JavaScript中this关键字用法实例分析

function a(){
  var user = "yao";
  console.log(this.user);//undefined
  console.log(this);//window
}
a();

function a(){
  var user = "yao";
  console.log(this.user);//undefined
  console.log(this);//window
}
window.a();

var o = {
  user:"yao",
  fn:function () {
    console.log(this.user);//yao
  }
}
o.fn();

var o = {
  user:"yao",
  fn:function () {
    console.log(this.user);//yao
  }
}
window.o.fn();

var o = {
  a:10,
  b:{
    a:12,
    fn:function () {
      console.log(this.a);//12
    }
  }
}
o.b.fn();

var o = {
  a:10,
  b:{
    a:12,
    fn:function () {
      console.log(this.a);//undefined
      console.log(this);//window
    }
  }
};
var j = o.b.fn;
j();

function fn(){
  this.user = "yao";
  return {};
}
var a = new fn;
console.log(a.user);//undefined

function fn(){
  this.user = "yao";
  return function(){};
}
var a = new fn;
console.log(a.user);//undefined

function fn(){
  this.user = "yao";
  return 1;
}
var a = new fn;
console.log(a.user);//yao

function fn(){
  this.user = "yao";
  return undefined;
}
var a = new fn;
console.log(a.user);//yao

function fn(){
  this.user = "yao";
  return null;
}
var a = new fn;
console.log(a.user);//yao