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

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

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

Valid cases

// 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

• Functions
• set