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 ===
.
Message
SyntaxError: Invalid left-hand side in assignment (V8-based) SyntaxError: invalid assignment left-hand side (Firefox) SyntaxError: Left side of assignment is not a reference. (Safari)
Error type
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.
Examples
Typical invalid assignments
js
if (Math.PI + 1 = 3 || Math.PI + 1 = 4) {
console.log("no way!");
}
// SyntaxError: invalid assignment left-hand side
const str = "Hello, "
+= "is it me "
+= "you're looking for?";
// SyntaxError: invalid assignment left-hand side
In the if
statement, you want to use an equality operator (===
), and for the string concatenation, the plus (+
) operator is needed.
js
if (Math.PI + 1 === 3 || Math.PI + 1 === 4) {
console.log("no way!");
}
const str = "Hello, "
+ "from the "
+ "other side!";
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 line is actually executed.
js
function foo() {
return { a: 1 };
}
foo() = 1; // ReferenceError: invalid assignment left-hand side
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.
js
function foo() {
return { a: 1 };
}
foo().a = 1;
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.
js
obj?.foo = 1; // SyntaxError: invalid assignment left-hand side
Instead, you have to first guard the nullish case.
js
if (obj) {
obj.foo = 1;
}