uncaught typeerror assignment to constant

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

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TypeError: invalid assignment to const "x"

Const and immutability, 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 is meant to appear in a function, for example?

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:

Document Tags and Contributors

• JavaScript basics
• JavaScript first steps
• JavaScript building blocks
• Introducing JavaScript objects
• Introduction
• Grammar and types
• Control flow and error handling
• Loops and iteration
• Expressions and operators
• Numbers and dates
• Text formatting
• Regular expressions
• Indexed collections
• Keyed collections
• Working with objects
• Details of the object model
• Iterators and generators
• Meta programming
• A re-introduction to JavaScript
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JavaScript Error: Assignment to Constant Variable

Last updated on October 3, 2023 at 3:29 PM

Krste Rajchevski

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TypeError: Assignment to constant variable

Have you ever encountered a JavaScript error like "TypeError: Assignment to constant variable"? Don't worry, you're not alone! This error occurs when you try to reassign a value to a variable that has been declared with the const keyword. In this blog post, we'll explore what this error means, why it happens, and how you can solve it.

There are a few common causes for the "TypeError: Assignment to constant variable" error. Let's take a closer look at each one and discuss possible solutions.

Cause 1: Attempting to Change a Constant Variable

In JavaScript, const is used to declare variables that are meant to remain constant and cannot be reassigned. Therefore, if you try to assign a new value to a constant variable, such as:

The above code will throw a "TypeError: Assignment to constant variable" error. The solution is simple: if you need to change the value of a variable, use the let keyword instead of const :

Cause 2: Accidental Re-declaration of a Constant Variable

Another common mistake that leads to this error is accidentally re-declaring a constant variable within the same scope. Consider the following example:

The above code will result in a "TypeError: Assignment to constant variable" error because the variable myConstant has already been declared using const . To fix this issue, choose a different name for the re-declaration or remove the second declaration altogether:

Cause 3: Trying to Update Properties of an Object Declared as Constant

In JavaScript, when we declare an object as a constant using the const keyword, we can still modify the properties of that object. However, we cannot assign a new object to the constant variable itself. Consider the following example:

In the code above, attempting to assign a new object to myObject will result in a "TypeError: Assignment to constant variable" error. However, updating the property of myObject itself, such as myObject.value = 20 , is allowed.

To fix the "TypeError: Assignment to constant variable" error, you need to identify the cause and apply the appropriate solution.

If you are trying to change the value of a variable, make sure it is declared using the let keyword instead of const .

If you accidentally re-declared a constant variable within the same scope, either choose a different name for the re-declaration or remove the second declaration.

If you need to modify the properties of an object declared as a constant, make sure you are not trying to assign a new object to the constant variable itself. Instead, update the properties of the existing object.

By following these solutions, you'll be able to overcome the "TypeError: Assignment to constant variable" error and write error-free JavaScript code.

The "TypeError: Assignment to constant variable" error occurs when you try to reassign a value to a variable declared with const . This error can be solved by understanding the causes and implementing the appropriate solutions. Remember to use let instead of const when you need to change the value of a variable, avoid re-declaring constants within the same scope, and update the properties of an object instead of assigning a new object to a constant variable. With these tips in mind, you'll be well equipped to handle this error and write more robust JavaScript code.

We hope this article has been helpful! If you have any questions or need further assistance, please feel free to leave a comment below. Happy coding!

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The JavaScript const keyword declares variables that cannot be reassigned, preventing their modification. It prohibits redeclaration and provides block scope, introduced in ES2015 (ES6) for defining immutable variables.

Properties:

• Cannot be reassigned.
• It has Block Scope
• It can be assigned to the variable on the declaration line.
• It’s a Primitive value.
• The property of a const object can be changed but it cannot be changed to a reference to the new object
• The values inside the const array can be changed, it can add new items to const arrays but it cannot reference a new array.
• Re-declaring of a const variable inside different block scopes is allowed.
• Cannot be Hoisted.
• Creates only read-only references to value.

Examples of JavaScript Const

Cannot be reassigned.

Example 1: It describes that the const variable cannot be reassigned. 

Example 2: It describes the const variable which contains the Block Scope. 

Variables must be Assigned

Example: It describes the const variable and assigned it after declaration. 

Output: 

Cannot be Hoisted

Example: It describes the const variable cannot be Hoisted. 

Const in Arrays

Example: It describes that the array values can be modified only reference to the array cannot be changed. 

Const in Objects

Example: It describes that the object properties can be modified only reference to the object cannot be changed. 

  Supported Browsers:

• Google Chrome
• Edge  

P.S: To clear your concept of var, const, and let please go through How to declare variables in different ways in JavaScript?

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[Fixed] TypeError: Assignment to constant variable in JavaScript

Table of Contents

Problem : TypeError: Assignment to constant variable

Rename the variable, change variable type to let or var, check if scope is correct, const and immutability.

TypeError: Assignment to constant variable in JavaScript occurs when we try to reassign value to const variable. If we have declared variable with const , it can’t be reassigned.

Let’s see with the help of simple example.

Solution : TypeError: Assignment to constant variable

If you are supposed to declare another constant, just declare another name.

If you are supposed to change the variable value, then it shouldn’t be declared as constant.

Change type to either let or var.

You can check if scope is correct as you can have different const in differnt scopes such as function.

This is valid declaration as scope for country1 is different.

const declaration creates read only reference. It means that you can not reassign it. It does not mean that you can not change values in the object.

Let’s see with help of simple example:

But, you can change the content of country object as below:

That’s all about how to fix TypeError: Assignment to constant variable in javascript.

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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:

Document Tags and Contributors

• JavaScript basics
• JavaScript first steps
• JavaScript building blocks
• Introducing JavaScript objects
• Introduction
• Grammar and types
• Control flow and error handling
• Loops and iteration
• Expressions and operators
• Numbers and dates
• Text formatting
• Regular expressions
• Indexed collections
• Keyed collections
• Working with objects
• Details of the object model
• Using promises
• Iterators and generators
• Meta programming
• Client-side web APIs
• A re-introduction to JavaScript
• JavaScript data structures
• Equality comparisons and sameness
• Inheritance and the prototype chain
• Strict mode
• JavaScript typed arrays
• Memory Management
• Concurrency model and Event Loop
• References:
• ArrayBuffer
• AsyncFunction
• Float32Array
• Float64Array
• GeneratorFunction
• InternalError
• Intl.Collator
• Intl.DateTimeFormat
• Intl.ListFormat
• Intl.NumberFormat
• Intl.PluralRules
• Intl.RelativeTimeFormat
• ReferenceError
• SharedArrayBuffer
• SyntaxError
• Uint16Array
• Uint32Array
• Uint8ClampedArray
• WebAssembly
• decodeURI()
• decodeURIComponent()
• encodeURI()
• encodeURIComponent()
• parseFloat()
• Arithmetic operators
• Array comprehensions
• Assignment operators
• Bitwise operators
• Comma operator
• Comparison operators
• Conditional (ternary) operator
• Destructuring assignment
• Expression closures
• Generator comprehensions
• Grouping operator
• Legacy generator function expression
• Logical operators
• Object initializer
• Operator precedence
• (currently at stage 1) allows the creation of chained function calls in a readable manner. Basically, the pipeline operator provides syntactic sugar on a function call with a single argument allowing you to write">Pipeline operator
• Property accessors
• Spread syntax
• async function expression
• class expression
• delete operator
• function expression
• function* expression
• in operator
• new operator
• void operator
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• async function
• for await...of
• for each...in
• function declaration
• import.meta
• try...catch
• Arrow functions
• Default parameters
• Method definitions
• Rest parameters
• The arguments object
• constructor
• element loaded from a different domain for which you violated the same-origin policy.">Error: Permission denied to access property "x"
• InternalError: too much recursion
• RangeError: argument is not a valid code point
• RangeError: invalid array length
• RangeError: invalid date
• RangeError: precision is out of range
• RangeError: radix must be an integer
• RangeError: repeat count must be less than infinity
• RangeError: repeat count must be non-negative
• ReferenceError: "x" is not defined
• ReferenceError: assignment to undeclared variable "x"
• ReferenceError: can't access lexical declaration`X' before initialization
• ReferenceError: deprecated caller or arguments usage
• ReferenceError: invalid assignment left-hand side
• ReferenceError: reference to undefined property "x"
• SyntaxError: "0"-prefixed octal literals and octal escape seq. are deprecated
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• SyntaxError: "x" is a reserved identifier
• SyntaxError: JSON.parse: bad parsing
• SyntaxError: Malformed formal parameter
• SyntaxError: Unexpected token
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• SyntaxError: a declaration in the head of a for-of loop can't have an initializer
• SyntaxError: applying the 'delete' operator to an unqualified name is deprecated
• SyntaxError: for-in loop head declarations may not have initializers
• SyntaxError: function statement requires a name
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Constants are block-scoped, much like variables declared using the let keyword. The value of a constant can't be changed through reassignment, and it can't be redeclared.

The constant's name, which can be any legal identifier .

The constant's value. This can be any legal expression , including a function expression.

The Destructuring Assignment syntax can also be used to declare variables.

Description

This declaration creates a constant whose scope can be either global or local to the block in which it is declared. Global constants do not become properties of the window object, unlike var variables.

An initializer for a constant is required. You must specify its value in the same statement in which it's declared. (This makes sense, given that it can't be changed later.)

The const 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 the case where the content is an object, this means the object's contents (e.g., its properties) can be altered.

All the considerations about the " temporal dead zone " apply to both let and const .

A constant cannot share its name with a function or a variable in the same scope.

Basic const usage

Constants can be declared with uppercase or lowercase, but a common convention is to use all-uppercase letters.

Block scoping

It's important to note the nature of block scoping.

const needs to be initialized

Const in objects and arrays.

const also works on objects and arrays.

Specifications

Browser compatibility.

• Constants in the JavaScript Guide

© 2005–2021 MDN contributors. Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/const

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Posted on Feb 27, 2019

Here is how I change the value of const keyword in Javascript

Every Javascript developer knows that var and let is reassignable but const cannot be reassigned or redeclared again.

But there is a little secret about const , let's look at some code.

As expected, we are unable to reassign val to another number. How about string?

Still no. How about the array and object?

Javascript: Nope nope nope you can't do that nope... But what if we do this:

What?! Let's continue and play around a bit...

OMG what just happened?

From MDN Web Docs, it describes:

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 the case where the content is an object, this means the object's contents (e.g., its properties) can be altered.

Who is the variable identifier/constant here? arrVariable , not the array itself.

MDN said variable identifier/constant cannot be reassigned, which means arrVariable cannot be reassigned . But what about the array? It doesn't have any effect, of course, it is still mutable.

const tells the reader that your variable cannot be reassigned, which is why it is highly recommended to use. It prevents us to create some unnecessary bugs and improve code readability.

Similar to object:

So next time if someone asks you about our friend const , you know what to say.

Still nope, anyway...

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Good one. enjoyed it

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关于“TypeError: Assignment to constant variable”的问题解决方案

在项目开发过程中，在使用变量声明时，如果不注意，可能会造成类型错误 比如：

Uncaught (in promise) TypeError: Assignment to constant variable. 未捕获的类型错误：赋值给常量变量。

我们使用 const 定义了变量且存在初始值。 后面又给这个变量赋值，所以报错了。

ES6 标准引入了新的关键字 const 来定义常量， const 与 let 都具有块级作用域：

• 使用 const 定义的常量，不能修改它的值，且定义的常量必须赋初值；
• let 定义的是变量，可以进行变量赋值操作，且不需要赋初值。

这个错误就是因为我们修改了常量而引起的错误，虽然某些浏览器不报错，但是无效果！

将 const 改为 let 进行声明。

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javascript エラー「Uncaught TypeError: Assignment to constant variable.」の原因

• 作成日 2020.10.21
• 更新日 2022.08.04

chromeでエラー「Uncaught TypeError: Assignment to constant variable.」が発生する原因と解決方法を記述をしてます。

• 5.1. オブジェクトの値を操作
• OS windows10 pro 64bit
• Apache 2.4.43
• ブラウザ chrome 103.0.5060.114

chrome103にて、以下のエラーが発生

発生したコード ※ その他の発生パターンは後述してます。

firefox102の場合は以下となります。

IE11の場合は以下となります。

constで、定数にしたものに値を代入しようとしたため

値を代入するのであれば、「let」 か 「var」を使用します。

もしくは別の定数を用意して、そこに代入します。

どうしても同じ変数名にしたい場合は、関数などを使用してスコープを変更します。

オブジェクトの値を操作

オブジェクトに、値を再割り当てすると同じエラーが発生します。

こちらも「let」か「var」を使用することが解決します。

ちなみに、配列は操作可能です。

「const」を使用して「if」文に「=」が足らない場合も発生します。

「==」か「===」を使用すれば解決します。

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