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

Question

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.

Answer 1

Use unshift. It's like push, except it adds elements to the beginning of the array instead of the end.

• unshift/push - add an element to the beginning/end of an array
• shift/pop - remove and return the first/last element of an array

A simple diagram...

   unshift -> array <- push
   shift   <- array -> pop
 

and chart:

          add  remove  start  end
   push    X                   X
    pop           X            X
unshift    X             X
  shift           X      X

Check out the MDN Array documentation. Virtually every language that has the ability to push/pop elements from an array will also have the ability to unshift/shift (sometimes called push_front/pop_front) elements, you should never have to implement these yourself.

---

As pointed out in the comments, if you want to avoid mutating your original array, you can use concat, which concatenates two or more arrays together. You can use this to functionally push a single element onto the front or back of an existing array; to do so, you need to turn the new element into a single element array:

const array = [3, 2, 1]

const newFirstElement = 4

const newArray = [newFirstElement].concat(array) // [ 4, 3, 2, 1 ]

console.log(newArray);

concat can also append items. The arguments to concat can be of any type; they are implicitly wrapped in a single-element array, if they are not already an array:

const array = [3, 2, 1]

const newLastElement = 0

// Both of these lines are equivalent:
const newArray1 = array.concat(newLastElement) // [ 3, 2, 1, 0 ]
const newArray2 = array.concat([newLastElement]) // [ 3, 2, 1, 0 ]

console.log(newArray1);
console.log(newArray2);

Answer 2

var a = [23, 45, 12, 67];
a.unshift(34);
console.log(a); // [34, 23, 45, 12, 67]

Answer 3

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)

Answer 4

Another way to do that through concat

var arr = [1, 2, 3, 4, 5, 6, 7];
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

Answer 5

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 ) 

Answer 6

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];
arr.splice(0, 0, 34)
console.log(arr);

Answer 7

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

const newArr = [item, ...oldArr]

Answer 8

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.

Answer 9

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.

Answer 10

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

Answer 11

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

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

Answer 12

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.

Answer 13

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]

Answer 14

Performance: For small arrays you can choose whichever you want, no significant performance difference. But for some tens of items, unshift() is much better than anything else.

See results here: https://jsben.ch/GDA3P

Answer 15

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);