Array.prototype.pop() by JavaScript convention.

let fruit = ['apple', 'orange', 'banana', 'tomato'];
let popped = fruit.pop();

console.log(popped); // "tomato"
console.log(fruit); // ["apple", "orange", "banana"]

You can use both Array.prototype.splice(start) and Array.prototype.slice(start, end) depending on if you want to mutate the array or not

Mutable method

Using Array.prototype.splice:

The splice() method of Array instances changes the contents of an array by removing or replacing existing elements and/or adding new elements in place.

const arr = ['a', 'b', 'c', 'd', 'e'];
const n = 3;

arr.splice(n - arr.length); // returns: ['d', 'e']

console.log(arr); // output: ['a', 'b', 'c']

Immutable method

Using Array.prototype.slice:

The slice() method of Array instances returns a shallow copy of a portion of an array into a new array object selected from start to end (end not included) where start and end represent the index of items in that array. The original array will not be modified.

const arr = ['a', 'b', 'c', 'd', 'e'];
const n = 3;

const result = arr.slice(0, n - arr.length);
console.log(result); // output: ['a', 'b', 'c']

console.log(arr); // output: ['a', 'b', 'c', 'd', 'e'] (arr is not mutated)

To keep the first ten items:

if (theArray.length > 10) theArray = theArray.slice(0, 10);

or, perhaps less intuitive:

if (theArray.length > 10) theArray.length = 10;

To keep the last ten items:

if (theArray.length > 10) theArray = theArray.slice(theArray.length - 10, 10);

You can use a negative value for the first parameter to specify length - n, and omitting the second parameter gets all items to the end, so the same can also be written as:

if (theArray.length > 10) theArray = theArray.slice(-10);

The splice method is used to remove items and replace with other items, but if you specify no new items it can be used to only remove items. To keep the first ten items:

if (theArray.length > 10) theArray.splice(10, theArray.length - 10);

To keep the last ten items:

if (theArray.length > 10) theArray.splice(0, theArray.length - 10);

The slice method is what you want. It returns a new object, so you must replace your existing object with the new one.

arr = arr.slice(-1 * n);

Alternatively, modify the existing array with splice().

arr.splice(0, arr.length - n);

Splice is the more efficient, since it is not copying elements.