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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

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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: 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:

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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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• 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. Message: TypeError: invalid assignment to const "x" (Firefox) TypeError: Assignment to constant variable. (Chrome) TypeErro 1 min read
• JavaScript TypeError - Property "X" is non-configurable and can't be deleted This JavaScript exception property is non-configurable and can't be deleted if the user tries to delete a property, and the property is non-configurable. Message: TypeError: Calling delete on 'x' is not allowed in strict mode (Edge) TypeError: property "x" is non-configurable and can't be deleted. ( 1 min read
• JavaScript TypeError - Can't redefine non-configurable property "x" This JavaScript exception can't redefine non-configurable property occurs if user tries to redefine a property, but that property is non-configurable. Message: TypeError: Cannot modify non-writable property {x} (Edge) TypeError: can't redefine non-configurable property "x" (Firefox) TypeError: Canno 1 min read
• JavaScript TypeError - Variable "x" redeclares argument This JavaScript exception variable redeclares argument occurs in strict-mode only and if the variable name which is also function parameter has been redeclared with the var keyword. Message: TypeError: variable "x" redeclares argument (Firefox) Error Type: TypeError Cause of the Error: A variable wh 1 min read
• JavaScript TypeError - Setting getter-only property "x" This JavaScript exception setting getter-only property works in strict-mode only and occurs if the user tries to set a new value to a property for which only a getter is specified. Message: TypeError: Assignment to read-only properties is not allowed in strict mode (Edge) TypeError: setting getter-o 2 min read
• JavaScript TypeError - Invalid 'instanceof' operand 'x' This JavaScript exception invalid 'instanceof' operand occurs if the right operand of the instanceof operator can not be used with a constructor object. It is an object that contains a prototype property and can be called. Message: TypeError: invalid 'instanceof' operand "x" (Firefox) TypeError: "x" 1 min read
• JavaScript TypeError - Invalid Array.prototype.sort argument This JavaScript exception invalid Array.prototype.sort argument occurs if the parameter of Array.prototype.sort() is not from either undefined or a function which sorts accordingly. Message: TypeError: argument is not a function object (Edge) TypeError: invalid Array.prototype.sort argument (Firefox 1 min read
• JavaScript TypeError - Cyclic object value This JavaScript exception cyclic object value occurs if the references of objects were found in JSON. JSON.stringify() fails to solve them. Message: TypeError: cyclic object value (Firefox) TypeError: Converting circular structure to JSON (Chrome and Opera) TypeError: Circular reference in value arg 1 min read
• JavaScript TypeError - Can't delete non-configurable array element This JavaScript exception can't delete non-configurable array element that occurs if there is an attempt to short array-length, and any one of the array's elements is non-configurable. Message: TypeError: can't delete non-configurable array element (Firefox) TypeError: Cannot delete property '2' of 1 min read

JS Other Errors

• JavaScript URIError | Malformed URI Sequence This JavaScript exception malformed URI sequence occurs if the encoding or decoding of URI is unsuccessful. Message: URIError: The URI to be encoded contains invalid character (Edge) URIError: malformed URI sequence (Firefox) URIError: URI malformed (Chrome) Error Type: URIError Cause of the Error: 1 min read
• JavaScript Warning - Date.prototype.toLocaleFormat is deprecated This JavaScript warning Date.prototype.toLocaleFormat is deprecated; consider using Intl.DateTimeFormat instead occurs if user is using the non-standard Date.prototype.toLocaleFormat method. Message: Warning: Date.prototype.toLocaleFormat is deprecated; consider using Intl.DateTimeFormat instead Err 1 min read
• Logging Script Errors in JavaScript In this article, we will learn to log script Errors using JavaScript. It is useful in case where scripts from any other source run/executes under the website (Ex - in an iframe) or cases where any of your viewer makes use of Browser's Console trying to alter the script or you want to monitor your JS 6 min read

JS Error Instance

• JavaScript Error message Property In JavaScript error message property is used to set or return the error message. Syntax: errorObj.message Return Value: It returns a string, representing the details of the error. More example codes for the above property are as follows: Below is the example of the Error message property. Example 1: 1 min read
• JavaScript Error name Property In JavaScript, the Error name property is used to set or return the name of an error. Syntax: errorObj.name Property values: This property contains six different values as described below: SyntaxError: It represents a syntax error.RangeError: It represents an error in the range.ReferenceError: It re 2 min read
• JavaScript Error.prototype.toString() Method The Error.prototype.toString() method is an inbuilt method in JavaScript that is used to return a string representing the specified Error object. Syntax: e.toString() Parameters: This method does not accept any parameters. Return value: This method returns a string representing the specified Error o 1 min read
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Uncaught TypeError: Assignment to constant variable

Good day, guys. In this post, we’ll look at how to solve the "Uncaught TypeError: Assignment to constant variable" programming puzzle.

Just remove the const from script which is same within the document. You can not update the value of declared const CODE. The value of a variable declared with const remains the same within its scope. It cannot be updated or re-declared. 

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