call方法和new对象的关系

var fn = function(a){
    this.a = a;
}
fn.prototype = {
    draw:function(){
        console.log(this.a);
    }
}

var obj = new fn(10);
obj.draw(); //10

var fn = function(a){
    this.a = a;
}
var obj = new fn();

var fn = function(a){
    this.a = a;
}
var obj = {};
继承
obj.__proto__ = fn.prototype;
改变this指向
fn.call(obj);