[javascript] How can I add new array elements at the beginning of an array in Javascript?

I have a need to add or prepend elements at the beginning of an array.

For example, if my array looks like below:

[23, 45, 12, 67]

And the response from my AJAX call is 34, I want the updated array to be like the following:

[34, 23, 45, 12, 67]

Currently I am planning to do it like this:

var newArray = [];
newArray.push(response);

for (var i = 0; i < theArray.length; i++) {
    newArray.push(theArray[i]);
}

theArray = newArray;
delete newArray;

Is there any better way to do this? Does Javascript have any built-in functionality that does this?

The complexity of my method is O(n) and it would be really interesting to see better implementations.

If you want to push elements that are in a array at the beginning of you array use <func>.apply(<this>, <Array of args>) :

const arr = [1, 2];
arr.unshift.apply(arr, [3, 4]);
console.log(arr); // [3, 4, 1, 2]

Another way to do that through concat

var arr = [1, 2, 3, 4, 5, 6, 7];_x000D_
console.log([0].concat(arr));

The difference between concat and unshift is that concat returns a new array. The performance between them could be found here.

function fn_unshift() {
  arr.unshift(0);
  return arr;
}

function fn_concat_init() {
  return [0].concat(arr)
}

Here is the test result

you can first reverse array and then add elem to array then reverse back it.

const arr = [2,3,4,5,6];
const arr2 = 1;
arr.reverse();
//[6,5,4,3,2]
arr.push(arr2);

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

good lock

ali alizadeh.

If you need to continuously insert an element at the beginning of an array, it is faster to use push statements followed by a call to reverse, instead of calling unshift all the time.

Benchmark test: http://jsben.ch/kLIYf

you can reverse your array and push the data , at the end again reverse it:

var arr=[2,3,4,5,6];
var arr2=1;
arr.reverse();
//[6,5,4,3,2]
arr.push(arr2);

Without Mutate

Actually, all unshift/push and shift/pop mutate the origin array.

The unshift/push add an item to the existed array from begin/end and shift/pop remove an item from the beginning/end of an array.

But there are few ways to add items to an array without a mutation. the result is a new array, to add to the end of array use below code:

const originArray = ['one', 'two', 'three'];
const newItem = 4;

const newArray = originArray.concat(newItem); // ES5
const newArray2 = [...originArray, newItem]; // ES6+

To add to begin of original array use below code:

const originArray = ['one', 'two', 'three'];
const newItem = 0;

const newArray = (originArray.slice().reverse().concat(newItem)).reverse(); // ES5
const newArray2 = [newItem, ...originArray]; // ES6+

With the above way, you add to the beginning/end of an array without a mutation.

With ES6 , use the spread operator ... :

DEMO

var arr = [23, 45, 12, 67];
arr = [34, ...arr]; // RESULT : [34,23, 45, 12, 67]

console.log(arr)

Using splice we insert an element to an array at the begnning:

arrName.splice( 0, 0, 'newName1' );

you have an array: var arr = [23, 45, 12, 67];

To add an item to the beginning, you want to use splice:

var arr = [23, 45, 12, 67];_x000D_
arr.splice(0, 0, 34)_x000D_
console.log(arr);

Quick Cheatsheet:

The terms shift/unshift and push/pop can be a bit confusing, at least to folks who may not be familiar with programming in C.

If you are not familiar with the lingo, here is a quick translation of alternate terms, which may be easier to remember:

* array_unshift()  -  (aka Prepend ;; InsertBefore ;; InsertAtBegin )     
* array_shift()    -  (aka UnPrepend ;; RemoveBefore  ;; RemoveFromBegin )

* array_push()     -  (aka Append ;; InsertAfter   ;; InsertAtEnd )     
* array_pop()      -  (aka UnAppend ;; RemoveAfter   ;; RemoveFromEnd ) 

Cheatsheet to prepend new element(s) into the array

1. Array#unshift

const list = [23, 45, 12, 67];

list.unshift(34);

console.log(list); // [34, 23, 45, 12, 67];

2. Array#splice

const list = [23, 45, 12, 67];

list.splice(0, 0, 34);

console.log(list); // [34, 23, 45, 12, 67];

3. ES6 spread...

const list = [23, 45, 12, 67];
const newList = [34, ...list];

console.log(newList); // [34, 23, 45, 12, 67];

4. Array#concat

const list = [23, 45, 12, 67];
const newList = [32].concat(list);

console.log(newList); // [34, 23, 45, 12, 67];

Note: In each of these examples, you can prepend multiple items by providing more items to insert.

Using ES6 destructuring: (avoiding mutation off the original array)

const newArr = [item, ...oldArr]