Array.prototype.pop()

The removed element from the array; undefined if the array is empty.

The pop() method removes the last element from an array and returns that value to the caller. If you call pop() on an empty array, it returns undefined.

Array.prototype.shift() has similar behavior to pop(), but applied to the first element in an array.

The pop() method is a mutating method. It changes the length and the content of this. In case you want the value of this to be the same, but return a new array with the last element removed, you can use arr.slice(0, -1) instead.

The pop() method is generic. It only expects the this value to have a length property and integer-keyed properties. Although strings are also array-like, this method is not suitable to be applied on them, as strings are immutable.

The following code creates the myFish array containing four elements, then removes its last element.

const myFish = ["angel", "clown", "mandarin", "sturgeon"];

const popped = myFish.pop();

console.log(myFish); // ['angel', 'clown', 'mandarin' ]

console.log(popped); // 'sturgeon'

The pop() method reads the length property of this. If the normalized length is 0, length is set to 0 again (whereas it may be negative or undefined before). Otherwise, the property at length - 1 is returned and deleted.

const arrayLike = {
  length: 3,
  unrelated: "foo",
  2: 4,
};
console.log(Array.prototype.pop.call(arrayLike));
// 4
console.log(arrayLike);
// { length: 2, unrelated: 'foo' }

const plainObj = {};
// There's no length property, so the length is 0
Array.prototype.pop.call(plainObj);
console.log(plainObj);
// { length: 0 }

push and pop are intentionally generic, and we can use that to our advantage — as the following example shows.

Note that in this example, we don't create an array to store a collection of objects. Instead, we store the collection on the object itself and use call on Array.prototype.push and Array.prototype.pop to trick those methods into thinking we're dealing with an array.

const collection = {
  length: 0,
  addElements(...elements) {
    // obj.length will be incremented automatically
    // every time an element is added.

    // Returning what push returns; that is
    // the new value of length property.
    return [].push.call(this, ...elements);
  },
  removeElement() {
    // obj.length will be decremented automatically
    // every time an element is removed.

    // Returning what pop returns; that is
    // the removed element.
    return [].pop.call(this);
  },
};

collection.addElements(10, 20, 30);
console.log(collection.length); // 3
collection.removeElement();
console.log(collection.length); // 2

• Indexed collections
• Array
• Array.prototype.push()
• Array.prototype.shift()
• Array.prototype.unshift()
• Array.prototype.concat()
• Array.prototype.splice()