invalid left hand side in assignment at htmlinputelement

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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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.

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: 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

• Skip to main content

• 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.

• Assignment operators
• Comparison operators
• JavaScript basics
• JavaScript first steps
• JavaScript building blocks
• Introducing JavaScript objects
• 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
• Using promises
• Iterators and generators
• Meta programming
• JavaScript modules
• Client-side web APIs
• 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:
• AggregateError
• ArrayBuffer
• AsyncFunction
• BigInt64Array
• BigUint64Array
• Float32Array
• Float64Array
• GeneratorFunction
• InternalError
• ReferenceError
• SharedArrayBuffer
• SyntaxError
• Uint16Array
• Uint32Array
• Uint8ClampedArray
• WebAssembly
• decodeURI()
• decodeURIComponent()
• encodeURI()
• encodeURIComponent()
• parseFloat()
• Arithmetic operators
• Bitwise operators
• Comma operator
• Conditional (ternary) operator
• Destructuring assignment
• Function expression
• Grouping operator
• Logical operators
• Nullish coalescing operator
• Object initializer
• Operator precedence
• Optional chaining
• Pipeline operator
• Property accessors
• Spread syntax
• async function expression
• class expression
• delete operator
• function* expression
• in operator
• new operator
• void operator
• async function
• for await...of
• function declaration
• import.meta
• try...catch
• Arrow function expressions
• Default parameters
• Method definitions
• Rest parameters
• The arguments object
• Class fields
• 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: invalid date
• 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: can't access lexical declaration "x" before initialization
• ReferenceError: deprecated caller or arguments usage
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• SyntaxError: "0"-prefixed octal literals and octal escape seq. are deprecated
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• SyntaxError: "x" is a reserved identifier
• SyntaxError: JSON.parse: bad parsing
• SyntaxError: Malformed formal parameter
• SyntaxError: Unexpected token
• SyntaxError: Using //@ to indicate sourceURL pragmas is deprecated. Use //# instead
• SyntaxError: a declaration in the head of a for-of loop can't have an initializer
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• SyntaxError: for-in loop head declarations may not have initializers
• SyntaxError: function statement requires a name
• SyntaxError: identifier starts immediately after numeric literal
• SyntaxError: illegal character
• SyntaxError: invalid regular expression flag "x"
• SyntaxError: missing ) after argument list
• SyntaxError: missing ) after condition
• SyntaxError: missing : after property id
• SyntaxError: missing ; before statement
• SyntaxError: missing = in const declaration
• SyntaxError: missing ] after element list
• SyntaxError: missing formal parameter
• SyntaxError: missing name after . operator
• SyntaxError: missing variable name
• SyntaxError: missing } after function body
• SyntaxError: missing } after property list
• SyntaxError: redeclaration of formal parameter "x"
• SyntaxError: return not in function
• SyntaxError: test for equality (==) mistyped as assignment (=)?
• SyntaxError: unterminated string literal
• TypeError: "x" has no properties
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• TypeError: "x" is not a constructor
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• JavaScript technologies overview
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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

© 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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JS Reference Error

• JavaScript ReferenceError - Can't access lexical declaration`variable' before initialization
• JavaScript ReferenceError - Invalid assignment left-hand side
• JavaScript ReferenceError - Assignment to undeclared variable
• JavaScript ReferenceError - Reference to undefined property "x"
• JavaScript ReferenceError - variable is not defined
• JavaScript ReferenceError Deprecated caller or arguments usage

JS Syntax Error

• JavaScript SyntaxError - Illegal character
• JavaScript SyntaxError - Identifier starts immediately after numeric literal
• JavaScript SyntaxError - Function statement requires a name
• JavaScript SyntaxError - Missing } after function body
• JavaScript SyntaxError - Missing } after property list
• JavaScript SyntaxError - Missing variable name
• JavaScript SyntaxError - Missing ] after element list
• JavaScript SyntaxError - Invalid regular expression flag "x"
• JavaScript SyntaxError "variable" is a reserved identifier
• JavaScript SyntaxError - Missing ':' after property id
• JavaScript SyntaxError - Missing ) after condition
• JavaScript SyntaxError - Missing formal parameter
• JavaScript SyntaxError - Missing ; before statement
• JavaScript SyntaxError - Missing = in const declaration
• JavaScript SyntaxError - Missing name after . operator
• JavaScript SyntaxError - Redeclaration of formal parameter "x"
• 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
• JavaScript TypeError - "X" is not a constructor
• JavaScript TypeError - "X" has no properties
• JavaScript TypeError - "X" is (not) "Y"
• JavaScript TypeError - "X" is not a function
• JavaScript TypeError - 'X' is not iterable
• JavaScript TypeError - More arguments needed
• JavaScript TypeError - "X" is read-only
• JavaScript TypeError - Reduce of empty array with no initial value
• JavaScript TypeError - Can't assign to property "X" on "Y": not an object
• JavaScript TypeError - Can't access property "X" of "Y"
• JavaScript TypeError - Can't define property "X": "Obj" is not extensible
• JavaScript TypeError - X.prototype.y called on incompatible type
• JavaScript TypeError - Invalid assignment to const "X"
• JavaScript TypeError - Property "X" is non-configurable and can't be deleted
• JavaScript TypeError - Can't redefine non-configurable property "x"
• JavaScript TypeError - Variable "x" redeclares argument
• JavaScript TypeError - Setting getter-only property "x"
• JavaScript TypeError - Invalid 'instanceof' operand 'x'
• JavaScript TypeError - Invalid Array.prototype.sort argument
• JavaScript TypeError - Cyclic object value
• JavaScript TypeError - Can't delete non-configurable array element

JS Other Errors

• JavaScript URIError | Malformed URI Sequence
• JavaScript Warning - Date.prototype.toLocaleFormat is deprecated
• Logging Script Errors in JavaScript

JS Error Instance

• JavaScript Error message Property
• JavaScript Error name Property
• JavaScript Error.prototype.toString() Method

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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The left-hand side of assignment expression may not be an optional property access

Last updated: Feb 28, 2024 Reading time · 4 min

# The left-hand side of assignment expression may not be an optional property access

The error "The left-hand side of an assignment expression may not be an optional property access" occurs when we try to use optional chaining (?.) to assign a property to an object.

To solve the error, use an if statement that serves as a type guard instead.

Here is an example of how the error occurs.

We aren't allowed to use the optional chaining (?.) operator on the left-hand side of an assignment.

# Use an if statement as a type guard to solve the error

To solve the error, use an if statement as a type guard before the assignment.

We used the loose not equals operator (!=), to check if the variable is NOT equal to null and undefined .

This works because when compared loosely, null is equal to undefined .

The if block is only run if employee doesn't store an undefined or a null value.

This is similar to what the optional chaining (?.) operator does.

# Using the non-null assertion operator to solve the error

You might also see examples online that use the non-null assertion operator to solve the error.

The exclamation mark is the non-null assertion operator in TypeScript.

When you use this approach, you basically tell TypeScript that this value will never be null or undefined .

Here is an example of using this approach to set a property on an object.

In most cases, you should use a simple if statement that serves as a type guard as we did in the previous code sample.

# Avoiding the error with a type assertion

You can also use a type assertion to avoid getting the error. However, this isn't recommended.

The (employee as Employee) syntax is called a type assertion.

Type assertions are used when we have information about the type of a value that TypeScript can't know about.

We effectively tell TypeScript that the employee variable will have a type of Employee and not to worry about it.

This could go wrong if the variable is null or undefined as accessing a property on a null or an undefined value would cause a runtime error.

# Using the logical AND (&&) operator to get around the error

You can also use the logical AND (&&) operator to avoid getting the error.

The logical AND (&&) operator checks if the value to the left is truthy before evaluating the statement in the parentheses.

If the employee variable stores a falsy value (e.g. null or undefined ), the code to the right of the logical AND (&&) operator won't run at all.

The falsy values in JavaScript are: false , undefined , null , 0 , "" (empty string), NaN (not a number).

All other values are truthy.

However, this approach can only be used to assign a single property at a time if the value is not equal to null and undefined .

# The optional chaining operator should only be used when accessing properties

The optional chaining (?.) operator short-circuits if the reference is equal to null or undefined .

The optional chaining (?.) operator will simply return undefined in the example because employee has a value of undefined .

The purpose of the optional chaining (?.) operator is accessing deeply nested properties without erroring out if a value in the chain is equal to null or undefined .

However, the optional chaining operator cannot be used on the left-hand side of an assignment expression.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• How to Check the Type of a Variable in TypeScript
• Exclamation Mark (non-null assertion) operator in TypeScript
• The ?. operator (optional chaining) in TypeScript
• Declare and Type a nested Object in TypeScript
• How to Add a property to an Object in TypeScript
• Check if a Property exists in an Object in TypeScript
• The left-hand side of an arithmetic operation must be type 'any', 'number', 'bigint' or an enum type

Borislav Hadzhiev

Web Developer

Copyright © 2024 Borislav Hadzhiev

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!

Maybe you are interested :

• TypeError: Assignment to Constant Variable in JavaScript
• Element type is invalid, expected a string (for built in components) or a class/function but got – How to solve?
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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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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 · 2 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

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

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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.

JavaScript初心者必見！Invalid assignment left-hand sideエラーを理解してコードを正確に書こう

Javascriptにおけるinvalid assignment left-hand sideエラー.

このエラーが発生する主な原因は以下の2つです。

• 左辺が未定義の変数である
• 左辺がオブジェクトのプロパティではない

Invalid assignment left-hand side エラーを回避するには、以下の点に注意する必要があります。

• 左辺が有効な変数であることを確認する
• 左辺がオブジェクトのプロパティである場合は、ドット記法を使用する

以下のコードは、 Invalid assignment left-hand side エラーが発生する例です。

Invalid assignment left-hand side エラーは、コードの構文エラーの一種です。このエラーが発生した場合は、コードを見直し、左辺が有効な変数またはオブジェクトのプロパティであることを確認する必要があります。

Invalid assignment left-hand side エラーが発生すると、以下のエラーメッセージが表示されます。

Invalid assignment left-hand side エラーは、JavaScriptの基礎的なエラーの一つです。このエラーを理解することで、より正確なコードを書くことができます。

JavaScript 例外 "invalid assignment left-hand side" は、どこかに予期しない割り当てがあった場合に発生します。 == または === の代わりに単一の = サインが使用された場合にトリガーされる可能性があります。

SyntaxError または ReferenceError (構文に応じて)。

どこかで予期せぬ任務があった。これは、たとえば assignment operator と equality operator の不一致が原因である可能性があります。単一の = 符号が変数に値を割り当てるのに対し、 == または === 演算子は値を比較します。

典型的な無効な割り当て

if ステートメントでは、等価演算子 ( === ) を使用する必要があり、文字列連結にはプラス演算子 ( + ) が必要です。

ReferenceError を生成する割り当て

無効な代入によって常に構文エラーが発生するとは限りません。場合によっては、構文がほぼ正しい場合でも、実行時に左側の式が参照ではなく値に評価されるため、割り当ては依然として無効になります。このようなエラーは、実行後のステートメントが実際に実行されるときに発生します。

関数呼び出し、 new 呼び出し、 super() 、および this はすべて、参照ではなく値です。これらを左側で使用したい場合は、代わりに代入ターゲットを生成された値のプロパティにする必要があります。

注: Firefox と Safari では、最初の例では非厳密モードで ReferenceError が生成され、 strict mode で SyntaxError が生成されます。Chrome は、厳密モードと非厳密モードの両方でランタイム ReferenceError をスローします。

割り当てターゲットとしてオプションのチェーンを使用する

Optional chaining は有効な割り当てのターゲットではありません。

代わりに、最初にnullish caseを保護する必要があります。

• 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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