JS Basic (2) filter filter ()

Filter is also a common operation, it is used to Array filter out some of the elements, and then return the remaining elements.

and map() Similarly, Array filter() a function is also received. And the map() difference is that the filter() incoming function acts on each element in turn, and then whether true false the element is persisted or discarded based on the return value.

For example, in one Array , delete an even number, keep only the odd number, and you can write:

var arr = [1, 2, 4, 5, 6, 9, 10, 15];var r = arr.filter(function (x) { return x % 2 !== 0;});r; // [1, 5, 9, 15]

Delete the Array empty string in one, so you can write:

var arr = [‘A‘, ‘‘, ‘B‘, null, undefined, ‘C‘, ‘ ‘];var r = arr.filter(function (s) { return s && s.trim(); // 注意：IE9以下的版本没有trim()方法});r; // [‘A‘, ‘B‘, ‘C‘]

filter()the key to using this higher-order function is to implement a "filter" function correctly.

callback function

filter()The received callback function can actually have multiple parameters. Usually we use only the first parameter, which represents an Array element. The callback function can also receive two additional parameters, representing the location of the element and the array itself:

var arr = [‘A‘, ‘B‘, ‘C‘];var r = arr.filter(function (element, index, self) { console.log(element); // 依次打印‘A‘, ‘B‘, ‘C‘ console.log(index); // 依次打印0, 1, 2 console.log(self); // self就是变量arr return true;});

Using filter , can subtly remove Array the repeating elements:

JS Basic (2) filter filter ()