Retrieving an element from a JavaScript array is most often done using its index. But what if we want to get the nth element from the end of the array? Or, perhaps, get all elements at an index that is a multiple of n? Let’s see how we can do that.
Get the nth element of a JavaScript array
Getting the nth element of a JavaScript array can be easily done, using Array.prototype.slice(). The only thing you need to keep in mind is negative indexes, which you can be used to get the last nth element of the array.
代码实现
const nthElement = (arr, n = 0) =>
(n === -1 ? arr.slice(n) : arr.slice(n, n + 1))[0];
nthElement(['a', 'b', 'c'], 1); // 'b'
nthElement(['a', 'b', 'b'], -3); // 'a'
nthElement(['a', 'b', 'b'], 3); // undefinedGet every nth element of a JavaScript array
If you need to get every nth element of a JavaScript array, you can use Array.prototype.filter(). Combined with the modulo operator (%), you can use it to keep only the elements that are at every nth position in the array.
使用样例
const everyNth = (arr, nth) =>
arr.filter((e, i) => i % nth === nth - 1);
everyNth([1, 2, 3, 4, 5, 6], 2); // [ 2, 4, 6 ]