assignment to constant variable. vue js

TypeError: Assignment to Constant Variable in JavaScript

Last updated: Mar 2, 2024 Reading time · 3 min

# TypeError: Assignment to Constant Variable in JavaScript

The "Assignment to constant variable" error occurs when trying to reassign or redeclare a variable declared using the const keyword.

When a variable is declared using const , it cannot be reassigned or redeclared.

Here is an example of how the error occurs.

# Declare the variable using let instead of const

To solve the "TypeError: Assignment to constant variable" error, declare the variable using the let keyword instead of using const .

Variables declared using the let keyword can be reassigned.

We used the let keyword to declare the variable in the example.

Variables declared using let can be reassigned, as opposed to variables declared using const .

You can also use the var keyword in a similar way. However, using var in newer projects is discouraged.

# Pick a different name for the variable

Alternatively, you can declare a new variable using the const keyword and use a different name.

We declared a variable with a different name to resolve the issue.

The two variables no longer clash, so the "assignment to constant" variable error is no longer raised.

# Declaring a const variable with the same name in a different scope

You can also declare a const variable with the same name in a different scope, e.g. in a function or an if block.

The if statement and the function have different scopes, so we can declare a variable with the same name in all 3 scopes.

However, this prevents us from accessing the variable from the outer scope.

# The const keyword doesn't make objects immutable

Note that the const keyword prevents us from reassigning or redeclaring a variable, but it doesn't make objects or arrays immutable.

We declared an obj variable using the const keyword. The variable stores an object.

Notice that we are able to directly change the value of the name property even though the variable was declared using const .

The behavior is the same when working with arrays.

Even though we declared the arr variable using the const keyword, we are able to directly change the values of the array elements.

The const keyword prevents us from reassigning the variable, but it doesn't make objects and arrays immutable.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• SyntaxError: Unterminated string constant in JavaScript
• TypeError (intermediate value)(...) is not a function in JS

Borislav Hadzhiev

Web Developer

Copyright © 2024 Borislav Hadzhiev

• ASP.NET Core
• Entity Framework

3 different ways to access constants in a Vue template

• by Karthik Chintala
• December 19, 2019 February 9, 2020

In this post, I’ll show how to access a constant in the vue <template> tag.

If we consume the constant directly in the vue template we’ll get an error when compiling the app.

[Vue warn]: Property or method “NumberOne” is not defined on the instance but referenced during render. Make sure that this property is reactive, either in the data option, or for class-based components, by initializing the property.

Here’s the example code with the above error.

The above example may not compile at all because NumberOne is defined but we are not using it anywhere.

error: ‘NumberOne’ is assigned a value but never used

If you are using the constant anywhere in the file, then you shouldn’t see the above error.

Okay, let’s fix accessing constant in the vue template section.

Adding a reactive property in data() — Not the best

One way to silence this is to add a local property in the data and use the local variable in the template instead of directly accessing the constant.

This solves the problem, but we don’t need to have a reactive variable for a constant. ( Vue change detection caveat )

Under the hood, vue creates getter and setter for numberOne . So, numberOne variable will be reactive.

For simple use cases like the above example, I think it’s okay to go with this approach. But, I don’t personally recommend.

Create private variables in created() hook — GOOD

We’ll have to create a variable in the created hook and utilize the variable in the template.

This is simple to do.

And this doesn’t add the variables to the reactive system because when the created() hook is called Vue has already finished the observation phase .

Note : all Vue API/internal properties start with either $ or _, so as long as your property doesn’t use these two prefixes it will be totally fine.

If you have very limited usage, I think this is the best way to use constants in the template.

A plugin to access constants — GOOD

If you have many constants that should be accessed across the files, it’s better to create a plugin and access these.

In the main.js file, add the following lines to use the plugin across.

and finally in the file(s), we’ll call the function.

This is the best way of having the constants if you wish to use them all over your files.

If you have many files that use the common constants, it’s better to go with this approach.

• creating private variables

Karthik is a passionate Full Stack developer working primarily on .NET Core, microservices, distributed systems, VUE and JavaScript. He also loves NBA basketball so you might find some NBA examples in his posts and he owns this blog.

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TypeError: invalid assignment to const "x"

The JavaScript exception "invalid assignment to const" occurs when it was attempted to alter a constant value. JavaScript const declarations can't be re-assigned or redeclared.

What went wrong?

A constant is a value that cannot be altered by the program during normal execution. It cannot change through re-assignment, and it can't be redeclared. In JavaScript, constants are declared using the const keyword.

Invalid redeclaration

Assigning a value to the same constant name in the same block-scope will throw.

Fixing the error

There are multiple options to fix this error. Check what was intended to be achieved with the constant in question.

If you meant to declare another constant, pick another name and re-name. This constant name is already taken in this scope.

const, let or var?

Do not use const if you weren't meaning to declare a constant. Maybe you meant to declare a block-scoped variable with let or global variable with var .

Check if you are in the correct scope. Should this constant appear in this scope or was it meant to appear in a function, for example?

const and immutability

The const declaration creates a read-only reference to a value. It does not mean the value it holds is immutable, just that the variable identifier cannot be reassigned. For instance, in case the content is an object, this means the object itself can still be altered. This means that you can't mutate the value stored in a variable:

But you can mutate the properties in a variable:

VueJs Variables: Data and Computed Properties in VueJs

James Sinkala

Last updated on July 6, 2022 .

Introduction

Just as any other JavaScript framework, Vue.js needs to work with data for it to be rendered useful. On this post, we are going to explore working with data inside Vue.js.

The Data Property

In Vue.js, data declaration is done inside the data property , it is a function that returns an object containing all the variables we can use on the template and inside other parts of the Vue.js app.

Let’s create a new Vue.js app to demonstrate this.

To display data inside our template as seen from the above example, we add the data property in the first argument of the createApp() function. We return an object containing our data variables, as we have done with firstName and lastName above. The data returned is reactive, meaning, any changes made to our variables will be reflected throughout the app instance. More on data reactivity in Vue.js will be covered in later posts.

We can simplify the data method from the above example as follows.

As we’ve seen in our example, firstName and lastName being top level properties returned by the data method will be accessed as independent variables inside our app. They will be accessible directly inside the template’s mustache syntax, and, in all other parts of the Vue app starting from the created hook and downwards on the Vue.js app lifecycle including methods inside this . We’ll explore Vue.js lifecycle hooks in a later post.

Beginning with the template, let’s see how the variables can be displayed inside the mustache delimiters.

In the above template, all paragraphs give the expected results just as expected in any other JavaScript environment. As explained in the previous post the expressions inside the mustache are subject to all JavaScript operations applicable to their specific data types, and we can see this in practice in this example.

Note that, all manipulations done to variables inside the mustache delimiters are solely for presentational purposes, the data values are never changed in this way. This is what we call one-way data binding , i.e data flows in one direction, in our case, from the data property to the template.

Vue.js does have support for two way binding, a subject that will be covered later .

If you have noticed, it takes a moment to figure out what some expressions within the above template are supposed to do, they are bloated and in being so become hard to maintain, especially if our app grows in features.

We can fix this with the aid of Vue.js computed properties .

The Computed Properties

Vue.js computed properties help simplify complex logic involving reactive data, enhancing its readability hence maintainability. The computed properties live inside the computed property of the Vue instance. Let’s see how this can help fix the situation that we’ve seen in our example above.

Instead of placing this cluttered code.

We can create a computed property for that logic as follows.

Afterwards, we can replace the cluttered template expression with a readable variable as follows.

From here on, whenever we need to have the name capitalized, we’ll just use the computed property placing it just as we would any data variable within our template code, no need to repeat the long expressions when we need the same complex logic applied on our data inside our templates.

The same can be done for the rest of the bloated expressions inside the template code.

Then, we’ll replace the cluttered code in our template with the cleaner computed properties.

On this post, we have covered how to work with data in Vue.js by declaring and using variables. We also got introduced to Vue.js computed properties, and saw how they can be used to complement variables or replace them when their expressions become complex within our templates, helping make our code readable and maintainable in the long run.

We will be covering Vue.js computed properties in depth in the next post .

Vue.js Computed Properties in Depth

Introduction to the VueJs Framework

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Troubleshooting and Fixing TypeError – Assignment to Constant Variable Explained

Introduction.

TypeError is a common error in JavaScript that occurs when a value is not of the expected type. One specific type of TypeError that developers often encounter is the “Assignment to Constant Variable” error. In this blog post, we will explore what a TypeError is and dive deep into understanding the causes of the “Assignment to Constant Variable” TypeError.

Causes of TypeError: Assignment to Constant Variable

The “Assignment to Constant Variable” TypeError is triggered when a developer attempts to modify a value assigned to a constant variable. There are a few common reasons why this error might occur:

Declaring a constant variable using the const keyword

One of the main causes of the “Assignment to Constant Variable” TypeError is when a variable is declared using the const keyword. In JavaScript, the const keyword is used to declare a variable that cannot be reassigned a new value. If an attempt is made to assign a new value to a constant variable, a TypeError will be thrown.

Attempting to reassign a value to a constant variable

Another cause of the “Assignment to Constant Variable” TypeError is when a developer tries to reassign a new value to a variable declared with the const keyword. Since constant variables are by definition, well, constant, trying to change their value will result in a TypeError.

Scoping issues with constant variables

Scoping plays an important role in JavaScript, and it can also contribute to the “Assignment to Constant Variable” TypeError. If a constant variable is declared within a specific scope (e.g., inside a block), any attempts to modify its value outside of that scope will throw a TypeError.

Common Scenarios and Examples

Scenario 1: declaring a constant variable and reassigning a value.

In this scenario, let’s consider a situation where a constant variable is declared, and an attempt is made to reassign a new value to it:

Explanation of the error:

When the above code is executed, a TypeError will be thrown, indicating that the assignment to the constant variable myVariable is not allowed.

Code example:

Troubleshooting steps:

• Double-check the declaration of the variable to ensure that it is indeed declared using const . If it is declared with let or var , reassigning a value is allowed.
• If you need to reassign a value to the variable, consider declaring it with let instead of const .

Scenario 2: Modifying a constant variable within a block scope

In this scenario, let’s consider a situation where a constant variable is declared within a block scope, and an attempt is made to modify its value outside of that block:

The above code will produce a TypeError, indicating that myVariable cannot be reassigned outside of the block scope where it is declared.

• Ensure that you are referencing the constant variable within the correct scope. Trying to modify it outside of that scope will result in a TypeError.
• If you need to access the variable outside of the block, consider declaring it outside the block scope.

Best Practices for Avoiding TypeError: Assignment to Constant Variable

Understanding when to use const vs. let.

In order to avoid the “Assignment to Constant Variable” TypeError, it’s crucial to understand the difference between using const and let to declare variables. The const keyword should be used when you know that the value of the variable will not change. If you anticipate that the value may change, consider using let instead.

Properly scoping constant variables

It’s essential to also pay attention to scoping when working with constant variables. Make sure that you are declaring the variables in the appropriate scope and avoid trying to modify them outside of that scope. This will help prevent the occurrence of the “Assignment to Constant Variable” TypeError.

Considerations for handling immutability

Immutability is a concept that is closely related to constant variables. Sometimes, using const alone may not be enough to enforce immutability. In such cases, you may need to use techniques like object freezing or immutable data structures to ensure that values cannot be modified.

In conclusion, the “Assignment to Constant Variable” TypeError is thrown when there is an attempt to modify a value assigned to a constant variable. By understanding the causes of this error and following best practices such as using const and let appropriately, scoping variables correctly, and considering immutability, you can write code that avoids this TypeError. Remember to pay attention to the specific error messages provided, as they can guide you towards the source of the problem and help you troubleshoot effectively.

By keeping these best practices in mind and understanding how to handle constant variables correctly, you can write cleaner and more reliable JavaScript code.

Related posts:

• Mastering Assignment to Constant Variable – Understanding the Syntax and Best Practices
• JavaScript Variable Types – Exploring the Differences Between var, let, and const
• Understanding the Differences – const vs let vs var – Which One to Use?
• Understanding Constant Variables in Java – A Complete Guide
• Understanding Assignment to Constant Variable – Tips and Strategies for Effective Programming

DEV Community

Posted on Jul 30, 2023 • Updated on Aug 3, 2023

Where to put constants in a .vue file

I often end up wishing there was a constants section in Vue, like data or methods . But something that useful will never be added into the framework, so instead I made a plugin to add it myself.

But lets explore the problem the plugin solves first:

This isn't great, because data is for reactive values that can be changed, and constants shouldn't be changed. The same problem happens if you return the constants from a setup function. So instead I could put them in computed , like so:

However, that ends up with a lot of boilerplate. So to get the best of both worlds, I created a Vue plugin to automate away all of that boilerplate.

Here's what the plugin lets you do

The plugin then runs the following code in every component during the beforeCreate lifecycle hook.

There is one downside to this approach, some IDEs/Editors will let you ctrl+click to go from the constant in the template to its definition in the script. This breaks that functionality. Though it should be possible to allow for it, I'm just not familiar with how. So if you know how, then create a PR to improve the plugin.

Here is the plugin, so you too can use this solution:

https://github.com/TheJaredWilcurt/vue-options-api-constants-plugin

Tell your friends. Let's get this to be a normal part of Vue.

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Add Application Constants in VueJS

Recently I was developing a web application using  VueJS , a clever javascript framework. We have had discussions at Gorges on how to choose between VueJS and  ReactJS , and I liked  Ted’s  observation that VueJS is easier to add to existing projects.

An early question I had was how to integrate application constants into VueJS. Web applications usually need some common constants, whether they be names, sizes, or just programming flags.

The constants could be placed into a VueJS component named Constants.vue, which would look something like this:

Admittedly this could instead be converted to a Javascript-only file named Constants.js. In other components or views, the constants would presumably be used like this:

This is acceptable, but the component would need to be referenced and loaded in every component. I later learned that a  plugin  would be easier.

Here is how a plugins/Constants.js file would look:

In my main.js file, I load the constants just once like this:

And reference these constants in various components:

There are probably dozens of ways to have accomplished this goal of referencing application-referencing wide constants in a VueJS program, but this approach seemed straightforward, easy, and intuitive.

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Reactivity in Depth ​

One of Vue’s most distinctive features is the unobtrusive reactivity system. Component state consists of reactive JavaScript objects. When you modify them, the view updates. It makes state management simple and intuitive, but it’s also important to understand how it works to avoid some common gotchas. In this section, we are going to dig into some of the lower-level details of Vue’s reactivity system.

What is Reactivity? ​

This term comes up in programming quite a bit these days, but what do people mean when they say it? Reactivity is a programming paradigm that allows us to adjust to changes in a declarative manner. The canonical example that people usually show, because it’s a great one, is an Excel spreadsheet:

Here cell A2 is defined via a formula of = A0 + A1 (you can click on A2 to view or edit the formula), so the spreadsheet gives us 3. No surprises there. But if you update A0 or A1, you'll notice that A2 automagically updates too.

JavaScript doesn’t usually work like this. If we were to write something comparable in JavaScript:

When we mutate A0 , A2 does not change automatically.

So how would we do this in JavaScript? First, in order to re-run the code that updates A2 , let's wrap it in a function:

Then, we need to define a few terms:

The update() function produces a side effect , or effect for short, because it modifies the state of the program.

A0 and A1 are considered dependencies of the effect, as their values are used to perform the effect. The effect is said to be a subscriber to its dependencies.

What we need is a magic function that can invoke update() (the effect ) whenever A0 or A1 (the dependencies ) change:

This whenDepsChange() function has the following tasks:

Track when a variable is read. E.g. when evaluating the expression A0 + A1 , both A0 and A1 are read.

If a variable is read when there is a currently running effect, make that effect a subscriber to that variable. E.g. because A0 and A1 are read when update() is being executed, update() becomes a subscriber to both A0 and A1 after the first call.

Detect when a variable is mutated. E.g. when A0 is assigned a new value, notify all its subscriber effects to re-run.

How Reactivity Works in Vue ​

We can't really track the reading and writing of local variables like in the example. There's just no mechanism for doing that in vanilla JavaScript. What we can do though, is intercept the reading and writing of object properties .

There are two ways of intercepting property access in JavaScript: getter / setters and Proxies . Vue 2 used getter / setters exclusively due to browser support limitations. In Vue 3, Proxies are used for reactive objects and getter / setters are used for refs. Here's some pseudo-code that illustrates how they work:

Code snippets here and below are meant to explain the core concepts in the simplest form possible, so many details are omitted, and edge cases ignored.

This explains a few limitations of reactive objects that we have discussed in the fundamentals section:

When you assign or destructure a reactive object's property to a local variable, accessing or assigning to that variable is non-reactive because it no longer triggers the get / set proxy traps on the source object. Note this "disconnect" only affects the variable binding - if the variable points to a non-primitive value such as an object, mutating the object would still be reactive.

The returned proxy from reactive() , although behaving just like the original, has a different identity if we compare it to the original using the === operator.

Inside track() , we check whether there is a currently running effect. If there is one, we lookup the subscriber effects (stored in a Set) for the property being tracked, and add the effect to the Set:

Effect subscriptions are stored in a global WeakMap<target, Map<key, Set<effect>>> data structure. If no subscribing effects Set was found for a property (tracked for the first time), it will be created. This is what the getSubscribersForProperty() function does, in short. For simplicity, we will skip its details.

Inside trigger() , we again lookup the subscriber effects for the property. But this time we invoke them instead:

Now let's circle back to the whenDepsChange() function:

It wraps the raw update function in an effect that sets itself as the current active effect before running the actual update. This enables track() calls during the update to locate the current active effect.

At this point, we have created an effect that automatically tracks its dependencies, and re-runs whenever a dependency changes. We call this a Reactive Effect .

Vue provides an API that allows you to create reactive effects: watchEffect() . In fact, you may have noticed that it works pretty similarly to the magical whenDepsChange() in the example. We can now rework the original example using actual Vue APIs:

Using a reactive effect to mutate a ref isn't the most interesting use case - in fact, using a computed property makes it more declarative:

Internally, computed manages its invalidation and re-computation using a reactive effect.

So what's an example of a common and useful reactive effect? Well, updating the DOM! We can implement simple "reactive rendering" like this:

In fact, this is pretty close to how a Vue component keeps the state and the DOM in sync - each component instance creates a reactive effect to render and update the DOM. Of course, Vue components use much more efficient ways to update the DOM than innerHTML . This is discussed in Rendering Mechanism .

The ref() , computed() and watchEffect() APIs are all part of the Composition API. If you have only been using Options API with Vue so far, you'll notice that Composition API is closer to how Vue's reactivity system works under the hood. In fact, in Vue 3 the Options API is implemented on top of the Composition API. All property access on the component instance ( this ) triggers getter / setters for reactivity tracking, and options like watch and computed invoke their Composition API equivalents internally.

Runtime vs. Compile-time Reactivity ​

Vue's reactivity system is primarily runtime-based: the tracking and triggering are all performed while the code is running directly in the browser. The pros of runtime reactivity are that it can work without a build step, and there are fewer edge cases. On the other hand, this makes it constrained by the syntax limitations of JavaScript, leading to the need of value containers like Vue refs.

Some frameworks, such as Svelte , choose to overcome such limitations by implementing reactivity during compilation. It analyzes and transforms the code in order to simulate reactivity. The compilation step allows the framework to alter the semantics of JavaScript itself - for example, implicitly injecting code that performs dependency analysis and effect triggering around access to locally defined variables. The downside is that such transforms require a build step, and altering JavaScript semantics is essentially creating a language that looks like JavaScript but compiles into something else.

The Vue team did explore this direction via an experimental feature called Reactivity Transform , but in the end we have decided that it would not be a good fit for the project due to the reasoning here .

Reactivity Debugging ​

It's great that Vue's reactivity system automatically tracks dependencies, but in some cases we may want to figure out exactly what is being tracked, or what is causing a component to re-render.

Component Debugging Hooks ​

We can debug what dependencies are used during a component's render and which dependency is triggering an update using the renderTracked onRenderTracked and renderTriggered onRenderTriggered lifecycle hooks. Both hooks will receive a debugger event which contains information on the dependency in question. It is recommended to place a debugger statement in the callbacks to interactively inspect the dependency:

Component debug hooks only work in development mode.

The debug event objects have the following type:

Computed Debugging ​

We can debug computed properties by passing computed() a second options object with onTrack and onTrigger callbacks:

• onTrack will be called when a reactive property or ref is tracked as a dependency.
• onTrigger will be called when the watcher callback is triggered by the mutation of a dependency.

Both callbacks will receive debugger events in the same format as component debug hooks:

onTrack and onTrigger computed options only work in development mode.

Watcher Debugging ​

Similar to computed() , watchers also support the onTrack and onTrigger options:

onTrack and onTrigger watcher options only work in development mode.

Integration with External State Systems ​

Vue's reactivity system works by deeply converting plain JavaScript objects into reactive proxies. The deep conversion can be unnecessary or sometimes unwanted when integrating with external state management systems (e.g. if an external solution also uses Proxies).

The general idea of integrating Vue's reactivity system with an external state management solution is to hold the external state in a shallowRef . A shallow ref is only reactive when its .value property is accessed - the inner value is left intact. When the external state changes, replace the ref value to trigger updates.

Immutable Data ​

If you are implementing an undo / redo feature, you likely want to take a snapshot of the application's state on every user edit. However, Vue's mutable reactivity system isn't best suited for this if the state tree is large, because serializing the entire state object on every update can be expensive in terms of both CPU and memory costs.

Immutable data structures solve this by never mutating the state objects - instead, it creates new objects that share the same, unchanged parts with old ones. There are different ways of using immutable data in JavaScript, but we recommend using Immer with Vue because it allows you to use immutable data while keeping the more ergonomic, mutable syntax.

We can integrate Immer with Vue via a simple composable:

Try it in the Playground

State Machines ​

State Machine is a model for describing all the possible states an application can be in, and all the possible ways it can transition from one state to another. While it may be overkill for simple components, it can help make complex state flows more robust and manageable.

One of the most popular state machine implementations in JavaScript is XState . Here's a composable that integrates with it:

RxJS is a library for working with asynchronous event streams. The VueUse library provides the @vueuse/rxjs add-on for connecting RxJS streams with Vue's reactivity system.

Connection to Signals ​

Quite a few other frameworks have introduced reactivity primitives similar to refs from Vue's Composition API, under the term "signals":

• Solid Signals
• Angular Signals
• Preact Signals
• Qwik Signals

Fundamentally, signals are the same kind of reactivity primitive as Vue refs. It's a value container that provides dependency tracking on access, and side-effect triggering on mutation. This reactivity-primitive-based paradigm isn't a particularly new concept in the frontend world: it dates back to implementations like Knockout observables and Meteor Tracker from more than a decade ago. Vue Options API and the React state management library MobX are also based on the same principles, but hide the primitives behind object properties.

Although not a necessary trait for something to qualify as signals, today the concept is often discussed alongside the rendering model where updates are performed through fine-grained subscriptions. Due to the use of Virtual DOM, Vue currently relies on compilers to achieve similar optimizations . However, we are also exploring a new Solid-inspired compilation strategy (Vapor Mode) that does not rely on Virtual DOM and takes more advantage of Vue's built-in reactivity system.

API Design Trade-Offs ​

The design of Preact and Qwik's signals are very similar to Vue's shallowRef : all three provide a mutable interface via the .value property. We will focus the discussion on Solid and Angular signals.

Solid Signals ​

Solid's createSignal() API design emphasizes read / write segregation. Signals are exposed as a read-only getter and a separate setter:

Notice how the count signal can be passed down without the setter. This ensures that the state can never be mutated unless the setter is also explicitly exposed. Whether this safety guarantee justifies the more verbose syntax could be subject to the requirement of the project and personal taste - but in case you prefer this API style, you can easily replicate it in Vue:

Angular Signals ​

Angular is undergoing some fundamental changes by foregoing dirty-checking and introducing its own implementation of a reactivity primitive. The Angular Signal API looks like this:

Again, we can easily replicate the API in Vue:

Compared to Vue refs, Solid and Angular's getter-based API style provide some interesting trade-offs when used in Vue components:

• () is slightly less verbose than .value , but updating the value is more verbose.
• There is no ref-unwrapping: accessing values always require () . This makes value access consistent everywhere. This also means you can pass raw signals down as component props.

Whether these API styles suit you is to some extent subjective. Our goal here is to demonstrate the underlying similarity and trade-offs between these different API designs. We also want to show that Vue is flexible: you are not really locked into the existing APIs. Should it be necessary, you can create your own reactivity primitive API to suit more specific needs.

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3 different ways to access constants in a Vue JS template

In this post, I’ll show how to access a constant in the vue <template> tag.

If we consume the constant directly in the vue template we’ll get an error when compiling the app.

[Vue warn]: Property or method “NumberOne” is not defined on the instance but referenced during render. Make sure that this property is reactive, either in the data option, or for class-based components, by initializing the property.

Here’s the example code with the above error.

The above example may not compile at all because NumberOne is defined but we are not using it anywhere.

error: ‘NumberOne’ is assigned a value but never used

If you are using the constant anywhere in the file, then you shouldn’t see the above error.

Okay, let’s fix accessing constant in the vue template section.

Adding a reactive property in data() – Not the best

One way to silence this is to add a local property in the data and use the local variable in the template instead of directly accessing the constant.

This solves the problem, but we don’t need to have a reactive variable for a constant. ( Vue change detection caveat )

Under the hood, vue creates getter and setter for numberOne . So, numberOne variable will be reactive.

For simple use cases like the above example, I think it’s okay to go with this approach. But, I don’t personally recommend.

Create private variables in created() hook – GOOD

We’ll have to create a variable in the created hook and utilize the variable in the template.

This is pretty simple to do.

And this doesn’t add the variables to the reactive system because when the created() hook is called Vue has already finished the observation phase .

Note all Vue API/internal properties start with either $ or _, so as long as your property doesn’t use these two prefixes it will be totally fine.

If you have very limited usage, I think this is the best way to use constants in the template.

A plugin to access constants – GOOD

If you have many constants that should be accessed across the files, it’s better to create a plugin and access these.

In the main.js file, add the following lines to use the plugin across.

This is the best way of having the constants if you wish to use them all over your files.

If you have many files that use the common constants, it’s better to go with this approach.

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How to define common constants in Vue.js?

• Post author By John Au-Yeung
• Post date March 12, 2022
• No Comments on How to define common constants in Vue.js?

Sometimes, we want to define common constants in Vue.js.

In this article, we’ll look at how to define common constants in Vue.js.

To define common constants in Vue.js, we can create a module that export an object with the constant values.

For instance, in deliverMethods.js , we write

to export an object with some constants.

Then in our components, we can import our constant file and use it by writing

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Typeerror assignment to constant variable

Doesn’t know how to solve the “Typeerror assignment to constant variable” error in Javascript?

Don’t worry because this article will help you to solve that problem

In this article, we will discuss the Typeerror assignment to constant variable , provide the possible causes of this error, and give solutions to resolve the error.

First, let us know what this error means.

What is Typeerror assignment to constant variable?

“Typeerror assignment to constant variable” is an error message that can occur in JavaScript code.

It means that you have tried to modify the value of a variable that has been declared as a constant.

In JavaScript, when you declare a variable using the const keyword, its value cannot be changed or reassigned.

Attempting to modify a constant variable, you will receive an error stating:

Here is an example code snippet that triggers the error:

In this example, we have declared a constant variable greeting and assigned it the value “Hello” .

When we try to reassign greeting to a different value (“Hi”) , we will get the error:

because we are trying to change the value of a constant variable.

Let us explore more about how this error occurs.

How does Typeerror assignment to constant variable occurs ?

This “ TypeError: Assignment to constant variable ” error occurs when you attempt to modify a variable that has been declared as a constant.

In JavaScript, constants are variables whose values cannot be changed once they have been assigned.

When you declare a variable using the const keyword, you are telling JavaScript that the value of the variable will remain constant throughout the program.

If you try to modify the value of a constant variable, you will get the error:

This error can occur in various situations, such as:

• Attempting to reassign a constant variable:

When you declare a variable using the const keyword, its value cannot be changed.

If you try to reassign the value of a constant variable, you will get this error.

Here is an example :

In this example, we declared a constant variable age and assigned it the value 30 .

When we try to reassign age to a different value ( 35 ), we will get the error:

• Attempting to modify a constant object:

If you declare an object using the const keyword, you can still modify the properties of the object.

However, you cannot reassign the entire object to a new value.

If you try to reassign a constant object, you will get the error.

For example:

In this example, we declared a constant object person with two properties ( name and age ).

We are able to modify the age property of the object without triggering an error.

However, when we try to reassign person to a new object, we will get the Typeerror.

• Using strict mode:

In strict mode, JavaScript throws more errors to help you write better code.

If you try to modify a constant variable in strict mode, you will get the error.

In this example, we declared a constant variable name and assigned it the value John .

However, because we are using strict mode, any attempt to modify the value of name will trigger the error.

Now let’s fix this error.

Typeerror assignment to constant variable – Solutions

Here are the alternative solutions that you can use to fix “Typeerror assignment to constant variable” :

Solution 1: Declare the variable using the let or var keyword:

If you need to modify the value of a variable, you should declare it using the let or var keyword instead of const .

Just like the example below:

Solution 2: Use an object or array instead of a constant variable:

If you need to modify the properties of a variable, you can use an object or array instead of a constant variable.

Solution 3: Declare the variable outside of strict mode:

If you are using strict mode and need to modify a variable, you can declare the variable outside of strict mode:

Solution 4: Use the const keyword and use a different name :

This allows you to keep the original constant variable intact and create a new variable with a different value.

Solution 5: Declare a const variable with the same name in a different scope :

This allows you to create a new constant variable with the same name as the original constant variable.

But with a different value, without modifying the original constant variable.

For Example:

You can create a new constant variable with the same name, without modifying the original constant variable.

By declaring a constant variable with the same name in a different scope.

This can be useful when you need to use the same variable name in multiple scopes without causing conflicts or errors.

So those are the alternative solutions that you can use to fix the TypeError.

By following those solutions, you can fix the “Typeerror assignment to constant variable” error in your JavaScript code.

Here are the other fixed errors that you can visit, you might encounter them in the future.

• typeerror unsupported operand type s for str and int
• typeerror: object of type int64 is not json serializable
• typeerror: bad operand type for unary -: str

In conclusion, in this article, we discussed   “Typeerror assignment to constant variable” , provided its causes and give solutions that resolve the error.

By following the given solution, surely you can fix the error quickly and proceed to your coding project again.

I hope this article helps you to solve your problem regarding a  Typeerror   stating  “assignment to constant variable” .

We’re happy to help you.

Happy coding! Have a Good day and God bless.

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Web Development

Mobile app development, nodejs typeerror: assignment to constant variable.

Published By: Divya Mahi

Published On: November 17, 2023

Published In: Development

Grasping and Fixing the 'NodeJS TypeError: Assignment to Constant Variable' Issue

Introduction.

Node.js, a powerful platform for building server-side applications, is not immune to errors and exceptions. Among the common issues developers encounter is the “NodeJS TypeError: Assignment to Constant Variable.” This error can be a source of frustration, especially for those new to JavaScript’s nuances in Node.js. In this comprehensive guide, we’ll explore what this error means, its typical causes, and how to effectively resolve it.

Understanding the Error

In Node.js, the “TypeError: Assignment to Constant Variable” occurs when there’s an attempt to reassign a value to a variable declared with the const keyword. In JavaScript, const is used to declare a variable that cannot be reassigned after its initial assignment. This error is a safeguard in the language to ensure the immutability of variables declared as constants.

Diving Deeper

This TypeError is part of JavaScript’s efforts to help developers write more predictable code. Immutable variables can prevent bugs that are hard to trace, as they ensure that once a value is set, it cannot be inadvertently changed. However, it’s important to distinguish between reassigning a variable and modifying an object’s properties. The latter is allowed even with variables declared with const.

Common Scenarios and Fixes

Example 1: reassigning a constant variable.

Javascript:

Fix: Use let if you need to reassign the variable.

Example 2: Modifying an Object's Properties

Fix: Modify the property instead of reassigning the object.

Example 3: Array Reassignment

Fix: Modify the array’s contents without reassigning it.

Example 4: Within a Function Scope

Fix: Declare a new variable or use let if reassignment is needed.

Example 5: In Loops

Fix: Use let for variables that change within loops.

Example 6: Constant Function Parameters

Fix: Avoid reassigning function parameters directly; use another variable.

Example 7: Constants in Conditional Blocks

Fix: Use let if the variable needs to change.

Example 8: Reassigning Properties of a Constant Object

Fix: Modify only the properties of the object.

Strategies to Prevent Errors

Understand const vs let: Familiarize yourself with the differences between const and let. Use const for variables that should not be reassigned and let for those that might change.

Code Reviews: Regular code reviews can catch these issues before they make it into production. Peer reviews encourage adherence to best practices.

Linter Usage: Tools like ESLint can automatically detect attempts to reassign constants. Incorporating a linter into your development process can prevent such errors.

Best Practices

Immutability where Possible: Favor immutability in your code to reduce side effects and bugs. Normally use const to declare variables, and use let only if you need to change their values later .

Descriptive Variable Names: Use clear and descriptive names for your variables. This practice makes it easier to understand when a variable should be immutable.

Keep Functions Pure: Avoid reassigning or modifying function arguments. Keeping functions pure (not causing side effects) leads to more predictable and testable code.

The “NodeJS TypeError: Assignment to Constant Variable” error, while common, is easily avoidable. By understanding JavaScript’s variable declaration nuances and adopting coding practices that embrace immutability, developers can write more robust and maintainable Node.js applications. Remember, consistent coding standards and thorough code reviews are your best defense against common errors like these.

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