大家帮我讲解下call的实际运用机制.

悬赏园豆：20 [已解决问题] 解决于 2012-05-31 14:41

fn.call(this,this.elements[i]); //..虽然看了call的手册,这里还是难于理解
而且这段代码中为什么还要这样写call呢?
上面代码是不是相当于:
function(el){
el.style[prop] = val;
}.call(this,this.elements[i]);
谁详细的讲解一下这个的执行流程?

<div id="box">mybox</div>

(function(){
        
        function _$(els){
            this.elements = [];
            for(var i=0, len = els.length; i<len; i++){
                var element = els[i];
                if(typeof element === "string"){
                    element = document.getElementById(element);
                }
                this.elements.push(element);
            }
        }
        
        _$.prototype = {
            each : function(fn){
                for(var i=0, len = this.elements.length; i<len; i++){
                    fn.call(this,this.elements[i]);
                    console.log(this.elements[i])
                }
            },
            setStyle : function(prop,val){
                this.each(function(el){
                    console.log(el)
                    el.style[prop] = val;
                })
            }
        }
        
        window.$ = function(){
            return new _$(arguments);
        }
    })();
    $("box").setStyle("color","red")