A sintaxe do parâmetro "rest" permite-nos representar um número indefinido de argumentoscomo um array.
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Sintaxe
function f(a, b, ...theArgs) { // ... }
Descrição
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) {
let normalArray = Array.prototype.slice.call(arguments)
// -- or --
let normalArray = [].slice.call(arguments)
// -- or --
let normalArray = Array.from(arguments)
let first = normalArray.shift() // OK, gives the first argument
let first = arguments.shift() // ERROR (arguments is not a normal array)
}
// Now, you can easily gain access to a normal array using a rest parameter
function f(...args) {
let normalArray = args
let first = normalArray.shift() // OK, gives the first argument
}
Destructuring rest parameters
Rest parameters can be destructured Arrays only (though objects will soon be supported). 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)
Fixme
Doing this is possible, but (afaik) there's no use-case, so it's just confusing the junior audience. The following code does exactly the same. There should at least be a note, that in theory you can do something like this, but there is no point in doing so. The example is contrived, but imagine that [a, b, c] were the return value of some function. Then the utility is clear.
Also, since Function arguments will never have named parameters (this is not Python), the statement "objects will soon be supported" is wrong. The preceding example may be mixing up destructuring with rest parameters. Please see the page on destructuring assignment for an example with object destructuring applied in the parameters area.
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 ...)
Exemplos
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 isn't provided, but manyMoreArgs
is still an array (albeit an empty one).
// 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 parameters after the first into 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
})
}
let 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) {
let sortedArgs = theArgs.sort()
return sortedArgs
}
console.log(sortRestArgs(5, 3, 7, 1)) // 1, 3, 5, 7
function sortArguments() {
let 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)
To use Array
methods on the arguments
object, it must be converted to a real array first.
function sortArguments() {
let args = Array.from(arguments)
let sortedArgs = args.sort()
return sortedArgs
}
console.log(sortArguments(5, 3, 7, 1)) // 1, 3, 5, 7
Especificações
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