Javascript: insert array in another array at a specific index : <p>I have two arrays:</p>

<pre><code>a = [1,2,3]
b = [4,5,6]
</code></pre>

<p>I'd like to insert b at index 1 of a, to have :</p>

<pre><code>c = [1,4,5,6,2,3]
</code></pre>

<p>Is there a builtin function to do this ?
I found the answer for a single element, but not for a whole array. 
I imagine something like <code>concat</code> but with an additional parameter which would be the index of insertion. </p>
