call()/apply()改变调用函数的作用域中的this指向
talk is cheap ,show me the code:
第一个参数为this,第二个参数为传输的值，例如
var a ="windowA";
var b = "windowB";
var str = "world";
var myObject = {a:"myA",b:"myB"};
function hello(s){
alert("a= "+this.a + ", b= "+this.b+" "+s);
}
//hello 中的this 指向window;
hello.call(null,str);//a ="windowA" b = "windowB" world
// hello 中的this.指向myObject；
hello.call(myObject,str);//a="myA" b="myB" world
如果第一个参数为null,则this指向window(在node环境中则指向global)
hello.call(null)//this 指向window
hello.call(window)//this同样指向window
