SyntaxError: parameter after rest parameter

The JavaScript exception "parameter after rest parameter" occurs when a rest parameter is followed by anything else in a parameter list, including another rest parameter, a formal parameter, or a trailing comma.

Message

SyntaxError: Rest parameter must be last formal parameter (V8-based)
SyntaxError: parameter after rest parameter (Firefox)
SyntaxError: Unexpected token ','. Rest parameter should be the last parameter in a function declaration. (Safari)

Error type

SyntaxError

What went wrong?

A rest parameter must be the last parameter in a function definition. This is because the rest parameter collects all the remaining arguments passed to the function, so it doesn't make sense to have any parameters after it. The next non-whitespace character must be the closing parenthesis of the parameter list.

Examples

Invalid cases

function replacer(match, ...groups, offset, string) {}

function doSomething(
  arg1,
  arg2,
  ...otherArgs, // Accidental trailing comma
) {}

Valid cases

function replacer(match, ...args) {
  const offset = args.at(-2);
  const string = args.at(-1);
}

function doSomething(arg1, arg2, ...otherArgs) {}