js中this的指向问题归纳总结

alert(this); // this -> window

function demo() {
 alert(this); // this -> window
}
demo();

function demo() {
 'use strict';
 alert(this); // undefined
}
demo();

function demo() {
 //alert(this); // this -> object
 this.testStr = 'this is a test';
}
let a = new demo();
alert(a.testStr); // 'this is a test'

function demo() {
 alert(this);
}
demo.call('abc'); // abc
demo.call(null); // this -> window
demo.call(undefined); // this -> window

setTimeout(function() {
 alert(this); // this -> window ，严格模式 也是指向window
},500)

window.onload = function() {
 let $btn = document.getElementById('btn');
 $btn.onclick = function(){
 alert(this); // this -> 当前触发
 }
}

window.onload = function() {
 let $btn = document.getElementById('btn');
 $btn.addEventListener('click',function() {
 alert(this); // window
 }.bind(window))
}

let name = 'finget'
let obj = {
 name: 'FinGet',
 getName: function() {
 alert(this.name);
 }
}
obj.getName(); // FinGet
---------------------------分割线----------------------------
let fn = obj.getName;
fn(); //finget this -> window

var x = 20;
var a = {
 x: 15,
 fn: function() {
 var x = 30;
 return function() {
  return this.x
 }
 }
}
console.log(a.fn());
console.log((a.fn())());
console.log(a.fn()());
console.log(a.fn()() == (a.fn())());
console.log(a.fn().call(this));
console.log(a.fn().call(a));