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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
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Why I get "Invalid left-hand side in assignment"?
There is code:
I get this error:
Why "&&" is wrong?
- ecmascript-6
- are you missing if before the first set of parentheses? – Dan O Commented Dec 1, 2017 at 3:32
- 1 result[id] = list[id] should be result[id] === list[id] as it is a condition – Aravind Commented Dec 1, 2017 at 3:33
- @Aravind that is not correct. – Pointy Commented Dec 1, 2017 at 3:38
- @DanO "if" must not be important, because all logical operator in ( ), not { } – Turar Abu Commented Dec 1, 2017 at 3:43
- @TurarAbu see my answer: the problem is that your assignment expression at the end of the && list is not in parentheses. – Pointy Commented Dec 1, 2017 at 3:43
2 Answers 2
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:
- The real problem is using logical operators for control flow. – user8897421 Commented Dec 1, 2017 at 3:50
- 1 @rockstar well sure :) But it's not erroneous and lots of hard-core library code does stuff like that, so people will see it in "respectable" places. – Pointy Commented Dec 1, 2017 at 3:55
There seems to be a typo in your code:
result[id] = list[id] should be result[id] == list[id] or result[id] === list[id] (if you're doing a strict comparison)
- No. The code posted is using JavaScript logical short-circuiting operators to conditionally perform an operation. The change you suggest would make the code do nothing at all. – Pointy Commented Dec 1, 2017 at 3:37
- The trouble at 4'th line. – Turar Abu Commented Dec 1, 2017 at 3:40
- There's no way = is a short hand for == or === . They have different meanings altogether. I checked in my console and it worked fine when I changed it to result[id] == list[id] – m-ketan Commented Dec 1, 2017 at 3:41
- @m-ketan he wants to use the assignment operator ( = ). If the statement does not perform an assignment, the code is useless. – Pointy Commented Dec 1, 2017 at 3:42
- @Pointy Oh I see it now. – m-ketan Commented Dec 1, 2017 at 3:44
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How to fix SyntaxError: invalid assignment left-hand side
Let me show you an example that causes this error and how I fix it.
How to reproduce this error
How to fix this error, other causes for this error.
You can also see this error when you use optional chaining as the assignment target.
Take your skills to the next level ⚡️
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
Create an assignment rule
Create an assignment rule and apply it to a single table. Assignment rules are\n designed to run at the time you open a record.
- \n Navigate to All > System Policy > Rules > Assignment and click New . \n
| Field | Description |
|---|---|
| Name | The descriptive name for the assignment rule. |
| Active | Specifies whether the assignment rule is active. Only\n active assignment rules take effect. |
| Applies to | |
| Table | The table with the records that the assignment rule\n applies to. \n\nThe list shows only tables and database views that\n are in the same scope as the assignment rule. If you\n select a custom table that extends the task table,\n and for the assignment rule to work properly, you\n must clear the instance cache by navigating to\n https://<instance_name>.service-now.com/cache.do. \n\n Clearing the system cache can\n affect overall performance, and degrade system\n response times. Do not run cache flushes during\n business hours, and do not trigger cache flushes\n automatically.\n |
| Conditions | The conditions under which the assignment rule\n applies. In the example, the assignment rule applies\n when an incident is in the\n Network category |
| Assign to | |
| User | The user the event is assigned to. |
| Group | The group the event is assigned to. |
| Script | |
| Script | A script to specify advanced assignment rule\n functionality. The current.variable_pool set of\n variables is available. Make sure the input in the\n script is correct, and that the input type matches\n the field type in the Assignment Rule script. For\n example, if the assignment rule script sets the\n value of an Integer field, and the value in the\n script is set to String, the assignment rule may\n yield unexpected results.\n |
| Other\n fields | |
| Match conditions | Choices are: \n |
| Execution Order | The order in which the assignment rule is processed.\n If assignment rules conflict, a rule with a lower-order\n value takes precedence over a rule with a higher value.\n If the order values are set to the same number, the\n assignment rule with the first matching condition takes\n precedence over the others. Only the first assignment\n rule with a matching condition runs against a\n record. |
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How to fix SyntaxError – ‘invalid assignment left-hand side in JavaScript’?
In JavaScript, a SyntaxError : Invalid Assignment Left-Hand Side occurs when the interpreter encounters an invalid expression or structure on the left side of an assignment operator ( = ).
This error typically arises when trying to assign a value to something that cannot be assigned, such as literals, expressions, or the result of function calls. Let’s explore this error in more detail and see how to resolve it with some examples.
Understanding an error
An invalid assignment left-hand side error occurs when the syntax of the assignment statement violates JavaScript’s rules for valid left-hand side expressions. The left-hand side should be a target that can receive an assignment like a variable, object property or any array element, let’s see some cases where this error can occur and also how to resolve this error.
Case 1: Error Cause: Assigning to Literals
When you attempt to assign a value to a literal like a number, string or boolean it will result in SyntaxError: Invalid Assignment Left-Hand Side .
Resolution of error
In this case values should be assigned to variables or expressions which will be on the left side of an equation and avoid assigning values directly to literals.
Case 2: Error Cause: Assigning to Function Calls
Assigning a value directly to the result of function call will give an invalid assignment left-hand side error.
Explanation : In this example, getX() returns a value but is not a valid target for assignment. Assignments should be made to variables, object properties, or array elements, not to the result of a function call.
Therefore, store it into a variable or at least place it on the left-hand side that is valid for assignment.
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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 ===.
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 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 === .
We would like to show you a description here but the site won’t allow us.
Incidents are being created without any value populated in the Assignment Group field (Assignment Rule is failing).
Unable to assign the Problem records to the user i.e Reference qualifier is not displaying the user for selection.
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.
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How to fix SyntaxError - 'invalid assignment left-hand side in JavaScript'? In JavaScript, a SyntaxError : Invalid Assignment Left-Hand Side occurs when the interpreter encounters an invalid expression or structure on the left side of an assignment operator (=).
An invalid assignment left-hand side error occurs when the syntax of the assignment statement violates JavaScript’s rules for valid left-hand side expressions.