Array.prototype.push()

elementN

The element(s) to add to the end of the array.

The new length property of the object upon which the method was called.

The push() method appends values to an array.

Array.prototype.unshift() has similar behavior to push(), but applied to the start of an array.

The push() 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 elements appended to the end, you can use arr.concat([element0, element1, /* ... ,*/ elementN]) instead. Notice that the elements are wrapped in an extra array — otherwise, if the element is an array itself, it would be spread instead of pushed as a single element due to the behavior of concat().

The push() 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 sports array containing two elements, then appends two elements to it. The total variable contains the new length of the array.

const sports = ["soccer", "baseball"];
const total = sports.push("football", "swimming");

console.log(sports); // ['soccer', 'baseball', 'football', 'swimming']
console.log(total); // 4

This example uses spread syntax to push all elements from a second array into the first one.

const vegetables = ["parsnip", "potato"];
const moreVegs = ["celery", "beetroot"];

// Merge the second array into the first one
vegetables.push(...moreVegs);

console.log(vegetables); // ['parsnip', 'potato', 'celery', 'beetroot']

Merging two arrays can also be done with the concat() method.

The push() method reads the length property of this. It then sets each index of this starting at length with the arguments passed to push(). Finally, it sets the length to the previous length plus the number of pushed elements.

const arrayLike = {
  length: 3,
  unrelated: "foo",
  2: 4,
};
Array.prototype.push.call(arrayLike, 1, 2);
console.log(arrayLike);
// { '2': 4, '3': 1, '4': 2, length: 5, unrelated: 'foo' }

const plainObj = {};
// There's no length property, so the length is 0
Array.prototype.push.call(plainObj, 1, 2);
console.log(plainObj);
// { '0': 1, '1': 2, length: 2 }

As mentioned above, push is intentionally generic, and we can use that to our advantage. Array.prototype.push can work on an object just fine, as this example shows.

Note that 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 to trick the method into thinking we are dealing with an array—and it just works, thanks to the way JavaScript allows us to establish the execution context in any way we want.

const obj = {
  length: 0,

  addElem(elem) {
    // obj.length is automatically incremented
    // every time an element is added.
    [].push.call(this, elem);
  },
};

// Let's add some empty objects just to illustrate.
obj.addElem({});
obj.addElem({});
console.log(obj.length); // 2

Note that although obj is not an array, the method push successfully incremented obj's length property just like if we were dealing with an actual array.

• Polyfill of Array.prototype.push in core-js with fixes of this method
• Indexed collections
• Array
• Array.prototype.pop()
• Array.prototype.shift()
• Array.prototype.unshift()
• Array.prototype.concat()
• Array.prototype.splice()