SyntaxError: invalid assignment left-hand side

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)

ReferenceError: Invalid left-hand side in assignment (V8-based)
ReferenceError: cannot assign to function call (Firefox)
ReferenceError: 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

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.

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 statement is actually executed.

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.

function foo() {
  return { a: 1 };
}
foo().a = 1;

Using optional chaining as assignment target

Optional chaining is not a valid target of assignment.

obj?.foo = 1; // SyntaxError: invalid assignment left-hand side

Instead, you have to first guard the nullish case.

if (obj) {
  obj.foo = 1;
}

See also

• Assignment operators
• Equality operators