Array: Insert an Item at a Specific Index with JavaScript

By  on  

There are many tasks related to arrays that sound quite simple but (1) aren't and (2) aren't required of a developer very often. I was encountered with one such task recently: inserting an item into an existing array at a specific index. Sounds easy and common enough but it took some research to figure it out.

// The original array
var array = ["one", "two", "four"];
// splice(position, numberOfItemsToRemove, item)
array.splice(2, 0, "three");

array;  // ["one", "two", "three", "four"]

If you aren't adverse to extending natives in JavaScript, you could add this method to the Array prototype:

Array.prototype.insert = function (index, item) {
  this.splice(index, 0, item);
};

I've tinkered around quite a bit with arrays, as you may have noticed:

Arrays are super useful -- JavaScript just makes some tasks a bit more ... code-heavy than they need to be. Keep these snippets in your toolbox for the future!

Recent Features

Incredible Demos

  • By
    Build a Toggling Announcement Slider Using MooTools 1.2

    A few of my customer have asked for me to create a subtle but dynamic (...I know...) way for them to advertise different specials on their website. Not something that would display on every page, but periodically or only the homepage. Using a trick...

  • By
    Use Custom Missing Image Graphics Using MooTools

    Missing images on your website can make you or your business look completely amateur. Unfortunately sometimes an image gets deleted or corrupted without your knowledge. You'd agree with me that IE's default "red x" icon looks awful, so why not use your own missing image graphic? The MooTools JavaScript Note that...

Discussion

  1. Man, isn’t this JavaScript powerful…

  2. MaxArt

    How about a little step further? Since splice takes an indefinite number of arguments, you can do something like this:

    Array.prototype.insert = function (index) {
      this.splice.apply(this, [index, 0].concat(this.slice.call(arguments, 1)));
    };
    

    so you can do

    array.insert(2, "three", "another three", "the last three");
    array;  // ["one", "two", "three", "another three", "the last three", "four"]
    
  3. Great tip, thanks. I knew this trick before but I forgot that splice could take more than 2 arguments, phew.

  4. Yes, it’s a very useful function. Last year I was delighted to find out that it exists – it made the project I was working on extremely useful.

Wrap your code in <pre class="{language}"></pre> tags, link to a GitHub gist, JSFiddle fiddle, or CodePen pen to embed!