ReferenceError: must call super constructor before using 'this' in derived class constructor

消息

ReferenceError: Must call super constructor in derived class before accessing 'this' or returning from derived constructor (V8-based)
ReferenceError: must call super constructor before using 'this' in derived class constructor (Firefox)
ReferenceError: 'super()' must be called in derived constructor before accessing |this| or returning non-object. (Safari)

错误类型

哪里出错了？

对于派生类构造函数的每个 new 调用，super() 最多只能被调用一次。通常，你需要精确地调用它一次，因为如果你不调用它，父构造函数无法初始化 this 的值，因此你无法在派生构造函数中访问 this，并且 this 不被视为有效的构造对象（如果派生构造函数在此状态下完成，则会抛出错误）。解决办法是从派生类构造函数返回一个对象，在这种情况下，返回的对象将用作构造对象而不是 this，从而允许你不调用 super()。不过，这种情况很少见。

示例

无效案例

class Base {
  constructor() {
    this.x = 1;
  }
}

class Derived extends Base {
  constructor() {
    console.log(this.x);
    // The Base constructor is not called yet, so this.x is undefined
    // ReferenceError: must call super constructor before using 'this' in derived class constructor
  }
}

有效情况

class Base {
  constructor() {
    this.x = 1;
  }
}

class Derived extends Base {
  constructor() {
    super();
    console.log(this.x); // 1
  }
}

另见

• 类
• super