I was studying the jQuery source when I found this (v1.5 line 2295):
namespace = new RegExp("(^|\\.)" +
jQuery.map( namespaces.slice(0).sort(), fcleanup ).join("\\.(?:.*\\.)?") + "(\\.|$)");
My question is, why use slice(0)
here?
sort()
modifies the array it's called on - and it isn't very nice to go around mutating stuff that other code might rely on.
slice()
always returns a new array - the array returned by slice(0)
is identical to the input, which basically means it's a cheap way to duplicate an array.
arr.slice(0)
makes a copy of the original array by taking a slice from the element at index 0 to the last element.
It was also used to convert array-like objects into arrays. For example, a DOM NodeList
(returned by several DOM methods like getElementsByTagName
) is not an array, but it is an array-like object with a length
field and is indexable in JavaScript. To convert it to an array, one often used:
var anchorArray = [].slice.call(document.getElementsByTagName('a'), 0)
Now, ES2015's spread syntax is used to convert iterable objects, including NodeList
and HTMLCollection
, to arrays:
const anchorArray = [...document.getElementsByTagName('a')]
slice(0)
creates a new array identical to the original array. Many times you want to preserve your original array and create a new one.
If you use slice(1)
, it will create a different array starting from index position 1.
Similar things hold for strings as well.
slice(0)
允许您返回您正在引用的现有数组的数组,在本例中为命名空间。
In addition to what @Anon said:
The slice()
method selects the elements starting at the given start argument, and ends at, but does not include, the given end argument.
Example1:
var fruits = ["Banana", "Orange", "Lemon", "Apple", "Mango"];
var citrus = fruits.slice(1, 3);
The result of citrus will be:
Orange,Lemon
Example2:
var fruits = ["Banana", "Orange", "Lemon", "Apple", "Mango"];
var citrus = fruits.slice(-3, -1);
The result of citrus will be:
Lemon,Apple
Further information can be found here .
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