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)

SyntaxError or ReferenceError, depending on the syntax.

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.

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!";

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;

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;
}

• Assignment operators
• Equality operators