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I parametri rest permettono di rappresentare un indefinito numero di argomenti come un array

Sintassi

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

Descrizione

Se l'ultimo argomento di una funzione è preceduto da ..., questo diventa un array i cui elementi, da 0 (incluso) a theArgs.length (escluso), sono forniti dagli argomenti effettivi passati alla funzione.

Nell'esempio precedente, theArgs ottiene il terzo parametro della funzione (perchè il primo è mappato ad a ed il secondo a b) e tutti quelli successivi.

Differenze tra i rest parameters e l'oggetto arguments

Ci sono tre differenze principali tra i rest parameters e l'oggetto arguments:

  • 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 extracted into distinct variables.

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 arguments from the second one to the end in an array. Each is then multipled them by the first and the array 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

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Function Definitions' in that specification.
Standard Initial definition
ECMAScript Latest Draft (ECMA-262)
The definition of 'Function Definitions' in that specification.
Living Standard  

Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari
Basic support 47 (Yes) 15.0 (15.0) Edge/10240 34 10
Destructuring (Yes) No support 52.0 (52.0) No support (Yes) ?
Feature Android Android Webview Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support No support 47 47 15.0 (15.0) No support No support No support
Destructuring ? ? ? 52.0 (52.0) ? ? ?

See also

Tag del documento e collaboratori

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