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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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
ReferenceError: Invalid left-hand side in assignment?
1 expert answer.
Brian E. answered 03/27/19
Senior Full Stack JavaScript Developer (Node & React)
JavaScript uses the "==" and "===" operators for equality. "=" is for assignment.
You can fix your error by using either "==" or "===". They are different. Please do a little research before deciding which fits your use case.
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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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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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Can anyone give me a clue as to why I a getting this error message in my Form "Invalid left-hand side in assignment"?
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ReferenceError: invalid assignment left-hand side
JavaScript の例外 "invalid assignment left-hand side" は、どこかで予想外の代入が行われたときに発生します。例えば、単一の " = " の記号が " == " や " === " の代わりに使用された場合です。
ReferenceError 。
どこかに予想外の代入があります。たとえば、 代入演算子 と 等値演算子 が合っていないからかもしれません。 " = " 記号が 1 つの場合は変数に値を割り当てる一方、" == " か " === " 演算子は値を比較します。
if 文では、等価演算子 ("==") が必要ですし、文字連結にはプラス ("+") 演算子が必要です。
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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7. The problem is that the assignment operator, =, is a low-precedence operator, so it's being interpreted in a way you don't expect. If you put that last expression in parentheses, it works: for(let id in list)(. (!q.id || (id == q.id)) &&. (!q.name || (list[id].name.search(q.name) > -1)) &&. (result[id] = list[id]) ); The real problem is ...
SyntaxError: invalid assignment left-hand side or SyntaxError: Invalid left-hand side in assignment 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.
What does "ReferenceError: Invalid left-hand side in assignment" mean?
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. js. function foo() { return { a: 1 }; } foo ...
mrjvdveen changed the title Allow nullable reference on the left hand side of an assignment Allow null conditional (?.) operator on left hand side of assignment Oct 15, 2019. Copy link Author. mrjvdveen commented Oct 15, 2019. ... Maybe we can get rid of null in c# by opt-in feature, instead of giving nullable to everything, and having ...
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🐛 bug report Description of the problem Getting an error: Invalid left-hand side in assignment. The root cause seems to be that this line from the stylis library: character = position > 0 ? charat(...
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.
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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. Message:
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 ...
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.
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I am trying to set a value in cell in a local table variable. I have done this in C# but I can't make it work in JavaScript. Nothing I have tried so far works.... It has to be something simple. Thanks!!! KeywordTests.Move_Order.Variables.VariableByName("tbl_QueryResults").SQLHeaderID(int_RowNumber) = 123456; Or
ReferenceError: invalid assignment left-hand side. JavaScript の例外 "invalid assignment left-hand side" は、どこかで予想外の代入が行われたときに発生します。. 例えば、単一の " = " の記号が " == " や " === " の代わりに使用された場合です。.
2. The problem is with using string.charAt () on the left hand side. That is not possible as you're trying to assign something to the result of a function, all in the same call. Store the value of string.charAt () in an intermediary variable and it should work.
"Uncaught SyntaxError: Invalid left-hand side in assignment" エラーが出たら ... 主にC#で仕事をしているプログラマーです。現在flutterの勉強をしています。 猫が好きです。
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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 ...