Question

there are a lot of places (e.g. How to use requestAnimationFrame?) that fix window.requestAnimationFrame as below. I do not understand why the right hand side of the assignment is wrapped in to a function call.

window.requestAnimFrame = (function(){
    return window.requestAnimationFrame ||
    window.webkitRequestAnimationFrame ||
    window.mozRequestAnimationFrame ||
    window.oRequestAnimationFrame ||
    window.msRequestAnimationFrame ||
    function(callback){
        window.setTimeout(callback, 1000 / 60);
    };
})();

Answer 1

I’ve wondered the same thing before. I’m pretty sure Google just really likes closures. Granted there is not one variable in there, it’s a ‘safe’ thing to do.

As far as I can tell, this is the exact same thing, without any change of interfering with namespaces (a general reason to use a closure like that):

window.requestAnimFrame = 
    window.requestAnimationFrame ||
    window.webkitRequestAnimationFrame ||
    window.mozRequestAnimationFrame ||
    window.oRequestAnimationFrame ||
    window.msRequestAnimationFrame ||
    function(callback){
        window.setTimeout(callback, 1000 / 60);
    };