Rest Parameter

Mit der rest parameter Syntax kann man beliebig viele Parameter als Array empfangen.

Syntax

function f(a, b, ...restArgs) {
  // ...
}

Description

Wenn vor dem letzten Argument einer Funktion ... steht wird aus diesem Parameter ein Array von 0 (inklusiv) bis zu restArgs.length (exklusiv), welches weitere der Funktion übergebene Parameter enthält. Im oberen Beispiel enthält das restArgs-Array alle weiteren argumente außer die ersten beiden, da diese bereits von den Variablen a und b erfasst werden.

Difference between rest parameters and the arguments object

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

• rest parameters are only the ones that haven't been given a separate name, while the arguments object contains all arguments passed to the function;
• 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 additional functionality specific to itself (like the callee property).

From arguments to an array

Rest parameters have been introduced to reduce the boilerplate code that was induced by the arguments

// Before rest parameters, the following could be found:
function f(a, b) {
  var args = Array.prototype.slice.call(arguments, f.length);

  // …
}

// to be equivalent of

function f(a, b, ...args) {
  
}

Destructuring rest parameters

Rest parameters can be destructured, that means that their data can be unpacked into distinct variables. See Destructuring assignment.

function f(...[a, b, c]) {
  return a + b + c;
}

f(1)          // NaN (b and c are undefined)
f(1, 2, 3)    // 6
f(1, 2, 3, 4) // 6 (the fourth parameter is not destructured)

Examples

Since theArgs is an array, a count of its elements is given by the length property:

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

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

In the next example, a rest parameter is used to collect all arguments after the first one in an array. Each one of them is then multiplied by the first parameter and the array is returned:

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

var arr = multiply(2, 1, 2, 3); 
console.log(arr); // [2, 4, 6]

The following example shows that Array methods can be used on rest parameters, but not on the arguments object:

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

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

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

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

In order to use Array methods on the arguments object, it must be converted to a real array first.

Specifications

Browser compatibility

See also

• Spread operator (also ‘...’)
• Arguments object
• Array
• Functions
• Original proposal at ecmascript.org
• JavaScript arguments object and beyond
• Destructuring assignment