SyntaxError: setter functions must have one argument

The JavaScript exception "setter functions must have one argument" occurs when a setter is declared and the parameter list is not consisted of exactly one formal parameter.

Message

SyntaxError: Setter must have exactly one formal parameter. (V8-based)
SyntaxError: Setter function argument must not be a rest parameter (V8-based)
SyntaxError: setter functions must have one argument (Firefox)
SyntaxError: Unexpected token ','. setter functions must have one parameter. (Safari)
SyntaxError: Unexpected token '...'. Expected a parameter pattern or a ')' in parameter list. (Safari)

Error type

SyntaxError

What went wrong?

The set property syntax looks like a function, but it is stricter and not all function syntax is allowed. A setter is always invoked with exactly one argument, so defining it with any other number of parameters is likely an error. This parameter can be destructured or have a default value, but it cannot be a rest parameter.

Note that this error only applies to property setters using the set syntax. If you define the setter using Object.defineProperty(), etc., the setter is defined as a normal function, although it's likely still an error if the setter expects any other number of arguments, as it will be called with exactly one.

Examples

Invalid cases

js
const obj = {
  set value() {
    this._value = Math.random();
  },
};

Valid cases

js
// You must declare one parameter, even if you don't use it
const obj = {
  set value(_ignored) {
    this._value = Math.random();
  },
};

// You can also declare a normal method instead
const obj = {
  setValue() {
    this._value = Math.random();
  },
};

See also