Get the first or last n elements in a JavaScript array

Array.prototype.slice() is a versatile tool in every JavaScript developer's arsenal. It retrieves the values of an array between two indices, handling edge cases like negative and out-of-bounds indices. This makes it ideal for retrieving the first or last n elements of an array.

For the first n elements, you can use a start value of 0 and an end value of n. For the last n elements, you can use a start value of -n and no end value.

const firstN = (arr, n = 1) => arr.slice(0, n);
const lastN = (arr, n = 1) => arr.slice(-n);

const arr = ['a', 'b', 'c', 'd'];

firstN(arr); // ['a']
firstN(arr, 2); // ['a', 'b']
firstN(arr, 5); // ['a', 'b', 'c', 'd']

lastN(arr); // ['d']
lastN(arr, 2); // ['c', 'd']
lastN(arr, 5); // ['a', 'b', 'c', 'd']

As you can see, it only takes a single line of code to get the first or last n elements of an array. This code also handles out-of-bounds indices, resulting in a shallow clone of the original array. Be mindful of negative n values, however, as the results will not make much sense.