liveedit compile failed uncaught syntaxerror invalid left hand side in assignment

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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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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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

Error Message:"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...

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

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Uncaught ReferenceError: Invalid left-hand side in assignment

I am trying to use the following fiddle in my code with few differences :

The HTML is :

In Javascript, since I have to use jQuery 1.6.1, the on method/function doesn’t work and hence I am using live as shown below:

When I click on Upload/Browse File , I get Uncaught ReferenceError: Invalid left-hand side in assignment error in the console. Please let me know what am I doing wrong here? Thanks

At the jQuery val documentation page you will see that assignment isn’t used.

Thanks. What would be a good strategy to replace this part of the JSFiddle:

with the jQuery using correct val() in my case:

Should I just change $("#uploadFile").val() = this.val(); to $("#uploadFile").val() ?

No, you should put what you want to assign to the upload file value inside of the parenthesis.

This topic was automatically closed 91 days after the last reply. New replies are no longer allowed.

Uncaught syntaxerror invalid left-hand side in assignment

The   uncaught syntaxerror invalid left-hand side in assignment   is an error message that is frequently encountered while working with JavaScript.

This error message is easy to fix however, if you’re not familiar with you’ll get confused about how to resolve it.

Fortunately, in this article, we’ll delve into the causes of this syntaxerror and solutions for the  invalid left-hand side in assignment expression .

What is uncaught syntaxerror “invalid left-hand side in assignment”?

The error message uncaught syntaxerror invalid left-hand side in assignment  happens in JavaScript when you make an unexpected assignment somewhere. 

For example:

Here’s another one:

This error is triggered if you use just one or single equal sign “ = ” instead of double “ == ” or triple equals “ === .”

In addition to that, this error message typically indicates that there is a problem with the syntax of an assignment statement.

Why does the “invalid left-hand side in assignment” syntaxerror occur?

The JavaScript exception   invalid assignment left-hand side  usually occurs when there was an unexpected assignment.

It is because you are using a single equal = sign rather than a double == or triple sign ===.

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 incorrect.

How to fix the “uncaught syntaxerror invalid left-hand side in assignment”?

To fix the  uncaught syntaxerror invalid left hand side in assignment expression   error, you need to identify where the unexpected assignment is happening in your code.

This error may be triggered when a single equal “= “ sign is being used instead of double “==” or triple “===.”

Ensure that you are using the correct operator for the intended operation.

A single equal sign “=” is used to assign a value to a variable. Meanwhile, the double equal sign “==” or triple “===” operators are used to compare values.

Here are the following solutions which you can use as your bases when troubleshooting the error.

Solution 1: Use double equals (==) or triple equals (===) when comparing values in JavaScript

Incorrect code:

Corrected code:

As what we mentioned above, in JavaScript, the single equals sign (=) is used for assigning a value to a variable, while double equals (==) or triple equals (===) are used for comparison operations.

The single equals sign is interpreted as an assignment operator, not a comparison operator.

Solution 2: Use correct operator for string concatenation

To resolve this error change the “+=” operator with the plus (+) operator for string concatenation

Note: The “+=” operator is used to add and assign a value to a variable, while the plus (+) operator is used for string concatenation.

In conclusion, the error message uncaught syntaxerror invalid left-hand side in assignment expression  happens in JavaScript when you make an unexpected assignment somewhere. 

To fix this   error, you need to identify where the unexpected assignment is happening in your code and ensure that you are using the correct operator for the intended operation.

This article already provides solutions to fix this error message. By executing the solutions above, you can master this  SyntaxError  with the help of this guide.

You could also check out other  SyntaxError  articles that may help you in the future if you encounter them.

• Syntaxerror: multiple exception types must be parenthesized
• Uncaught syntaxerror: invalid shorthand property initializer
• Expression.syntaxerror: token comma expected.

We are hoping that this article helps you fix the error.  Thank you for reading itsourcecoders 😊

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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

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• ReferenceError: invalid assignment left-hand side

The JavaScript exception "invalid assignment left-hand side" occurs when there was an unexpected assignment somewhere. For example, a single " = " sign was used instead of " == " or " === ".

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.

Typical invalid assignments

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

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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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JavaScript SyntaxError – Invalid left-hand side in postfix operation

A “SyntaxError: Invalid left-hand side in postfix operation” error occurs when JavaScript’s interpreter comes across an invalid expression that appears on the left hand side of a postfix increment (++) or decrement (–) operator, it is only variables or expressions which can take new values after this operation that are supposed to be used with postfix operators, let us examine this error, its causes and how it can be resolved using examples.

Understanding an error

An “Invalid left-hand side in postfix operation” error means that the expression on the left-hand side of a postfix increment or decrement operator (++ or –) is not a legal target for the modification, that variable, object property, or array element must be directly incremented or decremented by JavaScript, let’s some cases where this error can occur and how to resolve this error.

Case 1: Error Cause: Using a Literal or Constant as Left-Hand Side

If we try to do postfix increment or decrement operation for literal or constant it will give an “Invalid left-hand side in postfix operation” error.

Resolution of error

The use of variable names that can be re-assigned is what makes the postfix increment or decrement operator (++ or –) helpful.

Case 2: Error Cause: Using an Expression as Left-Hand Side

If you use a complex expression or function call on the left side of a postfix increment or decrement operation then you may run into this error.

Postfix operations (++) should only involve simple variables not complex expressions like function calls.

In conclusion if you want to prevent “Invalid left-hand side in postfix operation” errors in JavaScript, make sure to use valid targets that can be directly modified when you apply the postfix increment (++) and decrement (–) operators, these might include but are not limited to variables, object properties, and array elements, this approach promotes code coherence while at the same time adhering to JavaScript syntax rules governing use of post-fix operations.

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JS Invalid left-hand side expression in postfix operation?

I am playing with a javascript and am running into an error. The error is this:

Invalid left-hand side expression in postfix operation.

And the script is long but I think this is this issue. The weird thing is this works when I run it locally, but when it is packaged, using asset_packager, it fails.

Any ideas why I might be getting this error?

UPDATE: After doing more research I found this function. The error seems to happen after in the "while" statement and I assume it's the "++ + a + ". This is a plugin so I didn't want to go messing with the code...but do you thing this could be it?

• 2 This code works perfectly. I think the error is in another part of the code –  Danilo Valente Commented Jun 24, 2012 at 0:48
• 2 What is on the line the error is reported on? –  Niet the Dark Absol Commented Jun 24, 2012 at 0:48
• Why are you passing undefined as a parameter? –  Jared Farrish Commented Jun 24, 2012 at 0:55
• Is that what would cause something like that? –  Jeffrey Hunter Commented Jun 24, 2012 at 0:56
• 2 What if you try this.centerStack(id) instead of Stacks.centerStack(id) ? –  VisioN Commented Jun 24, 2012 at 1:03

3 Answers 3

You didn't say which plugin was that but I was dealing with the same problem, Jeffrey, with I think the same plugin, because my code was looking almost the same.

I followed your lead. The plugin was History.js, from page: https://github.com/browserstate/History.js/ and I was using bundled html4+html5 version, which was minimized.

I changed that fragment

And it did the job!

I started to wonder what exactly problem lied in. The most important suspect was of course minification "compression". In normal situation following code is correct

And returns "begining of string 1 the rest of string"

However the minification gets rid of white space and turns it into something that is understood by a browser as a:

What gives us error Uncaught ReferenceError: Invalid left-hand side expression in postfix operation

EDIT: As Sam pointed out, the issue was caused by minifcation, and not than gzip compression, of course :)

• Had the same problem with History.js and using WP Minify plugin for wordpress. Thanks very much! –  r8n5n Commented May 3, 2013 at 9:53
• 2 You must mean JS minification, not gzip compression. –  sam Commented Apr 18, 2015 at 2:55
• @sam I guess both methods can cause the same issue. In my case it was gzip ;) –  chmurson Commented Jun 30, 2016 at 13:08
• Had this same exact problem with jQuery form validator - could see the error in Chrome, could see the snippet in sources. So, thanks. –  Lazerblade Commented Aug 24, 2016 at 13:23
• I used this solution to resolve errors when I used Java Jawr to minify jquery-ui-1.9.2.custom.min.js. –  Michael Sobczak Commented Sep 15, 2016 at 21:55

This error is in reference to a ++ or -- following a non reference, such as a returned value. The problem is somewhere else in your code.

Before the increment always the variable to be incremented in calibraces

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Uncaught SyntaxError: invalid assignment left-hand side #140

abcmoritz commented Jul 11, 2022 • edited Loading

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