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.
Try it
Syntax
js
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.
js
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.
js
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 areArray
instances, meaning methods likesort()
,map()
,forEach()
orpop()
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
. Thearguments
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.
js
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.
js
// 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).
js
// 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.
js
// 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
.
js
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:
js
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:
js
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:
js
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:
js
function fn(...args) {
const normalArray = args;
const first = normalArray.shift(); // OK, gives the first argument
}
Specifications
Specification |
---|
ECMAScript Language Specification # sec-function-definitions |
Browser compatibility
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