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

问题:

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.

回答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 and 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.

回答2:

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

回答3:

With ES6 , use the spread operator ... :

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

回答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

回答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 ) 

回答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 = [