Array.prototype.push()

The push() method adds one or more elements to the end of an array and returns the new length of the array.

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.

push is intentionally generic. This method can be used with call() or apply() on objects resembling arrays. The push method relies on a length property to determine where to start inserting the given values. If the length property cannot be converted into a number, the index used is 0. This includes the possibility of length being nonexistent, in which case length will also be created.

Although strings are native, Array-like objects, they are not suitable in applications of this method, as strings are immutable. Similarly for the native, Array-like object arguments.

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.

let sports = ['soccer', 'baseball']
let 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.

let vegetables = ['parsnip', 'potato']
let 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.

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.

let obj = {
    length: 0,

    addElem: function 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.

• Array.prototype.pop()
• Array.prototype.shift()
• Array.prototype.unshift()
• Array.prototype.concat()