uncaught syntaxerror invalid left hand side in assignment webpack

How to fix SyntaxError: invalid assignment left-hand side

by Nathan Sebhastian

Posted on Jul 10, 2023

Reading time: 3 minutes

When running JavaScript code, you might encounter an error that says:

Both errors are the same, and they occured when you use the single equal = sign instead of double == or triple === equals when writing a conditional statement with multiple conditions.

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

How to reproduce this error

Suppose you have an if statement with two conditions that use the logical OR || operator.

You proceed to write the statement as follows:

When you run the code above, you’ll get the error:

This error occurs because you used the assignment operator with the logical OR operator.

An assignment operator doesn’t return anything ( undefined ), so using it in a logical expression is a wrong syntax.

How to fix this error

To fix this error, you need to replace the single equal = operator with the double == or triple === equals.

Here’s an example:

By replacing the assignment operator with the comparison operator, the code now runs without any error.

The double equal is used to perform loose comparison, while the triple equal performs a strict comparison. You should always use the strict comparison operator to avoid bugs in your code.

Other causes for this error

There are other kinds of code that causes this error, but the root cause is always the same: you used a single equal = when you should be using a double or triple equals.

For example, you might use the addition assignment += operator when concatenating a string:

The code above is wrong. You should use the + operator without the = operator:

Another common cause is that you assign a value to another value:

This is wrong because you can’t assign a value to another value.

You need to declare a variable using either let or const keyword, and you don’t need to wrap the variable name in quotations:

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

For example, suppose you want to add a property to an object only when the object is defined:

Here, we want to assign the age property to the person object only when the person object is defined.

But this will cause the invalid assignment left-hand side error. You need to use the old if statement to fix this:

Now the error is resolved.

The JavaScript error SyntaxError: invalid assignment left-hand side occurs when you have an invalid syntax on the left-hand side of the assignment operator.

This error usually occurs because you used the assignment operator = when you should be using comparison operators == or === .

Once you changed the operator, the error would be fixed.

I hope this tutorial helps. Happy coding!

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JavaScript Debugging Toolkit: Identifying and Fixing "Invalid assignment left-hand side"

This error arises when you attempt to assign a value to something that cannot be assigned to. JavaScript requires valid "left-hand sides" (targets) for assignments, which are typically variables, object properties, or array elements.

Correct Usage:

• Declared variables ( var , let , or const )
• Existing variables
• Object properties directly (without functions)
• Array elements using their numerical indices ( myArray[0] = 5 )

Incorrect Usage:

• Attempting to assign to expressions or values returned by functions
• Assigning to undeclared variables (causes ReferenceError )
• Using incorrect keywords or operators (e.g., using = for comparison instead of == or === )

Sample Code:

Precautions:

• Carefully check variable declaration (using var , let , or const ) to avoid undeclared variable errors.
• Remember that constants ( const ) cannot be reassigned after declaration.
• Use == or === for comparisons, not = for assignments.
• Be mindful of operator precedence (assignment has lower precedence than logical operators like && ).
• For object properties and array elements, ensure the object or array exists before assignment.
• ReferenceError: Occurs when trying to assign to an undeclared variable.
• TypeError: Occurs when trying to assign to a value that cannot be hold a value (e.g., modifying a constant or a returned function value).
• SyntaxError: Occurs if the code has incorrect syntax issues that prevent parsing.

Key Points:

• Understand the different assignment operators and when to use them.
• Declare variables before using them (except var , which has hoisting).
• Be mindful of object property and array element accessibility.
• Use strict equality comparison ( === ) or loose equality ( == ) instead of single assignment ( = ) for comparisons.
• Practice debugging techniques to identify and fix assignment errors.
• Consider using linters or code analysis tools to catch potential errors early.

By following these guidelines and carefully avoiding incorrect assignment scenarios, you can write clearer, more robust JavaScript code.

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

Demystifying Milliseconds: Using JavaScript's Date.getMilliseconds() Effectively

Date. getMilliseconds() is a method of the Date object in JavaScript.It returns the number of milliseconds (0-999) that have elapsed since the beginning of the day (midnight) for the specified date according to local time

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• JavaScript Date.parse : Handle Dates Like a Pro (With Pitfalls to Avoid)
• Demystifying JavaScript's continue : Usage, Precautions, and Best Practices

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What went wrong?

ReferenceError .

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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• ReferenceError
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• Introducing JavaScript objects
• Introduction
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• Control flow and error handling
• Loops and iteration
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• Text formatting
• Regular expressions
• Indexed collections
• Keyed collections
• Working with objects
• Details of the object model
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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

You may also like.

Dec 28, 2016 8:00:56 AM | JavaScript Error Handling - ReferenceError: assignment to undeclared variable “x”

Feb 15, 2017 7:41:35 am | javascript error handling: syntaxerror: "use strict" not allowed in function with non-simple parameters, may 21, 2017 9:00:51 am | javascript errors - syntaxerror: test for equality mistyped as assignment.

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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

Document Tags and Contributors

• ReferenceError
• Introduction
• Grammar and types
• Control flow and error handling
• Loops and iteration
• Expressions and operators
• Numbers and dates
• Text formatting
• Regular expressions
• Indexed collections
• Keyed collections
• Working with objects
• Details of the object model
• Iterators and generators
• Meta programming
• JavaScript basics
• JavaScript technologies overview
• Introduction to Object Oriented JavaScript
• A re-introduction to JavaScript
• JavaScript data structures
• Equality comparisons and sameness
• Inheritance and the prototype chain
• Strict mode
• JavaScript typed arrays
• Memory Management
• Concurrency model and Event Loop
• References:
• ArrayBuffer
• AsyncFunction
• Float32Array
• Float64Array
• GeneratorFunction
• InternalError
• Intl.Collator
• Intl.DateTimeFormat
• Intl.NumberFormat
• ParallelArray
• SIMD.Bool16x8
• SIMD.Bool32x4
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• decodeURIComponent()
• encodeURI()
• encodeURIComponent()
• parseFloat()
• Arithmetic operators
• Array comprehensions
• Bitwise operators
• Comma operator
• Conditional (ternary) Operator
• Destructuring assignment
• Expression closures
• Generator comprehensions
• Grouping operator
• Legacy generator function expression
• Logical Operators
• Object initializer
• Operator precedence
• Property accessors
• Spread syntax
• async function expression
• class expression
• delete operator
• function expression
• function* expression
• in operator
• new operator
• void operator
• Legacy generator function
• async function
• for each...in
• try...catch
• Arguments object
• Arrow functions
• Default parameters
• Method definitions
• Rest parameters
• constructor
• Error: Permission denied to access property "x"
• InternalError: too much recursion
• RangeError: argument is not a valid code point
• RangeError: invalid array length
• RangeError: precision is out of range
• RangeError: radix must be an integer
• RangeError: repeat count must be less than infinity
• RangeError: repeat count must be non-negative
• ReferenceError: "x" is not defined
• ReferenceError: assignment to undeclared variable "x"
• ReferenceError: deprecated caller or arguments usage
• ReferenceError: reference to undefined property "x"
• SyntaxError: "use strict" not allowed in function with non-simple parameters
• SyntaxError: "x" is not a legal ECMA-262 octal constant
• SyntaxError: JSON.parse: bad parsing
• SyntaxError: Malformed formal parameter
• SyntaxError: Unexpected token
• SyntaxError: Using //@ to indicate sourceURL pragmas is deprecated. Use //# instead
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• SyntaxError: missing ] after element list
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• JavaScript ReferenceError - Can't access lexical declaration`variable' before initialization
• JavaScript ReferenceError - Invalid assignment left-hand side
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JS Syntax Error

• JavaScript SyntaxError - Illegal character
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• JavaScript SyntaxError - Missing ) after argument list
• JavaScript SyntaxError - Return not in function
• JavaScript SyntaxError: Unterminated string literal
• JavaScript SyntaxError - Applying the 'delete' operator to an unqualified name is deprecated
• JavaScript SyntaxError - Using //@ to indicate sourceURL pragmas is deprecated. Use //# instead
• JavaScript SyntaxError - Malformed formal parameter
• JavaScript SyntaxError - "0"-prefixed octal literals and octal escape sequences are deprecated
• JavaScript SyntaxError - Test for equality (==) mistyped as assignment (=)?
• JavaScript SyntaxError - "x" is not a legal ECMA-262 octal constant

JS Type Error

• JavaScript TypeError - "X" is not a non-null object
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• Logging Script Errors in JavaScript

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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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How to solve “Invalid left-hand side in assignment” in JavaScript

The “Invalid left-hand side in assignment” in JavaScript is a syntax error that often occurs in comparing values, such as using the “=” sign to compare. This article will give examples of everyday situations and how to fix them.

Table of Contents

What causes the “Invalid left-hand side in assignment” in JavaScript?

This is a very common syntax error. There are many causes of errors, such as wrong comparison signs, using “=” to compare two values, and not creating a variable to receive a value from the function. Here are examples of some errors.

Using the wrong comparison sign

“Invalid left-hand side in assignment” is an error caused by misspelled operator when comparing two values.

Using “=” to compare two values

This error also happens because instead of using “===”, you use “=” to compare.

Not creating a variable to receive a value from the function

This case leaves the wrong position of the variable.

Do not use square brackets when accessing object properties

For properties like this, we need to use square brackets.

Solution for the error “Invalid left-hand side in assignment” in JavaScript

Use the correct operator.

We need to pay attention to the comparison signs in expressions.

Pay attention to the position on either side of the “=”

To get the value of a function, we need to create a variable to the left of the “=” sign.

Use square brackets when accessing properties

For properties of objects with two or more words, we use square brackets.

The article has given some examples that lead to the error “Invalid left-hand side in assignment” in JavaScript. These are just syntax errors , pay attention to the process of writing code, and we will avoid such errors. We hope you can fix it quickly through this article. Good luck to you!

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Carolyn Hise has three years of software development expertise. Strong familiarity with the following languages is required: Python, Typescript/Nodejs, .Net, Java, C++, and a strong foundation in Object-oriented programming (OOP).

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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

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 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.

• Assignment operators
• Equality operators

© 2005–2021 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" when building with certain dependencies #45

gnestor commented Nov 30, 2016

• 👍 1 reaction

blink1073 commented Nov 30, 2016

Sorry, something went wrong.

blink1073 commented Dec 1, 2016

gnestor commented Dec 1, 2016 • edited

Gnestor commented dec 5, 2016.

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