Moon Moon - 2 months ago 6
Javascript Question

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

I have a need to add elements (I retrieve on timely basis using AJAX) at the top 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 (int 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

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.

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