Rest Parameters

Champions: Allen Wirfs-Brock, Brendan Eich, Erik Arvidsson
ES Wiki

Rest parameters provide a cleaner way of dealing with functions that takes arbitrary number of parameters than using arguments.

Details: full proposal

A better arguments

The arguments object is one of the strangest objects in ES5. It looks like an array but it isn't. It also magically updates the named parameters when it is changed. The rest parameter feature allows a function to declare that it is variadic. 

function sprintf(format, ...args) {}

The rest parameter is a real Array and it is a snapshot of the arguments passed to the function. Changing the array has no strange side effects.

Example: Below is one incomplete implementation of sprintf. 

function sprintf(format, ...args) {
  var i = 0;
  return format.replace(/%(.)/, function(_, c) {
    switch (c) {
      case '%':
        return '%';
      case 's':
        return String(args[i++]);
    }
  });
}

format('Name: %s', 'Hello World')

Evaluation

When a function with a rest parameter is called a new Array is populated with all the trailing arguments. This is always an Array (never null or undefined) even if too few arguments were passed. 

function f(x, ...rest) {
  assert(Array.isArray(rest));
  assert(rest.length === 0);
}
f();

Trailing

The rest parameter is only valid as the last parameter of a function. 

function f(...rest, more) {}  // SyntaxError

Arity

The arity (length) of the function does not include the rest parameter. 

(function(a) {}).length  // 1
(function(...a) {}).length  // 0
(function(a, ...b) {}).length  // 1

See Also