`this.some_property` 在匿名回调函数中变得未定义

Question

所以我不太明白为什么变量 this.tasks 在我的目标对象内部的添加事件侦听器中变得未定义。我有一种感觉，它可能与异步编程有关（我仍然不完全理解）。抱歉，我有点 JS 菜鸟，但如果你们能向我解释我做错了什么，以及什么可能是一个更好的解决方案，那就太棒了！谢谢。

function Goal(name) {
        this.gDiv =  document.createElement('div');
        this.name = name || "goal";
        this.tasks = document.createElement('ul');
        //Sets the styling and content and adds it to the parent element
        this.initialize = function() {
            this.gDiv.className = "default";
            this.gDiv.setAttribute("id", this.name);
            this.gDiv.innerHTML = this.name;
            elem.appendChild(this.gDiv);

            this.gDiv.parentNode.insertBefore(this.tasks, this.gDiv.nextSibling);
            this.tasks.style.display = "none";


        };  
        //Creates a list underneath the a dive associated with the Goal object
        this.addTask = function(task) {
            var newLi = document.createElement('li');
                newLi.innerHTML = task;
                this.tasks.appendChild(newLi);
        };

        this.gDiv.addEventListener('click', function(){
            alert(this.tasks);              
        });

    }

谢谢你们！你们都回答了我的问题！我一直在为此挠头。向大家致敬！

Answer 1

当您输入匿名闭包并且“this”发生变化时，范围会发生变化。你可以通过这样做来破解它

var self = this;

然后用 self 代替这个（例如）：

function Goal(name) {
    var self = this;

    /* ... */

    this.gDiv.addEventListener('click', function(){
        alert(self.tasks);              
    });

如果你使用 jQuery，你可以做得更好：

this.gDiv.addEventListener('click', $.proxy(function() {
    alert(this.tasks);
 }, this));

任何一种方式都很好。

编辑：在 ES6 中，可以使用箭头函数代替，因为它们不绑定自己的“this”，所以它变得更加简单：

this.gDiv.addEventListener('click', () => {
    alert(this.tasks);
 });

Answer 2

这里是一些方法的比较（包括你的问题），给你一个品尝者，并试着解释一下。

// This is the problem that you have,
// where `this` inside the anonymous function
// is a different scope to it's parent
function Test1(something) {
  // `this` here refers to Test1's scope
  this.something = something;
  setTimeout(function() {
    // `this` here refers to the anonymous function's scope
    // `this.something` is `undefined` here
    console.log(this.something);
  }, 1000);
};

new Test1('Hello');

// This solution captures the parent `this` as `test2This`,
// which can then be used inside the anonymous function
function Test2(something) {
  var test2This = this;

  this.something = something;
  setTimeout(function() {
    console.log(test2This.something);
  }, 1000);
}

new Test2('World');

// This solution captures `this` as `test3This` in an `IIFE closure`
// which can then be used in the anonymous function
// but is not available outside of the `IIFE closure` scope
function Test3(something) {
  this.something = something;
  (function(test3This) {
    setTimeout(function() {
      console.log(test3This.something);
    }, 1000);
  }(this));
}

new Test3('Goodbye');

// This method requires that you load an external library: jQuery
// and then use it's `$.proxy` method to achieve the basics of 
// Test3 but instead of being referred to as `test3This` the
// outer scope `this` becomes the inner scope `this`
// Ahh, that's much clearer?
function Test4(something) {
  this.something = something;
  setTimeout($.proxy(function() {
    console.log(this.something);
  }, this), 1000);
}

new Test4('Mum');

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>

// This is approximately what jQuery's `$.proxy` does
// but without having to load the whole library
function Test5(something) {
  this.something = something;
  setTimeout((function(func, context) {
    return function() {
      func.call(context);
    };
  }(function() {
    console.log(this.something);
  }, this)), 1000);
}

new Test5('Dad');

// Lets create the proxy method as a reuseable
function proxy(func, context) {
  var args = Array.prototype.slice.call(arguments, 2);

  return function() {
    return func.apply(
      context, 
      args.concat(Array.prototype.slice.call(arguments))
    );
  };
}

// and now using it
function Test6(something) {
  this.something = something;
  setTimeout(proxy(function() {
    console.log(this.something);
  }, this), 1000);
}

new Test6('Me want cookies');

然后我们有Function#bind

function Test7(something) {
  this.something = something;
  setTimeout(function() {
    // `this` was bound to the parent's `this` using bind
    console.log(this.something);
  }.bind(this), 1000);
};

new Test7('Num num');

<script src="https://cdnjs.cloudflare.com/ajax/libs/es5-shim/4.5.9/es5-shim.min.js"></script>

最近ES2015 Arrow functions

function Test8(something) {
  this.something = something;
  setTimeout(() => console.log(this.something), 1000);
};

new Test8('Whoop');

Answer 3

在 ES6 中，引入了箭头函数，它们不绑定自己的 this。

MDN for reference.

因此，使用箭头语法创建匿名函数可能是当今解决此问题的最简单方法。目前所有主流浏览器都支持，除了IE。

Answer 4

关键字'this'改变了它对构造函数的事件处理程序的含义

请参考MDN

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this#As_a_DOM_event_handler