assignment to constant variable

TypeError: Assignment to Constant Variable in JavaScript

Last updated: Jul 25, 2022 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 © 2023 Borislav Hadzhiev

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The const declaration declares block-scoped local variables. The value of a constant can't be changed through reassignment using the assignment operator , but if a constant is an object , its properties can be added, updated, or removed.

The name of the variable to declare. Each must be a legal JavaScript identifier or a destructuring binding pattern .

Initial value of the variable. It can be any legal expression.

Description

The const declaration is very similar to let :

• const declarations are scoped to blocks as well as functions.
• const declarations can only be accessed after the place of declaration is reached (see temporal dead zone ). For this reason, const declarations are commonly regarded as non-hoisted .
• const declarations do not create properties on globalThis when declared at the top level of a script.
• const declarations cannot be redeclared by any other declaration in the same scope.

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

The const declaration creates an immutable 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. You should understand const declarations as "create a variable whose identity remains constant", not "whose value remains constant" — or, "create immutable bindings ", not "immutable values".

Many style guides (including MDN's ) recommend using const over let whenever a variable is not reassigned in its scope. This makes the intent clear that a variable's type (or value, in the case of a primitive) can never change. Others may prefer let for non-primitives that are mutated.

The list that follows the const keyword is called a binding list and is separated by commas, where the commas are not comma operators and the = signs are not assignment operators . Initializers of later variables can refer to earlier variables in the list.

Basic const usage

Constants can be declared with uppercase or lowercase, but a common convention is to use all-uppercase letters, especially for primitives because they are truly immutable.

Block scoping

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

const in objects and arrays

const also works on objects and arrays. Attempting to overwrite the object throws an error "Assignment to constant variable".

However, object keys are not protected, so the following statement is executed without problem.

You would need to use Object.freeze() to make an object immutable.

The same applies to arrays. Assigning a new array to the variable throws an error "Assignment to constant variable".

Still, it's possible to push items into the array and thus mutate it.

Declaration with destructuring

The left-hand side of each = can also be a binding pattern. This allows creating multiple variables at once.

For more information, see Destructuring assignment .

Specifications

Browser compatibility.

BCD tables only load in the browser with JavaScript enabled. Enable JavaScript to view data.

• Constants in the JavaScript Guide

How to Fix the ‘TypeError: invalid assignment to const “x” ‘ Error in Our JavaScript App?

• Post author By John Au-Yeung
• Post date August 22, 2021
• No Comments on How to Fix the ‘TypeError: invalid assignment to const “x” ‘ Error in Our JavaScript App?

Sometimes, we may run into the ‘TypeError: invalid assignment to const "x"’ when we’re developing JavaScript apps.

In this article, we’ll look at how to fix the ‘TypeError: invalid assignment to const "x"’ when we’re developing JavaScript apps.

Fix the ‘TypeError: invalid assignment to const "x"’ When Developing JavaScript Apps

To fix the ‘TypeError: invalid assignment to const "x"’ when we’re developing JavaScript apps, we should make sure we aren’t assigning a variable declared with const to a new value.

On Firefox, the error message for this error is TypeError: invalid assignment to const "x" .

On Chrome, the error message for this error is TypeError: Assignment to constant variable.

And on Edge, the error message for this error is TypeError: Assignment to const .

For example, the following code will throw this error:

We tried to assign 100 to COLUMNS which is declared with const , so we’ll get this error.

To fix this, we write:

to declare 2 variables, or we can use let to declare a variable that we can reassign a value to:

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By John Au-Yeung

Web developer specializing in React, Vue, and front end development.

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JavaScript TypeError – Invalid assignment to const “X”

This JavaScript exception invalid assignment to const occurs if a user tries to change a constant value. Const declarations in JavaScript can not be re-assigned or re-declared.

Error Type:

Cause of Error: A const value in JavaScript is changed by the program which can not be altered during normal execution. 

Example 1: In this example, the value of the variable(‘GFG’) is changed, So the error has occurred.

Output(in console):

Example 2: In this example, the value of the object(‘GFG_Obj’) is changed, So the error has occurred.

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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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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.

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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