What's the point of .slice(0) here?
JavascriptJqueryType ConversionSliceJavascript Problem Overview
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?
Javascript Solutions
Solution 1 - Javascript
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.
Solution 2 - Javascript
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')]
Solution 3 - Javascript
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.
Solution 4 - Javascript
slice(0) allows you to return an array of the existing array you're referencing, in this case namespaces.
Solution 5 - Javascript
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.