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
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Copyright © 2024 Borislav Hadzhiev
How to fix SyntaxError: invalid assignment left-hand side
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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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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
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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.
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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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Invalid left-hand side in assignment expression
Hello. I am attempting to create a self-generating biology question that randomly generates three numbers for the problem question, then asks a yes or no question. When I was attempting to create the function that checks for the answer to the question and compared it to the student input, I get the “Invalid left-hand side in assignment expression”
My code is here, line 33 in the JavaScript window: https://codepen.io/KDalang/pen/OJpEdQB
Here is the specific line in question: if (chiTotal <= 3.841 && input=“Yes”) What did I do wrong?
= is assignment of a value to a variable == is weak comparison (with type coercion) === is strong comparison (probably what you want)
Hey thanks for the quick reply! I actually want it to be a “less than or equal to” and I used <=. <== and <=== don’t do anything either.
Edit: Nevermind, I understand now.
Do you try to compare values or do you try to assign a value?
Oh my gosh! Sorry its 2a.m. over here I understand what you and JeremyLT are saying now. Thanks so much!
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TypeScript: Invalid left-hand side in assignment expression (2:0) #7638
lukescott commented Mar 27, 2018 • edited
| Bug num: number | undefined num! += 1
No Errors
I discovered this while pulling in a piece of code we were compiling with tsc. If you copy this code into the TypeScript playground it works fine. My work-around for now was to change it to: num: number | undefined num = num! + 1
The text was updated successfully, but these errors were encountered:  babel-bot commented Mar 27, 2018
Sorry, something went wrong.  Successfully merging a pull request may close this issue. 
Collectives™ on Stack OverflowFind centralized, trusted content and collaborate around the technologies you use most. Q&A for work Connect and share knowledge within a single location that is structured and easy to search. Get early access and see previews of new features. ReferenceError: Invalid left-hand side in assignmentmy code for a rock paper scissors game (called toss) is as follows: When I enter in the parameters I get this error code: "ReferenceError: Invalid left-hand side in assignment" How to fix it? What this error means and what other cases when this error can happen? 
3 Answers 3You have to use == to compare (or even === , if you want to compare types). A single = is for assignment. 
Common reasons for the error:
In this particular case you want to use == (or better === - What exactly is Type Coercion in Javascript? ) to check for equality (like if(one === "rock" && two === "rock") , but it the actual reason you are getting the error is trickier. The reason for the error is Operator precedence . In particular we are looking for && (precedence 6) and = (precedence 3). Let's put braces in the expression according to priority - && is higher than = so it is executed first similar how one would do 3+4*5+6 as 3+(4*5)+6 : Now we have expression similar to multiple assignments like a = b = 42 which due to right-to-left associativity executed as a = (b = 42) . So adding more braces: Finally we arrived to actual problem: ("rock" && two) can't be evaluated to l-value that can be assigned to (in this particular case it will be value of two as truthy ). Note that if you'd use braces to match perceived priority surrounding each "equality" with braces you get no errors. Obviously that also producing different result than you'd expect - changes value of both variables and than do && on two strings "rock" && "rock" resulting in "rock" (which in turn is truthy) all the time due to behavior of logial && : For even more details on the error and other cases when it can happen - see specification: LeftHandSideExpression = AssignmentExpression ... Throw a SyntaxError exception if the following conditions are all true: ... IsStrictReference(lref) is true Left-Hand-Side Expressions and The Reference Specification Type explaining IsStrictReference: ... function calls are permitted to return references. This possibility is admitted purely for the sake of host objects. No built-in ECMAScript function defined by this specification returns a reference and there is no provision for a user-defined function to return a reference... 
The same happened for me with eslint module. EsLinter throw Parsing error: Invalid left-hand side in assignment expression for await in second if statement. As strange as it sounds what fixed this error was to add ; semicolon at the end of line where await occurred.  Your AnswerReminder: Answers generated by artificial intelligence tools are not allowed on Stack Overflow. Learn more Sign up or log inPost as a guest. Required, but never shown By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy . Not the answer you're looking for? Browse other questions tagged javascript logic or ask your own question .
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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 ...
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 ...
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.
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.
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.
Today we examine the invalid assignment error, which is thrown, as the name implies, when code attempts to perform an invalid assignment somewhere.
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.
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 "===". ... Errors: Deprecated expression closures. Errors: Deprecated octal. Errors: Deprecated source map pragma ...
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 "===". ... SyntaxError: invalid regular expression flag "x" SyntaxError: missing ) after argument list; SyntaxError: missing ) after condition;
const object = {}; object?. property = 1; // SyntaxError: Invalid left-hand side in assignment Short-circuiting When using optional chaining with expressions, if the left operand is null or undefined , the expression will not be evaluated.
The Left Shift Assignment Operator is represented by "<<=". This operator moves the specified number of bits to the left and assigns that result to the variable. We can fill the vacated place by 0. The left shift operator treats the integer stored in the variable to the operator's left as a 32-bit binary number. This can also be explained as
Hello. I am attempting to create a self-generating biology question that randomly generates three numbers for the problem question, then asks a yes or no question. When I was attempting to create the function that checks for the answer to the question and compared it to the student input, I get the "Invalid left-hand side in assignment expression" My code is here, line 33 in the JavaScript ...
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Does anyone have any ideas of why I continue to get the Invalid left-hand side expression in prefix operation error? Any advice would be much appreciated. The text was updated successfully, but these errors were encountered: All reactions. Copy link Owner. shama ...
TypeScript: Invalid left-hand side in assignment expression (2:0) #7638. Closed lukescott opened this issue Mar 27, 2018 · 1 comment · Fixed by #7888. ... Invalid left-hand side in assignment expression (2:0) Context. I discovered this while pulling in a piece of code we were compiling with tsc. If you copy this code into the TypeScript ...
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。.
0. You're first approach is about retrieval data. Whereas the second part is about assigning the data. Here You're are assigning value to something function variable. May you need to change like below. function something(id, val){. document.getElementById(id).value = val; return document.getElementById(id).value; }
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SyntaxError: unparenthesized unary expression can't appear on the left-hand side of '**' SyntaxError: unterminated string literal; SyntaxError: Using //@ to indicate sourceURL pragmas is deprecated. Use //# instead ... // ReferenceError: invalid assignment left-hand side var str = 'Hello, ...