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SyntaxError: invalid assignment left-hand side

The JavaScript exception "invalid assignment left-hand side" occurs when there was an unexpected assignment somewhere. It may be triggered when a single = sign was used instead of == or === .

SyntaxError or ReferenceError , depending on the syntax.

What went wrong?

There was an unexpected assignment somewhere. This might be due to a mismatch of an assignment operator and an equality operator , for example. While a single = sign assigns a value to a variable, the == or === operators compare a value.

Typical invalid assignments

In the if statement, you want to use an equality operator ( === ), and for the string concatenation, the plus ( + ) operator is needed.

Assignments producing ReferenceErrors

Invalid assignments don't always produce syntax errors. Sometimes the syntax is almost correct, but at runtime, the left hand side expression evaluates to a value instead of a reference , so the assignment is still invalid. Such errors occur later in execution, when the statement is actually executed.

Function calls, new calls, super() , and this are all values instead of references. If you want to use them on the left hand side, the assignment target needs to be a property of their produced values instead.

Note: In Firefox and Safari, the first example produces a ReferenceError in non-strict mode, and a SyntaxError in strict mode . Chrome throws a runtime ReferenceError for both strict and non-strict modes.

Using optional chaining as assignment target

Optional chaining is not a valid target of assignment.

Instead, you have to first guard the nullish case.

• Assignment operators
• Equality operators

How to fix SyntaxError: invalid assignment left-hand side

Let me show you an example that causes this error and how I fix it.

How to reproduce this error

How to fix this error, other causes for this error.

You can also see this error when you use optional chaining as the assignment target.

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Invalid left-hand side in assignment in JavaScript [Solved]

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

# Invalid left-hand side in assignment in JavaScript [Solved]

The "Invalid left-hand side in assignment" error occurs when we have a syntax error in our JavaScript code.

The most common cause is using a single equal sign instead of double or triple equals in a conditional statement.

To resolve the issue, make sure to correct any syntax errors in your code.

Here are some examples of how the error occurs.

# Use double or triple equals when comparing values

The most common cause of the error is using a single equal sign = instead of double or triple equals when comparing values.

The engine interprets the single equal sign as an assignment and not as a comparison operator.

We use a single equals sign when assigning a value to a variable.

However, we use double equals (==) or triple equals (===) when comparing values.

# Use bracket notation for object properties that contain hyphens

Another common cause of the error is trying to set an object property that contains a hyphen using dot notation.

You should use bracket [] notation instead, e.g. obj['key'] = 'value' .

# Assigning the result of calling a function to a value

The error also occurs when trying to assign the result of a function invocation to a value as shown in the last example.

If you aren't sure where to start debugging, open the console in your browser or the terminal in your Node.js application and look at which line the error occurred.

The screenshot above shows that the error occurred in the index.js file on line 25 .

You can hover over the squiggly red line to get additional information on why the error was thrown.

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How to fix SyntaxError – ‘invalid assignment left-hand side in JavaScript’?

In JavaScript, a SyntaxError : Invalid Assignment Left-Hand Side occurs when the interpreter encounters an invalid expression or structure on the left side of an assignment operator ( = ).

This error typically arises when trying to assign a value to something that cannot be assigned, such as literals, expressions, or the result of function calls. Let’s explore this error in more detail and see how to resolve it with some examples.

Understanding an error

An invalid assignment left-hand side error occurs when the syntax of the assignment statement violates JavaScript’s rules for valid left-hand side expressions. The left-hand side should be a target that can receive an assignment like a variable, object property or any array element, let’s see some cases where this error can occur and also how to resolve this error.

Case 1: Error Cause: Assigning to Literals

When you attempt to assign a value to a literal like a number, string or boolean it will result in SyntaxError: Invalid Assignment Left-Hand Side .

Resolution of error

In this case values should be assigned to variables or expressions which will be on the left side of an equation and avoid assigning values directly to literals.

Case 2: Error Cause: Assigning to Function Calls

Assigning a value directly to the result of function call will give an invalid assignment left-hand side error.

Explanation : In this example, getX() returns a value but is not a valid target for assignment. Assignments should be made to variables, object properties, or array elements, not to the result of a function call.

Therefore, store it into a variable or at least place it on the left-hand side that is valid for assignment.

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Demystifying JavaScript‘s "Invalid Assignment Left-Hand Side" Error

Assignment operations are fundamental in JavaScript – we use them all the time to assign values to variables. However, occasionally you may come across a confusing error:

This "Invalid Assignment Left-Hand Side" error occurs when you try to assign a value to something that JavaScript will not allow. At first glance, this doesn‘t seem to make sense – isn‘t assignment valid in JS?

In this comprehensive guide, we‘ll demystify exactly when and why this error occurs and equip you with the knowledge to resolve it.

Assignment and Equality Operators in JavaScript

To understand this error, we first need to understand the role of assignment and equality operators in JavaScript.

The Assignment Operator

The assignment operator in JS is the single equals sign = . It is used to assign a value to a variable, like so:

This stores the value 10 in the variable x . Simple enough!

The Equality Operator

The equality operator == checks if two values are equal to each other. For example:

The equality operator == is different from the assignment operator = – it compares values rather than assigning them.

Mixing up assignment and equality is a common source of bugs in JS programs.

Immutable vs Mutable Values in JavaScript

In JavaScript, some values are immutable – they cannot be changed or reassigned. The most common immutable values are:

• Constants like Math.PI
• Primitive values like undefined or null

Trying to reassign an immutable value will lead to our error.

On the other hand, mutable values like variables can be reassigned:

Keeping mutable vs immutable values in mind is key to avoiding "Invalid Assignment" errors.

When and Why This Error Occurs

There are two main situations that cause an "Invalid Assignment Left-Hand Side" error:

1. Attempting to Mutate an Immutable Constant

Immutable constants in JavaScript cannot be reassigned. For example:

Core language constants like Math.PI are immutable. Trying to alter them with the assignment operator = will throw an error.

You‘ll also get an error trying to reassign a declared const variable:

2. Accidentally Using Assignment = Instead of Equality ==

Another common source of this error is accidentally using the single = assignment operator when you meant to use the == equality operator:

This can lead to logical errors, as you are assigning 10 to x rather than checking if x equals 10 .

According to a 2020 survey, over 40% of JavaScript developers have made this mistake that led to bugs in their code.

Example Error Message

When an invalid assignment occurs, you‘ll see an error like:

This tells us there is an invalid assignment on line 2 of myScript.js . The full error message gives us an important clue that an assignment operation is causing the issue.

Let‘s look at a full code example:

Running this would result in our error:

Now that we‘ve seen the error, let‘s walk through debugging techniques.

Debugging an Invalid Assignment

When the "Invalid Assignment Left-Hand Side" error appears, follow these steps:

• Identify the exact line causing the issue from the error stack trace
• Check if the line is trying to reassign a constant value
• If so, use a variable instead of a constant
• Otherwise, verify = is intended and not == for equality

Let‘s demonstrate with our code example:

The error said line 2 was invalid, so we examine it:

Aha! We‘re trying to assign to the constant PI . Since constants are immutable, this causes an error.

To fix, we need to use a mutable variable instead:

That‘s all there is to debugging simple cases like this. Now let‘s look at some tips to avoid the problem altogether.

Avoiding the "Invalid Assignment" Error

With knowledge of assignments and equality in JavaScript, you can avoid these errors with:

• Using const for true constants – Avoid reassignment by default
• Declaring variables rather than trying to mutate language builtins
• Take care with = vs == – Understand what each one does
• Use a linter – Catches many invalid assignments before runtime
• Improve testing – Catch assumption errors around assignments early
• Refactor code – Make invalid assignments impossible through design

Avoiding mutations and validating equality logic will steer you clear of this problem.

Why This Error Matters

At first glance, the "Invalid Assignment Left-Hand Side" error may seem minor. However, it points to flawed assumptions around assignments and equality in JavaScript that can cause major issues down the line.

That‘s why understanding this error is about more than just fixing that one line of code. It represents a milestone in solidifying your mental models around immutable values, variables, assignment and equality in JavaScript.

Making assignments consciously and validating them through linting and testing will level up your code quality and make you a more proficient JS developer.

Key Takeaways

To recap, the core takeaways around the "Invalid Assignment Left-Hand Side" error are:

• It occurs when trying to assign a value to a constant or immutable value
• Accidentally using = instead of == for equality checks is another common cause
• The error message directly states "invalid assignment" which provides a clue
• Debug by checking for assignment to constants or verifying equality checks
• Declare variables and use const properly to avoid reassignment errors
• Differentiate between = assignment and == equality checks

Learning to debug and avoid this error will improve your fundamental JavaScript skills. With time, you‘ll handle invalid assignments with ease!

Dealing with "Invalid Assignment Left-Hand Side" errors may seem cryptic initially. But by leveraging the error message itself and understanding assignments in JavaScript, you can swiftly resolve them.

Immutable values and equality logic are at the heart of these errors. With care and awareness around assignments, you can sidestep these issues in your code going forward.

Debugging and resolving errors like this are an important part of the JavaScript learning journey. Each one makes you a little wiser! So don‘t get discouraged when you run into an "Invalid Assignment" error. Leverage the techniques in this guide to level up your skills.

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Invalid left-hand side in assignment #29629

randalpinto Oct 3, 2021

Beta Was this translation helpful? Give feedback.

right, after a tortuous path I finally found the culprit. We use Sentry and deep in their docs it says that the webpack configuration is not compatible with serverless environments: https://docs.sentry.io/platforms/javascript/guides/nextjs/manual-setup/ . This can be closed.

Replies: 4 comments · 3 replies

Patrickchodev oct 3, 2021.

ijjk Oct 5, 2021 Maintainer

{{editor}}'s edit

Randalpinto oct 6, 2021 author.

zigang93 Apr 11, 2023

KosGrillis Nov 10, 2023

favll May 5, 2024

This discussion was converted from issue #29582 on October 05, 2021 03:50.

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Javascript error : invalid assignment left-hand side

Using javascript in Acrobat XI. For some reason, I keep getting the following error:

invalid assignment left-hand side at 9: line 10

My code is pretty simple and looks spot on AFAICT. Please review and tell me I'm not crazy. (Or tell me I am, but you have a solution :))

• 5 boo on the -3. This is a legitimate question about equality in javascript that doesn't seem to make total sense. See answer and comments below. –  Scott Holtzman Commented Sep 25, 2013 at 21:14
• 2 It's not a question about equality, it's "please check my JavaScript". It could be a good question if you at the very least pointed out which line is line 10 and asked a legit question about why using = there isn't allowed. –  JJJ Commented Sep 25, 2013 at 21:17
• @Juhana -> fair point. I didn't learn it was a question about equality until I got the answer. However, had I know it was the = sign was throwing it off I would have asked, but I didn't know, so it's hard to ask what you don't know. Also, pointing out line 10 seemed overkill with such short code and only breaking on line 10. –  Scott Holtzman Commented Sep 25, 2013 at 21:28
• 1 Reason #26 why VB* is evil: It's one of the few language families left that uses the same operator for equality and assignment. If you hadn't gotten used to its brain-damaged syntax, the error would have jumped out at you. –  cHao Commented Sep 25, 2013 at 21:36
• @cHao -> and wouldn't you know that VB is where I started, so the bad habits are deeply ingrained! –  Scott Holtzman Commented Sep 25, 2013 at 21:37

Two equal signs:

I used three equal signs in my code above for keeping your coding style consistent.

As vol7ron suggested, you should also add parentheses in your IF statements. This greatly improves readability in my opinion.

• this one as well if (event.target === f2 || event.target = f3 && event.value = "On") –  Kyle Commented Sep 25, 2013 at 21:08
• and before the f3 ; actually, I think you just mistyped ; edited –  vol7ron Commented Sep 25, 2013 at 21:09
• 1 ... or better still, use strict comparison === , following the rest of the code. –  Reinstate Monica -- notmaynard Commented Sep 25, 2013 at 21:09
• 2 @ScottHoltzman it's also helpful to use parentheses ( () ) when using && and || together or in the presence of complex conditionals . When bouncing between languages you'll find that some have different precedence rules, whereas parens are generally high up on OoO –  vol7ron Commented Sep 25, 2013 at 21:18
• 1 @ScottHoltzman f2.value = "Off" is per se correct, but you shouldn't type it in an IF condition (why do you would need to do so?). –  ComFreek Commented Sep 26, 2013 at 18:18

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