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
    Introducing MooTools ScrollSpy

    I've been excited to release this plugin for a long time. MooTools ScrollSpy is a unique but simple MooTools plugin that listens to page scrolling and fires events based on where the user has scrolled to in the page. Now you can fire specific...

  • By
    Checkbox Filtering Using MooTools ElementFilter

    When I first wrote MooTools ElementFilter, I didn't think much of it. Fast forward eight months later and I've realized I've used the plugin a billion times. Hell, even one of the "big 3" search engines is using it for their maps application.

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!