javascript error invalid left hand side in assignment

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.

Borislav Hadzhiev

Web Developer

Copyright © 2024 Borislav Hadzhiev

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.

Take your skills to the next level ⚡️

Fixing Assignment Errors in JavaScript: 'Invalid left-hand side'

• "Invalid assignment left-hand side"

What it Means:

This error arises in JavaScript when you attempt to assign a value to something that cannot be assigned to. In simpler terms, you're trying to put data (the value on the right side of the equal sign) into a location (the left side of the equal sign) that doesn't accept it.

Common Causes:

Here are some frequent scenarios that lead to this error:

Mistaking Comparison for Assignment:

• You might accidentally use a single equal sign ( = ) instead of a comparison operator (like == or === ) within an if statement or similar conditional block.
• For example: if (x = 5 ) { // Incorrect - trying to assign within an if statement console .log( "This won't work" ); }
• The correct way to compare would be: if (x === 5 ) { // Correct - using comparison operator console .log( "This works" ); }

Assigning to Read-Only Values:

• JavaScript has certain built-in values or properties that cannot be changed directly. Trying to assign to these will trigger the error.
• Constants declared with const .

Incorrect Object Property Access:

Fixing the Error:

Double-Check Comparisons:

Verify Object Properties:

• ReferenceError: Invalid left-hand side in assignment: This error might appear in similar scenarios, often when you attempt to assign to a variable that hasn't been declared or is inaccessible. Double-check variable declarations and scope.
• TypeError: Cannot set property 'x' of undefined: This arises when you try to assign to a property of an undefined variable or object. Ensure the variable or object exists before accessing its properties.

Troubleshooting Tips:

• Use Browser Developer Tools: When you encounter these errors, inspect your code using your browser's developer tools (usually F12 key). The console will pinpoint the exact line causing the issue, making debugging easier.
• Read Error Messages Carefully: Error messages often provide valuable clues about the source of the problem. Pay attention to the line number mentioned in the error and the specific details it reveals.
• Break Down Complex Code: If you're working with intricate logic, try breaking it down into smaller, more manageable chunks. This can help isolate the part where the invalid assignment is occurring.
• Console Logging for Debugging: Strategically place console.log statements throughout your code to inspect variable values and the flow of your program. This can aid in visualizing the state of your variables at different points.
• Consider Linting Tools: Tools like ESLint or JSHint can help you identify potential errors and enforce code style guidelines, which might catch these issues early on.

Additional Cautions:

• Strict vs. Non-Strict Mode: JavaScript has strict mode, which enforces stricter rules on variable declarations and other aspects. Some errors you might encounter in non-strict mode might become SyntaxErrors in strict mode.
• Asynchronous Code: When dealing with asynchronous code (like using promises or callbacks), be cautious about assigning values before they're available. Understand the timing and flow of your asynchronous operations.

Related Example Codes for "Invalid assignment left-hand side" Errors in JavaScript:

A. document.getElementById Result:

B. const Variables:

Fix the Issue: Depending on the cause, you might need to:

• Use comparison operators: If you're accidentally using an assignment operator ( = ) in a conditional statement, use the correct comparison operator (like == , === , != , etc.).
• Avoid read-only assignments: If you're trying to modify a read-only value (like document.getElementById result or a const variable), consider alternative approaches (e.g., modify element content using properties like innerHTML or declare a new variable).
• Ensure object properties exist: If you're assigning to a non-existent property of an object, check if the property exists before assigning.

Here's a different way to think about it:

Imagine the "Invalid assignment left-hand side" error as a warning sign. It's pointing out a potential problem in your code. By addressing this issue, you'll end up with code that works as intended.

"Errors: Invalid date" is a common error you might encounter in JavaScript code when you try to work with dates but provide invalid data

This error occurs when you use the instanceof operator incorrectly in your JavaScript code. The instanceof operator is used to check whether an object was created by a specific constructor function

"Errors: is not iterable" indicates that you're trying to use a loop or other construct that expects a sequence of values (iterable) on something that JavaScript can't iterate over

JavaScript: JavaScript is a programming language commonly used to create interactive web pages and applications.JSON: JavaScript Object Notation (JSON) is a lightweight data format that resembles JavaScript objects

What it is: This error indicates that JavaScript encountered a malformed (incorrectly formatted) Uniform Resource Identifier (URI) while trying to process a web address or URL

"Errors: Missing initializer in const"What it Means:This error occurs when you declare a variable using the const keyword (used for constants) but don't assign a value to it at the same time

"Missing parenthesis after argument list"Meaning:This error arises in JavaScript when you call a function or method but forget to include the closing parenthesis ')' after the list of arguments you're passing to it

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• ReferenceError: invalid assignment left-hand side

ReferenceError .

What went wrong?

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

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

• Assignment operators
• Comparison operators

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Jan 26, 2017 6:00:03 AM | JavaScript - ReferenceError: invalid assignment left-hand side

Today we examine the invalid assignment error, which is thrown, as the name implies, when code attempts to perform an invalid assignment somewhere.

Next on the list in our extensive JavaScript Error Handling series we're going to examine the Invalid Left-Hand Assignment error in greater detail. The Invalid Left-Hand Assignment error is a sub-object of ReferenceError and is thrown, as the name implies, when code attempts to perform an invalid assignment somewhere.

In this post we'll look at a few code examples to illustrate some common methods of producing an Invalid Left-Hand Assignment error, as well as examine how to handle this error when it rears its ugly head. Let the party begin!

The Technical Rundown

• All JavaScript error objects are descendants of the  Error  object, or an inherited object therein.
• The  ReferenceError  object is inherited from the  Error  object.
• The Invalid Left-Hand Assignment error is a specific type of ReferenceError object.

When Should You Use It?

As one of the simplest JavaScript errors to understand, the Invalid Left-Hand Assignment error appears in only a handful of situations in which code is attempting to pass an assignment incorrectly. While this is generally thought of as a syntactic issue, JavaScript defines this particular assignment error as a ReferenceError, since the engine effectively assumes an assignment to a non-referenced variable is being attempted.

The most common example of an Invalid Left-Hand Assignment error is when attempting to compare a value using a assignment operator (=), rather than using a proper comparison operator (== or ===). For example, here we're attempting to perform a basic comparison of the variable name with the values John or Fred. Unfortunately, we've made the mistake of using the assignment operator =, instead of a comparison operator such as == or ===:

try { var name = 'Bob'; if (name = 'John' || name = 'Fred') { console.log(`${name} returns!`) } else { console.log(`Just ${name} this time.`) } } catch (e) { if (e instanceof ReferenceError) { printError(e, true); } else { printError(e, false); } }

Sure enough, rather than giving us an output, the JavaScript engine produces the expected Invalid Left-Hand Assignment error:

It's worth noting that catching an Invalid Left-Hand Assignment error with a typical try-catch block is particular difficult, because the engine parses the code from inside out, meaning inner code blocks are parsed and executed before outer blocks. Since the issue of using a = assignment operator instead of a == comparison operator means the actual structure of the code is changed from the expected, the outer try-catch fails to be parsed and properly executed. In short, this means Invalid Left-Hand Assignment errors are always "raw", without any simple means of catching them.

Another common method for producing an Invalid Left-Hand Assignment error is when attempting to concatenate a string value onto a variable using the addition assignment += operator, instead of the concatenation operator +. For example, below we're attempting to perform concatenation on the name variable on multiple lines, but we've accidentally used the += operator:

try { var name = 'Bob' += ' Smith';

console.log(`Name is ${name}.`); } catch (e) { if (e instanceof ReferenceError) { printError(e, true); } else { printError(e, false); } }

This isn't the syntax JavaScript expects when concatenating multiple values onto a string, so an Invalid Left-Hand Assignment error is thrown:

To resolve this, we simply need to replace += with the concatenation operator +:

try { var name = 'Bob' + ' Smith';

Now we skip the Invalid Left-Hand Assignment error entirely and get our expected output indicating the full name stored in the name variable:

To dive even deeper into understanding how your applications deal with JavaScript Errors, check out the revolutionary Airbrake JavaScript error tracking tool for real-time alerts and instantaneous insight into what went wrong with your JavaScript code.

Written By: Frances Banks

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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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JavaScript ReferenceError – Invalid assignment left-hand side

This JavaScript exception invalid assignment left-hand side occurs if there is a wrong assignment somewhere in code. A single “=” sign instead of “==” or “===” is an Invalid assignment.

Error Type:

Cause of the error: There may be a misunderstanding between the assignment operator and a comparison operator.

Basic Example of ReferenceError – Invalid assignment left-hand side, run the code and check the console

Example 1: In this example, “=” operator is misused as “==”, So the error occurred.

Example 2: In this example, the + operator is used with the declaration, So the error has not occurred.

Output: 

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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

© 2005–2023 MDN contributors. Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_assignment_left-hand_side

Invalid left-hand side in assignment expression

Hello. I am attempting to create a self-generating biology question that randomly generates three numbers for the problem question, then asks a yes or no question. When I was attempting to create the function that checks for the answer to the question and compared it to the student input, I get the “Invalid left-hand side in assignment expression”

My code is here, line 33 in the JavaScript window: https://codepen.io/KDalang/pen/OJpEdQB

Here is the specific line in question: if (chiTotal <= 3.841 && input=“Yes”) What did I do wrong?

= is assignment of a value to a variable == is weak comparison (with type coercion) === is strong comparison (probably what you want)

Hey thanks for the quick reply! I actually want it to be a “less than or equal to” and I used <=. <== and <=== don’t do anything either.

Edit: Nevermind, I understand now.

Do you try to compare values or do you try to assign a value?

Oh my gosh! Sorry its 2a.m. over here I understand what you and JeremyLT are saying now. Thanks so much!

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what is the wrong in my code it showing error " Invalid left-hand side in assignment "?

it is showing error Invalid left-hand side in assignment? what is wrong in this code can any once help ?

• 1 Don't submit code in a image- it makes it harder for other users to run that code, or use it in an answer. See the formatting guide . –  battery.cord Commented Mar 24, 2017 at 13:03
• remove a==1 and similar. it is not needed –  Ashwani Commented Mar 24, 2017 at 14:07

2 Answers 2

The problem is that you have expressions returning true , which you then attempt to assign values.

What you meant to do is:

The switch statement handles the comparisons for you. case 1 is the case where a is equal to 1.

Also, you want to get the element with the id "number", and instead of switch(a) , you want switch(+a) .

Problem is with your JavaScript. Syntax for Switch case is wrong. This is how it should be:

JavaScript documentation

• @popoornimapuligadda replace your switch case code with above code. –  Rohit Mourya Commented Mar 24, 2017 at 13:03

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Syntax Error: Invalid left-hand side in assignment expression

I have received data from backend which contains a student object.

The student contains a field named grade . I would like to explicitly update that value of the student object in my code:

When I run my app, I ended up with error "Invalid left-hand side in assignment expression" for the code showed above. Why? What is the correct way to update that value then?

=== About using the optional syntax student?.grade ===

The reason why I use optional syntax is that the student at the beginning is undefined or null until it is loaded. If I remove the optional ? I would end up with another error "undefined is not an object" when evaluating student.grade .

• react-native

• @SuperStormer please see my update, I explained why using optional syntax. –  user842225 Commented May 27, 2021 at 11:57
• you can't just wrap the whole thing inside an if (student) { const {grade} .... } ? –  user3282374 Commented May 27, 2021 at 12:00
• "the student at the beginning is undefined or null until it is loaded" And what would you expect to happen if you try to assign to student?.grade when student is null ? Should it silently skip the assignment? Should it populate student with a new object? Optional chaining is for accessing properties, not assigning to them. –  Felix Kling Commented May 27, 2021 at 12:01
• try with student! –  Thiluxan Commented May 27, 2021 at 12:04
• @Thiluxan that's typescript (non-null assertion operator), not javascript. –  user3282374 Commented May 27, 2021 at 12:06

Optional chaining should be used when reading the value, not when assigning it. You can update the piece of code to this:

Edit: Mistake pointed out by Felix King

• I think it should just be if (student) . Currently you are only assigning to student.grade if student.grade is already set. –  Felix Kling Commented May 27, 2021 at 12:03
• You should probably provide a reference why optional chaining is for retrieval only. –  user3282374 Commented May 27, 2021 at 12:04

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