javascript error assignment to constant variable

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Error "Assignment to constant variable" in ReactJS

I did follow a tutorial of how to integrate mailchimp with node backend. I have never touched back end, so am pretty lame at it. When I POST to their API I get the subscriber's credentials, but I get an error back - "Assignment to constant variable". Reading through the web and other SO questions, it seems like I am trying to reassign a CONST value.

I had a goooooooooood look at my code and the only thing I have noticed that might be issues here is

I have noticed I am creating an empty object called "resObj" , then trying to assign a value to it. I have tried changing the CONST to LET , but I get an error saying: "resObj is not defined" .

Here is my front end code:

And the Back end code:

If anyone could help I would be very grateful. The subscription form works, but I need to clear that bug in order for my front end to work correctly onto submission of the form.

• variable-assignment

• 1 Can't reassign a const . const resObj = {}; should be let resObj = {}; –  Nick Commented May 27, 2020 at 22:08
• 1 Does this answer your question? Const in JavaScript: when to use it and is it necessary? –  Emile Bergeron Commented May 27, 2020 at 22:08
• @Nick I have tried it and it doesn't work for some reason... error: " There was an error with your request" message: "respObj is not defined" –  Denis Denchev Commented May 27, 2020 at 22:24

Maybe what you are looking for is Object.assign(resObj, { whatyouwant: value} )

This way you do not reassign resObj reference (which cannot be reassigned since resObj is const), but just change its properties.

Reference at MDN website

Edit: moreover, instead of res.send(respObj) you should write res.send(resObj) , it's just a typo

• I still get error: " There was an error with your request" message: "respObj is not defined" –  Denis Denchev Commented May 27, 2020 at 22:30
• 1 mmmm... It look like a different problem. Wait, I think you just misspelled resObj with respObj –  Luca Fabbian Commented May 28, 2020 at 0:11
• Yes, thank you guys, you are legends! Such a silly mistake and I have been staring at the screen for 2 days, trying to solve it... I have changed the CONST to LET and fixed the typo and it all works now perfectly, thank you ! Thank you @Luca Fabbian ! –  Denis Denchev Commented May 28, 2020 at 10:48

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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

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:

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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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Fixing the Uncaught TypeError: Assignment to Constant Variable in JavaScript

Rakesh Purohit

Frequently asked questions, what does it mean when javascript says "assignment to constant variable", can i change the properties of an object declared with const, is it possible to declare a constant without initializing it, how can i fix the "assignment to constant variable" error.

In the dynamic world of JavaScript development, encountering errors is a part of the learning curve that every developer must navigate. Among these, the "Uncaught TypeError: Assignment to Constant Variable" stands out as a common yet puzzling hurdle. This error not only interrupts the normal execution of your code but also serves as a crucial learning opportunity to understand JavaScript declarations better.

This blog dives deep into the causes of this error, explores solutions, and offers best practices to avoid it in the future, ensuring your development process is smoother and more efficient.

Understanding the Error

What is the "uncaught typeerror: assignment to constant variable".

The JavaScript exception “Assignment to constant variable” occurs when there's an attempt to alter a constant value. Constants, declared using the const keyword, are meant to be a read-only reference to a value. Unlike var or let, constants cannot be re-assigned or redeclared, making them a staple for values that should remain unchanged throughout the execution of a program.

Why does JavaScript throw this exception?

JavaScript enforces the immutability of constants to ensure code predictability and integrity. When a constant variable is attempted to be modified, JavaScript throws this error to signal a breach of its fundamental rules. Consider the following snippet:

This code attempts to re-assign a new value to const x, leading to the mentioned TypeError.

Causes and Solutions

How can assigning a value to the same constant name cause an error.

Assigning a new value to a constant within the same block scope is a direct violation of the const declaration's principle. Block scope, in JavaScript, refers to the area within a function or a block where the variable or constant is accessible. Since const creates a block-scoped variable, any attempt to re-assign its value within the same scope leads to an error.

What does block scope mean in the context of this error?

Block scope is crucial in understanding this error. It delineates the boundaries within which a constant is valid. For instance:

In this example, a is block-scoped within the if statement, and re-assigning it triggers the error.

Strategies to avoid and resolve this error.

To circumvent this error, developers have multiple options:

• Use let for variables that need re-assignment.

• Ensure that constants are not mistakenly re-assigned within their scope.

• Re-evaluate the need for the variable to be a constant if re-assignment is necessary.

Best Practices for Using Const

What does the const declaration imply for variable mutability.

The const declaration creates a read-only reference, meaning the variable identifier cannot be re-assigned. However, if the constant is an object, the object's properties can still be mutated. This distinction is vital for using const effectively without running into the "assignment to constant variable" error.

Guidelines for using const effectively in JavaScript code.

• Use const for all variables that do not require re-assignment.

• Understand the scope of your constants to avoid invalid assignments.

• Remember, const does not make the value immutable, just the variable identifier.

Debugging and Troubleshooting

Steps to identify and fix the "assignment to constant variable" error..

When faced with this error, the first step is to locate the constant that is being incorrectly re-assigned. Review your code to ensure that you are not trying to alter a constant within its scope. Utilizing debugging tools and console logs can help identify the exact line where the error occurs.

Utilizing debugging tools to isolate and resolve the issue.

Modern IDEs and browsers come equipped with powerful debugging tools that can pinpoint where the re-assignment attempt is happening. Breakpoints and step-through execution allow developers to observe the state of their variables and constants at various execution points, making it easier to understand and fix the error.

Understanding and resolving the "Uncaught TypeError: Assignment to Constant Variable" error is a stepping stone towards mastering JavaScript. By adhering to best practices and utilizing effective debugging strategies, developers can ensure their code is robust, error-free, and maintainable.

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