The rest parameter syntax allows to represent an indefinite number of arguments as an array.
Syntax
function(a, b, ...theArgs) {
// ...
}
Description
If the last named argument of a function is prefixed with ..., it becomes an array whose elements from 0 to theArgs.length are supplied by the actual arguments passed to the function.
In the above example, theArgs would collect the third argument of the function (because the first one is mapped to a, and the second to b) and all the consecutive arguments.
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
argumentsobject contains all arguments passed to the function; - the
argumentsobject is not a real array, while rest parameters areArrayinstances, meaning methods likesort,map,forEachorpopcan be applied on it directly; - the
argumentsobject has additional functionality specific to itself (like thecalleeproperty).
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(a, b, ...args) {
}
Examples
Since theArgs is an array, you can get the count of its elements by using the length property:
function fun1(...theArgs) {
console.log(theArgs.length);
}
fun1(); // 0
fun1(5); // 1
fun1(5, 6, 7); // 3
In the next example, we use the rest parameters to collect arguments from the second one to the end. We then multiply them by the first one:
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 you can use Array methods on rest params, 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, you would need to convert it to a real array first.
Specifications
| Specification | Status | Comment |
|---|---|---|
| {{SpecName('ES6', '#sec-function-definitions', 'Function Definitions')}} | {{Spec2('ES6')}} | Initial definition. |
Browser compatibility
| Feature | Chrome | Edge | Firefox (Gecko) | Internet Explorer | Opera | Safari |
|---|---|---|---|---|---|---|
| Basic support | {{CompatChrome(45)}} | {{CompatVersionUnknown}} | {{CompatGeckoDesktop("15.0")}} | {{CompatNo}} | {{CompatNo}} | {{CompatNo}} |
| Feature | Android | Android Webview | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile | Chrome for Android |
|---|---|---|---|---|---|---|---|
| Basic support | {{CompatNo}} | {{CompatChrome(45)}} | {{CompatGeckoMobile("15.0")}} | {{CompatNo}} | {{CompatNo}} | {{CompatNo}} | {{CompatChrome(45)}} |