Rest parameters

The rest parameter syntax allows a function to accept an indefinite number of arguments as an array, providing a way to represent variadic functions in JavaScript.

Syntax

function f(a, b, ...theArgs) {
  // …
}

Description

A function definition's last parameter can be prefixed with ... (three U+002E FULL STOP characters), which will cause all remaining (user supplied) parameters to be placed within an Array object.

function myFun(a, b, ...manyMoreArgs) {
  console.log("a", a);
  console.log("b", b);
  console.log("manyMoreArgs", manyMoreArgs);
}

myFun("one", "two", "three", "four", "five", "six");

// Console Output:
// a, one
// b, two
// manyMoreArgs, ["three", "four", "five", "six"]

A function definition can only have one rest parameter, and the rest parameter must be the last parameter in the function definition.

function wrong1(...one, ...wrong) {}
function wrong2(...wrong, arg2, arg3) {}

The rest parameter is not counted towards the function's length property.

The difference between rest parameters and the arguments object

There are three main differences between rest parameters and the arguments object:

The arguments object is not a real array, while rest parameters are Array instances, meaning methods like sort(), map(), forEach() or pop() can be applied on it directly.
The arguments object has the additional (deprecated) callee property.
In a non-strict function with simple parameters, the arguments object syncs its indices with the values of parameters. The rest parameter array never updates its value when the named parameters are re-assigned.
The rest parameter bundles all the extra parameters into a single array, but does not contain any named argument defined before the ...restParam. The arguments object contains all of the parameters — including the parameters in the ...restParam array — bundled into one array-like object.

Examples

Using rest parameters

In this example, the first argument is mapped to a and the second to b, so these named arguments are used as normal.

However, the third argument, manyMoreArgs, will be an array that contains the third, fourth, fifth, sixth, …, nth — as many arguments as the user specifies.

function myFun(a, b, ...manyMoreArgs) {
  console.log("a", a);
  console.log("b", b);
  console.log("manyMoreArgs", manyMoreArgs);
}

myFun("one", "two", "three", "four", "five", "six");

// a, "one"
// b, "two"
// manyMoreArgs, ["three", "four", "five", "six"] <-- an array

Below, even though there is just one value, the last argument still gets put into an array.

// Using the same function definition from example above

myFun("one", "two", "three");

// a, "one"
// b, "two"
// manyMoreArgs, ["three"] <-- an array with just one value

Below, the third argument isn't provided, but manyMoreArgs is still an array (albeit an empty one).

// Using the same function definition from example above

myFun("one", "two");

// a, "one"
// b, "two"
// manyMoreArgs, [] <-- still an array

Below, only one argument is provided, so b gets the default value undefined, but manyMoreArgs is still an empty array.

// Using the same function definition from example above

myFun("one");

// a, "one"
// b, undefined
// manyMoreArgs, [] <-- still an array

Argument length

Since theArgs is an array, a count of its elements is given by the length property. If the function's only parameter is a rest parameter, restParams.length will be equal to arguments.length.

function fun1(...theArgs) {
  console.log(theArgs.length);
}

fun1(); // 0
fun1(5); // 1
fun1(5, 6, 7); // 3

Using rest parameters in combination with ordinary parameters

In the next example, a rest parameter is used to collect all parameters after the first parameter into an array. Each one of the parameter values collected into the array is then multiplied by the first parameter, and the array is returned:

function multiply(multiplier, ...theArgs) {
  return theArgs.map((element) => multiplier * element);
}

const arr = multiply(2, 15, 25, 42);
console.log(arr); // [30, 50, 84]

From arguments to an array

Array methods can be used on rest parameters, but not on the arguments object:

function sortRestArgs(...theArgs) {
  const sortedArgs = theArgs.sort();
  return sortedArgs;
}

console.log(sortRestArgs(5, 3, 7, 1)); // 1, 3, 5, 7

function sortArguments() {
  const sortedArgs = arguments.sort();
  return sortedArgs; // this will never happen
}

console.log(sortArguments(5, 3, 7, 1));
// throws a TypeError (arguments.sort is not a function)

Rest parameters were introduced to reduce the boilerplate code that was commonly used for converting a set of arguments to an array.

Before rest parameters, arguments need to be converted to a normal array before calling array methods on them:

function fn(a, b) {
  const normalArray = Array.prototype.slice.call(arguments);
  // — or —
  const normalArray2 = [].slice.call(arguments);
  // — or —
  const normalArrayFrom = Array.from(arguments);

  const first = normalArray.shift(); // OK, gives the first argument
  const firstBad = arguments.shift(); // ERROR (arguments is not a normal array)
}

Now, you can easily gain access to a normal array using a rest parameter:

function fn(...args) {
  const normalArray = args;
  const first = normalArray.shift(); // OK, gives the first argument
}