The rest parameter syntax allows us to represent an indefinite number of arguments as an array.
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Syntax
function f(a, b, ...theArgs) { // ... }
Description
A function's last parameter can be prefixed with ...
which will cause all remaining (user supplied) arguments to be placed within a "standard" javascript array. Only the last parameter can be a "rest parameter".
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]
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 (i.e. formally defined in function expression), while the
arguments
object contains all arguments passed to the function; - the
arguments
object is not a real array, while rest parameters areArray
instances, meaning methods likesort
,map
,forEach
orpop
can be applied on it directly; - the
arguments
object has additional functionality specific to itself (like thecallee
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, "arguments" could be converted to a normal array using: function f(a, b) { var normalArray = Array.prototype.slice.call(arguments); // -- or -- var normalArray = [].slice.call(arguments); // -- or -- var normalArray = Array.from(arguments); var first = normalArray.shift(); // OK, gives the first argument var first = arguments.shift(); // ERROR (arguments is not a normal array) } // Now we can easily gain access to a normal array using a rest parameter function f(...args) { var normalArray = args; var first = normalArray.shift(); // OK, gives the first argument }
Destructuring rest parameters
Rest parameters can be destructured (arrays only), 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
In this example, the first argument is mapped to "a"
and the second to "b"
, so these named arguments are used like normal. However the third argument "manyMoreArgs"
will be an array that contains the 3rd, 4th, 5th, 6th ... nth -- as many arguments that the user includes.
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]
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]
Below... the third argument wasn't provided, yet "manyMoreArgs" is still an array (although empty).
// using the same function definition from example above myFun("one", "two"); // a, one // b, two // manyMoreArgs, []
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]
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)); // 1, 3, 5, 7 function sortArguments() { var sortedArgs = arguments.sort(); return sortedArgs; // this will never happen } console.log(sortArguments(5, 3, 7, 1)); // TypeError (arguments.sort is not a function)
To use Array
methods on the arguments
object, it must be converted to a real array first.
function sortArguments() { var args = Array.from(arguments); var sortedArgs = args.sort(); return sortedArgs; } console.log(sortArguments(5, 3, 7, 1)); // 1, 3, 5, 7
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. |
Draft |
Browser compatibility
Desktop | Mobile | Server | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Basic support | Chrome Full support 47 | Edge Full support 12 | Firefox Full support 15 | IE No support No | Opera Full support 34 | Safari Full support 10 | WebView Android Full support 47 | Chrome Android Full support 47 | Edge Mobile Full support 12 | Firefox Android Full support 15 | Opera Android Full support 34 | Safari iOS Full support 10 | Samsung Internet Android Full support 5.0 | nodejs
Full support
6.0.0
|
Destructuring rest parameters | Chrome Full support 49 | Edge No support No | Firefox Full support 52 | IE No support No | Opera Full support 36 | Safari ? | WebView Android Full support 49 | Chrome Android Full support 49 | Edge Mobile No support No | Firefox Android Full support 52 | Opera Android Full support 36 | Safari iOS ? | Samsung Internet Android Full support 5.0 | nodejs Full support Yes |
Legend
- Full support
- Full support
- No support
- No support
- Compatibility unknown
- Compatibility unknown
- User must explicitly enable this feature.
- User must explicitly enable this feature.