Array.prototype.unshift()

elementN

The elements to add to the front of the arr.

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

The unshift() method inserts the given values to the beginning of an array-like object.

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

Please note that, if multiple elements are passed as parameters, they're inserted in chunk at the beginning of the object, in the exact same order they were passed as parameters. Hence, calling unshift() with n arguments once, or calling it n times with 1 argument (with a loop, for example), don't yield the same results.

See example:

let arr = [4, 5, 6];

arr.unshift(1, 2, 3);
console.log(arr);
// [1, 2, 3, 4, 5, 6]

arr = [4, 5, 6]; // resetting the array

arr.unshift(1);
arr.unshift(2);
arr.unshift(3);

console.log(arr);
// [3, 2, 1, 4, 5, 6]

The unshift() 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 unshift() method reads the length property of this. It shifts all indices in the range 0 to length - 1 right by the number of arguments (incrementing their values by this number). Then, it sets each index starting at 0 with the arguments passed to unshift(). Finally, it sets the length to the previous length plus the number of prepended elements.

const arrayLike = {
  length: 3,
  unrelated: "foo",
  2: 4,
};
Array.prototype.unshift.call(arrayLike, 1, 2);
console.log(arrayLike);
// { '0': 1, '1': 2, '4': 4, length: 5, unrelated: 'foo' }

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

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