get first element of javascript array

Question

I have an array of objects in javascript. I use jquery.

How do i get the first element in the array? I cant use the array index - as I assign each elements index when I am adding the objects to the array. So the indexes arent 0, 1, 2 etc.

Just need to get the first element of the array?

Answer 1

If you don't use sequentially numbered elements, you'll have to loop through until you hit the first one:

var firstIndex = 0;
while (firstIndex < myarray.length && myarray[firstIndex] === undefined) {
    firstIndex++;
}
if (firstIndex < myarray.length) {
    var firstElement = myarray[firstIndex];
} else {
    // no elements.
}

or some equivalently silly construction. This gets you the first item's index, which you might or might not care about it.

If this is something you need to do often, you should keep a lookaside reference to the current first valid index, so this becomes an O(1) operation instead of O(n) every time. If you're frequently needing to iterate through a truly sparse array, consider another data structure, like keeping an object alongside it that back-maps ordinal results to indexes, or something that fits your data.

Answer 2

Shift();

See http://www.bennadel.com/blog/1796-Javascript-Array-Methods-Unshift-Shift-Push-And-Pop-.htm

quixoto is right, this won't work in your case due to shift returning undefined.

How about something simple like this:

var result = myArray.filter(function(e, i, a) { return e != undefined;})[0];

That should return the first non-undefined entry in the array regardless of index.

Answer 3

If you find yourself needing to do manipulation of arrays a lot, you might be interested in the Underscore library. It provides utility methods for manipulating arrays, for example compact:

var yourArray = [];
yourArray[10] = "foo";
var firstValue = _.compact(yourArray)[0];

However, it does sound like you are doing something strange when you are constructing your array. Perhaps Array.push would help you out?