, string, string] are respectively subtypes of the first and second elements of [number, string] , and the “trailing” string type is assignable to the union of element types from [number, string] . However, after examining real world usage of tuples, we noticed that most situations in which this was permitted was typically undesirable.

Thanks to a pull request from Kiara Grouwstra , tuple types now encode their arity into the type of their respective length property, and tuples of different arities are no longer assignable to each other. This is accomplished by leveraging numeric literal types, which now allow tuples to be distinct from tuples of different arities.

Conceptually, you might consider the type [number, string] to be equivalent to the following declaration of NumStrTuple :

Note that this is a breaking change. If you need to resort to the original behavior in which tuples only enforce a minimum size, you can use a similar declaration that does not explicitly define a length property, falling back to number .

TypeScript 2.7 brings two new changes to type narrowing – the ability to get a more specific type for a value by running certain types of checks called “type guards”.

First, the instanceof operator is now leverages the inheritance chain instead of

relying on structural compatibility, more accurately reflecting whether how instanceof may behave at runtime. This can help avoid certain complex issues when instanceof narrows from structurally similar (but unrelated) types.

Second, thanks to GitHub user IdeaHunter , the in operator now acts as a type guard , narrowing out types that don’t explicitly declare properties of a given name.

There are certain patterns in JavaScript where users will omit properties so that all uses of those properties are effectively undefined .

TypeScript used seek the best common supertype between { value: number } and {} , ending up with {} . While being technically correct, this wasn’t very useful.

Starting with version 2.7, TypeScript now “normalizes” each object literal’s type to account for every property, inserting an optional property of type undefined on each object type, and unioning them together.

From the above example, the new type of foo would be { value: number } | { value?: undefined } . Combined with the ways TypeScript can narrow, this lets us write expressive code that TypeScript can still understand. As another example, take the following:

Here, TypeScript is able to narrow the type of bar based on checking its b property, allowing us to access the bData property.

Breaking Changes

This release brings some minor breaking changes:

• Tuples now have fixed numeric length properties.
• instanceof and in now have slightly different narrowing behavior.
• Inferences from generic signatures now use base constraint types of type parameters instead of any .
• The setSelectionRange API now only accepts "forward" | "backward" | "none" .
• allowSyntheticDefaultImports no longer synthesizes default imports from TypeScript implementation files (i.e. .ts and .tsx ).

Additionally, as mentioned above, users with the --strict setting on will automatically be opted in to --strictPropertyInitialization which errors on properties which are not directly initialized on their declarations or in constructor bodies. While easy to opt out of by explicitly turning this check off, your code may be impacted.

You can get a detailed look from our list of breaking changes issues for TypeScript 2.7 on GitHub, or keep track of general breaking changes on our Breaking Changes wiki page .

What’s next?

We always try to bring more joy to the TypeScript experience. We hope this release continues the tradition of making you more productive and expressive to bring that joy to the core experience.

Now that 2.7 is out, we’ve got some fantastic things in mind for TypeScript 2.8, including conditional types ! While plans are still currently in flux, you can keep a close watch on our roadmap to get an idea of what’s on the TypeScript horizon.

Let us know what you think of this release over on Twitter or in the comments below, and feel free to report issues and suggestions filing a GitHub issue .

Happy Hacking!

Daniel Rosenwasser Senior Program Manager, TypeScript

Discussion is closed.

Strict Property Initialization in TypeScript

TypeScript 2.7 introduced a new compiler option for strict property initialization checks in classes. If the --strictPropertyInitialization flag is enabled, the type checker verifies that each instance property declared in a class either

• has a type that includes undefined ,
• has an explicit initializer , or
• is definitely assigned to in the constructor.

The --strictPropertyInitialization option is part of the family of compiler options that is enabled automatically when the --strict flag is set. As with all the other strict compiler options, you can set --strict to true and selectively opt out of strict property initialization checks by setting --strictPropertyInitialization to false .

Note that the --strictNullChecks flag must be set (either directly or indirectly via --strict ) in order for --strictPropertyInitialization to have any effect.

Alright, let's see strict property initialization checks in action. Without the --strictPropertyInitialization flag enabled, the following code type-checks just fine, but produces a TypeError at runtime:

The reason for the runtime error is that the username property holds the value undefined because there's no assignment to that property. Therefore, the call to the toLowerCase() method fails.

If we enable --strictPropertyInitialization , the type checker raises an error:

Let's look at four different ways we can properly type our User class to make the type error go away.

# Solution #1: Allowing undefined

One way to make the type error go away is to give the username property a type that includes undefined :

Now, it's perfectly valid for the username property to hold the value undefined . Whenever we want to use the username property as a string, though, we first have to make sure that it actually holds a string and not the value undefined , e.g. using typeof :

Alternatively, we can use optional chaining (the ?. operator) to only call the toLowerCase() method if the username property holds a non-nullish value. We can combine that with nullish coalescing (the ?? operator) to provide the fallback value:

# Solution #2: Explicit Property Initializer

Another way to make the type error go away is to add an explicit initializer to the username property. This way, the property holds a string value right away and is not observably undefined :

# Solution #3: Assignment in the Constructor

Perhaps the most useful solution is to add a username parameter to the constructor, which is then assigned to the username property. This way, whenever an instance of the User class is constructed, the caller has to provide the username as an argument:

We could simplify the User class by removing the explicit assignment to the class field and adding the public modifier to the username constructor parameter:

Note that strict property initialization requires each property to be definitely assigned in all possible code paths in the constructor. The following (contrived) example is therefore not type-correct because in some cases, we leave the username property uninitialized:

# Solution #4: Definite Assignment Assertion

If a class property neither has an explicit initializer nor a type including undefined , the type checker requires that property to be initialized directly within the constructor; otherwise, strict property initialization checks will fail. This is problematic if you want to initialize a property within a helper method or have a dependency injection framework initialize it for you. In these cases, you have to add a definite assignment assertion ( ! ) to that property's declaration:

By adding a definite assignment assertion to the username property, we're telling the type checker that it can expect the username property to be initialized, even if it cannot detect that on its own. It is now our responsibility to make sure the property is definitely assigned to after the constructor returns, so we have to careful; otherwise, the username property can be observably undefined and we're back to the TypeError at runtime.

This article and 44 others are part of the TypeScript Evolution series. Have a look!

The art of simplicity

Search this blog, typescript 2.7–definite assignment assertions.

While looking through some TypeScript 2.7 samples, I noticed the following syntax:

• has a type that includes undefined ,
• has an explicit initializer
• is initialized within the constructor

This is problematic if you want to initialize a property within a helper method or have a dependency injection framework do it for you. To solve this situation you can use a definite assignment assertion using the exclamation mark: (!) . Now the type checker will no longer complain.

Remark: By using a definite assignment assertion it becomes your responsibility to make sure the property gets set.

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What Does "!" Symbol Mean in Typescript?

An introduction to definite assignment assertions in typescript..

In this short guide, I will introduce you to the Definite Assignment Assertion feature in typescript and demonstrate to you an interesting use case to implement this feature.

Prerequisites

This guide assumes you have basic knowledge of Typescript , you have a basic understanding of the Stripe API , and you have installed the Typescript error translator extension.

The Problem

When working with the Stripe API in one of my Next.js & Typescript projects, I came across an error where typescript didn't recognize my environment variable as a string. It later displayed the error message below.

The Solution

The solution in this case is to use the "!" symbol which is a feature in Typescript known as the definite assignment assertion.

As defined in the official Typescript Documentation , the definite assignment assertion feature is used to tell the compiler that the variable assigned is indeed valid for all intents and purposes, even if Typescript's analyses cannot detect so.

The "!" symbol is used and is declared after an instance property and/or variable declarations.

With the definite assignment assertion, we can assert that indeed in this case we have environment variables ( process.env.STRIPE_SECRET_KEY ) in string format and there is nothing to worry about.

This workaround still leaves us with weaker type safety since any misconfiguration (in our case the environment variables) could lead to errors that cannot be caught at compile time. These errors can only be caught at runtime which beats the logic of having a strict and functional type checking system. With this in mind it's your responsibility to ensure correctness.

It's also good to note that previous Typescript versions used the "!" symbol as a non-null assertion operator. You can read more about this in the Typescript Documentation about Non-null assertion operator .

Thank you for reading this far and happy coding!

21 Type assertions (related to casting)

• 21.1.1  Alternative syntax for type assertions
• 21.1.2  Example: asserting an interface
• 21.1.3  Example: asserting an index signature
• 21.2.1  Non-nullish assertion operator (postfix ! )
• 21.2.2  Definite assignment assertions

This chapter is about type assertions in TypeScript, which are related to type casts in other languages and performed via the as operator.

21.1 Type assertions

A type assertion lets us override a static type that TypeScript has computed for a value. That is useful for working around limitations of the type system.

Type assertions are related to type casts in other languages, but they don’t throw exceptions and don’t do anything at runtime (they do perform a few minimal checks statically).

In line A, we widen the type of the Array to object .

In line B, we see that this type doesn’t let us access any properties ( details ).

In line C, we use a type assertion (the operator as ) to tell TypeScript that data is an Array. Now we can access property .length .

Type assertions are a last resort and should be avoided as much as possible. They (temporarily) remove the safety net that the static type system normally gives us.

Note that, in line A, we also overrode TypeScript’s static type. But we did it via a type annotation. This way of overriding is much safer than type assertions because we are much more constrained: TypeScript’s type must be assignable to the type of the annotation.

21.1.1 Alternative syntax for type assertions

TypeScript has an alternative “angle-bracket” syntax for type assertions:

I recommend avoiding this syntax. It has grown out of style and is not compatible with React JSX code (in .tsx files).

21.1.2 Example: asserting an interface

In order to access property .name of an arbitrary object obj , we temporarily change the static type of obj to Named (line A and line B).

21.1.3 Example: asserting an index signature

In the following code (line A), we use the type assertion as Dict , so that we can access the properties of a value whose inferred type is object . That is, we are overriding the static type object with the static type Dict .

21.2 Constructs related to type assertions

21.2.1 non-nullish assertion operator (postfix ).

If a value’s type is a union that includes the types undefined or null , the non-nullish assertion operator (or non-null assertion operator ) removes these types from the union. We are telling TypeScript: “This value can’t be undefined or null .” As a consequence, we can perform operations that are prevented by the types of these two values – for example:

21.2.1.1 Example – Maps: .get() after .has()

After we use the Map method .has() , we know that a Map has a given key. Alas, the result of .get() does not reflect that knowledge, which is why we have to use the nullish assertion operator:

We can avoid the nullish assertion operator whenever the values of a Map can’t be undefined . Then missing entries can be detected by checking if the result of .get() is undefined :

21.2.2 Definite assignment assertions

If strict property initialization is switched on, we occasionally need to tell TypeScript that we do initialize certain properties – even though it thinks we don’t.

This is an example where TypeScript complains even though it shouldn’t:

The errors go away if we use definite assignment assertions (exclamation marks) in line A and line B:

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Improved Definite Assignment Analysis

• 5 contributors

This article is a feature specification. The specification serves as the design document for the feature. It includes proposed specification changes, along with information needed during the design and development of the feature. These articles are published until the proposed spec changes are finalized and incorporated in the current ECMA specification.

There may be some discrepancies between the feature specification and the completed implementation. Those differences are captured in the pertinent language design meeting (LDM) notes .

You can learn more about the process for adopting feature speclets into the C# language standard in the article on the specifications .

Definite assignment §9.4 as specified has a few gaps which have caused users inconvenience. In particular, scenarios involving comparison to boolean constants, conditional-access, and null coalescing.

Related discussions and issues

csharplang discussion of this proposal: https://github.com/dotnet/csharplang/discussions/4240

Probably a dozen or so user reports can be found via this or similar queries (i.e. search for "definite assignment" instead of "CS0165", or search in csharplang). https://github.com/dotnet/roslyn/issues?q=is%3Aclosed+is%3Aissue+label%3A%22Resolution-By+Design%22+cs0165

I have included related issues in the scenarios below to give a sense of the relative impact of each scenario.

As a point of reference, let's start with a well-known "happy case" that does work in definite assignment and in nullable.

Comparison to bool constant

• https://github.com/dotnet/csharplang/discussions/801
• Links to 4 other issues where people were affected by this.

Comparison between a conditional access and a constant value

• https://github.com/dotnet/roslyn/issues/33559
• https://github.com/dotnet/csharplang/discussions/4214
• https://github.com/dotnet/csharplang/issues/3659
• https://github.com/dotnet/csharplang/issues/3485

This scenario is probably the biggest one. We do support this in nullable but not in definite assignment.

Conditional access coalesced to a bool constant

• https://github.com/dotnet/csharplang/discussions/916
• https://github.com/dotnet/csharplang/issues/3365

This scenario is very similar to the previous one. This is also supported in nullable but not in definite assignment.

Conditional expressions where one arm is a bool constant

• https://github.com/dotnet/roslyn/issues/4272

It's worth pointing out that we already have special behavior for when the condition expression is constant (i.e. true ? a : b ). We just unconditionally visit the arm indicated by the constant condition and ignore the other arm.

Also note that we haven't handled this scenario in nullable.

Specification

. (null-conditional operator) expressions.

We introduce a new section ?. (null-conditional operator) expressions . See the null-conditional operator specification ( §11.7.7 )and Precise rules for determining definite assignment §9.4.4 for context.

As in the definite assignment rules linked above, we refer to a given initially unassigned variable as v .

We introduce the concept of "directly contains". An expression E is said to "directly contain" a subexpression E 1 if it is not subject to a user-defined conversion §10.5 whose parameter is not of a non-nullable value type, and one of the following conditions holds:

• E is E 1 . For example, a?.b() directly contains the expression a?.b() .
• If E is a parenthesized expression (E2) , and E 2 directly contains E 1 .
• If E is a null-forgiving operator expression E2! , and E 2 directly contains E 1 .
• If E is a cast expression (T)E2 , and the cast does not subject E 2 to a non-lifted user-defined conversion whose parameter is not of a non-nullable value type, and E 2 directly contains E 1 .

For an expression E of the form primary_expression null_conditional_operations , let E 0 be the expression obtained by textually removing the leading ? from each of the null_conditional_operations of E that have one, as in the linked specification above.

In subsequent sections we will refer to E 0 as the non-conditional counterpart to the null-conditional expression. Note that some expressions in subsequent sections are subject to additional rules that only apply when one of the operands directly contains a null-conditional expression.

• The definite assignment state of v at any point within E is the same as the definite assignment state at the corresponding point within E0 .
• The definite assignment state of v after E is the same as the definite assignment state of v after primary_expression .

We use the concept of "directly contains" to allow us to skip over relatively simple "wrapper" expressions when analyzing conditional accesses that are compared to other values. For example, ((a?.b(out x))!) == true is expected to result in the same flow state as a?.b == true in general.

We also want to allow analysis to function in the presence of a number of possible conversions on a conditional access. Propagating out "state when not null" is not possible when the conversion is user-defined, though, since we can't count on user-defined conversions to honor the constraint that the output is non-null only if the input is non-null. The only exception to this is when the user-defined conversion's input is a non-nullable value type. For example:

This also includes lifted conversions like the following:

When we consider whether a variable is assigned at a given point within a null-conditional expression, we simply assume that any preceding null-conditional operations within the same null-conditional expression succeeded.

For example, given a conditional expression a?.b(out x)?.c(x) , the non-conditional counterpart is a.b(out x).c(x) . If we want to know the definite assignment state of x before ?.c(x) , for example, then we perform a "hypothetical" analysis of a.b(out x) and use the resulting state as an input to ?.c(x) .

Boolean constant expressions

We introduce a new section "Boolean constant expressions":

For an expression expr where expr is a constant expression with a bool value:

• If expr is a constant expression with value true , and the state of v before expr is "not definitely assigned", then the state of v after expr is "definitely assigned when false".
• If expr is a constant expression with value false , and the state of v before expr is "not definitely assigned", then the state of v after expr is "definitely assigned when true".

We assume that if an expression has a constant value bool false , for example, it's impossible to reach any branch that requires the expression to return true . Therefore variables are assumed to be definitely assigned in such branches. This ends up combining nicely with the spec changes for expressions like ?? and ?: and enabling a lot of useful scenarios.

It's also worth noting that we never expect to be in a conditional state before visiting a constant expression. That's why we do not account for scenarios such as " expr is a constant expression with value true , and the state of v before expr is "definitely assigned when true".

?? (null-coalescing expressions) augment

We augment section §9.4.4.29 as follows:

For an expression expr of the form expr_first ?? expr_second :

• If expr_first directly contains a null-conditional expression E , and v is definitely assigned after the non-conditional counterpart E 0 , then the definite assignment state of v after expr is the same as the definite assignment state of v after expr_second .

The above rule formalizes that for an expression like a?.M(out x) ?? (x = false) , either the a?.M(out x) was fully evaluated and produced a non-null value, in which case x was assigned, or the x = false was evaluated, in which case x was also assigned. Therefore x is always assigned after this expression.

This also handles the dict?.TryGetValue(key, out var value) ?? false scenario, by observing that v is definitely assigned after dict.TryGetValue(key, out var value) , and v is "definitely assigned when true" after false , and concluding that v must be "definitely assigned when true".

The more general formulation also allows us to handle some more unusual scenarios, such as:

• if (x?.M(out y) ?? (b && z.M(out y))) y.ToString();
• if (x?.M(out y) ?? z?.M(out y) ?? false) y.ToString();

?: (conditional) expressions

We augment section §9.4.4.30 as follows:

For an expression expr of the form expr_cond ? expr_true : expr_false :

• If the state of v after expr_true is "definitely assigned when true", and the state of v after expr_false is "definitely assigned when true", then the state of v after expr is "definitely assigned when true".
• If the state of v after expr_true is "definitely assigned when false", and the state of v after expr_false is "definitely assigned when false", then the state of v after expr is "definitely assigned when false".

This makes it so when both arms of a conditional expression result in a conditional state, we join the corresponding conditional states and propagate it out instead of unsplitting the state and allowing the final state to be non-conditional. This enables scenarios like the following:

This is an admittedly niche scenario, that compiles without error in the native compiler, but was broken in Roslyn in order to match the specification at the time.

==/!= (relational equality operator) expressions

We introduce a new section ==/!= (relational equality operator) expressions .

The general rules for expressions with embedded expressions §9.4.4.23 apply, except for the scenarios described below.

For an expression expr of the form expr_first == expr_second , where == is a[predefined comparison operator ( §11.11 ) or a lifted operator ( §11.4.8 ), the definite assignment state of v after expr is determined by:

• If expr_first directly contains a null-conditional expression E and expr_second is a constant expression with value null , and the state of v after the non-conditional counterpart E 0 is "definitely assigned", then the state of v after expr is "definitely assigned when false".
• If expr_first directly contains a null-conditional expression E and expr_second is an expression of a non-nullable value type, or a constant expression with a non-null value, and the state of v after the non-conditional counterpart E 0 is "definitely assigned", then the state of v after expr is "definitely assigned when true".
• If expr_first is of type boolean , and expr_second is a constant expression with value true , then the definite assignment state after expr is the same as the definite assignment state after expr_first .
• If expr_first is of type boolean , and expr_second is a constant expression with value false , then the definite assignment state after expr is the same as the definite assignment state of v after the logical negation expression !expr_first .

For an expression expr of the form expr_first != expr_second , where != is a predefined comparison operator ( §11.11 ) or a lifted operator (( §11.4.8 )), the definite assignment state of v after expr is determined by:

• If expr_first directly contains a null-conditional expression E and expr_second is a constant expression with value null , and the state of v after the non-conditional counterpart E 0 is "definitely assigned", then the state of v after expr is "definitely assigned when true".
• If expr_first directly contains a null-conditional expression E and expr_second is an expression of a non-nullable value type, or a constant expression with a non-null value, and the state of v after the non-conditional counterpart E 0 is "definitely assigned", then the state of v after expr is "definitely assigned when false".
• If expr_first is of type boolean , and expr_second is a constant expression with value true , then the definite assignment state after expr is the same as the definite assignment state of v after the logical negation expression !expr_first .
• If expr_first is of type boolean , and expr_second is a constant expression with value false , then the definite assignment state after expr is the same as the definite assignment state after expr_first .

All of the above rules in this section are commutative, meaning that if a rule applies when evaluated in the form expr_second op expr_first , it also applies in the form expr_first op expr_second .

The general idea expressed by these rules is:

• if a conditional access is compared to null , then we know the operations definitely occurred if the result of the comparison is false
• if a conditional access is compared to a non-nullable value type or a non-null constant, then we know the operations definitely occurred if the result of the comparison is true .
• since we can't trust user-defined operators to provide reliable answers where initialization safety is concerned, the new rules only apply when a predefined == / != operator is in use.

We may eventually want to refine these rules to thread through conditional state which is present at the end of a member access or call. Such scenarios don't really happen in definite assignment, but they do happen in nullable in the presence of [NotNullWhen(true)] and similar attributes. This would require special handling for bool constants in addition to just handling for null /non-null constants.

Some consequences of these rules:

• if (a?.b(out var x) == true)) x() else x(); will error in the 'else' branch
• if (a?.b(out var x) == 42)) x() else x(); will error in the 'else' branch
• if (a?.b(out var x) == false)) x() else x(); will error in the 'else' branch
• if (a?.b(out var x) == null)) x() else x(); will error in the 'then' branch
• if (a?.b(out var x) != true)) x() else x(); will error in the 'then' branch
• if (a?.b(out var x) != 42)) x() else x(); will error in the 'then' branch
• if (a?.b(out var x) != false)) x() else x(); will error in the 'then' branch
• if (a?.b(out var x) != null)) x() else x(); will error in the 'else' branch

is operator and is pattern expressions

We introduce a new section is operator and is pattern expressions .

For an expression expr of the form E is T , where T is any type or pattern

• The definite assignment state of v before E is the same as the definite assignment state of v before expr .
• If E directly contains a null-conditional expression, and the state of v after the non-conditional counterpart E 0 is "definitely assigned", and T is any type or a pattern that does not match a null input, then the state of v after expr is "definitely assigned when true".
• If E directly contains a null-conditional expression, and the state of v after the non-conditional counterpart E 0 is "definitely assigned", and T is a pattern that matches a null input, then the state of v after expr is "definitely assigned when false".
• If E is of type boolean and T is a pattern which only matches a true input, then the definite assignment state of v after expr is the same as the definite assignment state of v after E.
• If E is of type boolean and T is a pattern which only matches a false input, then the definite assignment state of v after expr is the same as the definite assignment state of v after the logical negation expression !expr .
• Otherwise, if the definite assignment state of v after E is "definitely assigned", then the definite assignment state of v after expr is "definitely assigned".

This section is meant to address similar scenarios as in the == / != section above. This specification does not address recursive patterns, e.g. (a?.b(out x), c?.d(out y)) is (object, object) . Such support may come later if time permits.

Additional scenarios

This specification doesn't currently address scenarios involving pattern switch expressions and switch statements. For example:

It seems like support for this could come later if time permits.

There have been several categories of bugs filed for nullable which require we essentially increase the sophistication of pattern analysis. It is likely that any ruling we make which improves definite assignment would also be carried over to nullable.

https://github.com/dotnet/roslyn/issues/49353 https://github.com/dotnet/roslyn/issues/46819 https://github.com/dotnet/roslyn/issues/44127

It feels odd to have the analysis "reach down" and have special recognition of conditional accesses, when typically flow analysis state is supposed to propagate upward. We are concerned about how a solution like this could intersect painfully with possible future language features that do null checks.

Alternatives

Two alternatives to this proposal:

• Introduce "state when null" and "state when not null" to the language and compiler. This has been judged to be too much effort for the scenarios we are trying to solve, but that we could potentially implement the above proposal and then move to a "state when null/not null" model later on without breaking people.
• Do nothing.

Unresolved questions

There are impacts on switch expressions that should be specified: https://github.com/dotnet/csharplang/discussions/4240#discussioncomment-343395

Design meetings

https://github.com/dotnet/csharplang/discussions/4243

C# feature specifications

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Additional resources

• Assignment Statement

An Assignment statement is a statement that is used to set a value to the variable name in a program .

Assignment statement allows a variable to hold different types of values during its program lifespan. Another way of understanding an assignment statement is, it stores a value in the memory location which is denoted by a variable name.

The symbol used in an assignment statement is called as an operator . The symbol is ‘=’ .

Note: The Assignment Operator should never be used for Equality purpose which is double equal sign ‘==’.

The Basic Syntax of Assignment Statement in a programming language is :

variable = expression ;

variable = variable name

expression = it could be either a direct value or a math expression/formula or a function call

Few programming languages such as Java, C, C++ require data type to be specified for the variable, so that it is easy to allocate memory space and store those values during program execution.

data_type variable_name = value ;

In the above-given examples, Variable ‘a’ is assigned a value in the same statement as per its defined data type. A data type is only declared for Variable ‘b’. In the 3 rd line of code, Variable ‘a’ is reassigned the value 25. The 4 th line of code assigns the value for Variable ‘b’.

Assignment Statement Forms

This is one of the most common forms of Assignment Statements. Here the Variable name is defined, initialized, and assigned a value in the same statement. This form is generally used when we want to use the Variable quite a few times and we do not want to change its value very frequently.

Tuple Assignment

Generally, we use this form when we want to define and assign values for more than 1 variable at the same time. This saves time and is an easy method. Note that here every individual variable has a different value assigned to it.

(Code In Python)

Sequence Assignment

(Code in Python)

Multiple-target Assignment or Chain Assignment

In this format, a single value is assigned to two or more variables.

Augmented Assignment

In this format, we use the combination of mathematical expressions and values for the Variable. Other augmented Assignment forms are: &=, -=, **=, etc.

Browse more Topics Under Data Types, Variables and Constants

• Concept of Data types
• Built-in Data Types
• Constants in Programing Language 
• Access Modifier
• Variables of Built-in-Datatypes
• Declaration/Initialization of Variables
• Type Modifier

Few Rules for Assignment Statement

Few Rules to be followed while writing the Assignment Statements are:

• Variable names must begin with a letter, underscore, non-number character. Each language has its own conventions.
• The Data type defined and the variable value must match.
• A variable name once defined can only be used once in the program. You cannot define it again to store other types of value.
• If you assign a new value to an existing variable, it will overwrite the previous value and assign the new value.

FAQs on Assignment Statement

Q1. Which of the following shows the syntax of an  assignment statement ?

• variablename = expression ;
• expression = variable ;
• datatype = variablename ;
• expression = datatype variable ;

Answer – Option A.

Q2. What is an expression ?

• Same as statement
• List of statements that make up a program
• Combination of literals, operators, variables, math formulas used to calculate a value
• Numbers expressed in digits

Answer – Option C.

Q3. What are the two steps that take place when an  assignment statement  is executed?

• Evaluate the expression, store the value in the variable
• Reserve memory, fill it with value
• Evaluate variable, store the result
• Store the value in the variable, evaluate the expression.

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Data Types, Variables and Constants

• Variables in Programming Language
• Concept of Data Types
• Declaration of Variables
• Type Modifiers
• Access Modifiers
• Constants in Programming Language

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