“未捕获的引用错误:未定义"在类构造函数中 [英] "Uncaught ReferenceError: this is not defined" in class constructor
问题描述
我正在使用 JavaScript/ES6 中的新东西.我在我的代码中收到一个 Uncaught ReferenceError: this is not defined(...) player.js:5.据我所知，这里没有错误！这是一个错误吗?有什么解决方法吗?
I am playing with the new stuff in JavaScript/ES6. I get an Uncaught ReferenceError: this is not defined(...) player.js:5 in my code. As far as I see, there are no errors here! Is this a bug? Any workarounds?
index.html
<html>
<head>
<script type="text/javascript" src="js/entity.js"></script>
<script type="text/javascript" src="js/player.js"></script>
<link href="css/style.css" rel="stylesheet" type="text/css">
<title>Test</title>
</head>
<body>
<canvas id="screen" width=500 height=500></canvas>
<script type="text/javascript">initialize();</script>
</body>
</html>
entity.js
"use strict";
class Entity {
constructor() {
console.log("Entity");
}
}
player.js
"use strict";
class Player extends Entity {
constructor() {
console.log("Created"); // <- error here
}
}
推荐答案
这是新类语法的事实.您的子类需要调用 super() 才能正确初始化该类，例如
This is a fact of the new class syntax. Your subclass needs to call super() in order for the class to be properly initialized, e.g.
super(arg1, arg2, argN);
使用父构造函数需要的任何参数.
with whatever arguments the parent constructor needs.
要求，如果执行到constructor函数的末尾，this的值需要被初始化为某个东西.你要么需要在一个基类中(其中 this 是自动初始化的)，已经调用了 super() 所以 this 被初始化，或者返回一个替代对象.
It is required that, if execution reaches the end of a constructor function, the value of this needs to have been initialized to something. You either need to be in a base class (where this is auto-initialized), have called super() so this is initialized, or returned an alternative object.
class Player extends Entity {
constructor() {
super();
console.log("Created"); ;// error here
}
}
你可以把它想象成 constructor 函数在它们的末尾有一个自动 return this.
You can think of it like constructor functions kind of have an automatic return this at the end of them.
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