Using a global variable in JavaScript

By using var when setting number = '10', you are declaring number as a local variable each time. Try this:

var number = null;

function playSong(artist, title, song, id)
{
    alert('old number was: ' + [number] + '');

    number = '10';

    alert('' + [number] + '');
}

Remove the var in front of number in your function. You are creating a local variable by

var number = 10;

You just need

number = 10;

The problem is that you're declaring a new variable named number inside of the function. This new variable hides the global number variable, so the line number = 10 assigns only to this new local variable.

You need to remove the var keyword from var number = 10.