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

訊息

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)

錯誤型別

哪裡出錯了？

對於派生類建構函式的每個 new 呼叫，super() 最多隻能被呼叫一次。通常，你需要精確地呼叫它一次，因為如果你不呼叫它，父建構函式無法初始化 this 的值，因此你無法在派生建構函式中訪問 this，並且 this 不被視為有效的構造物件（如果派生建構函式在此狀態下完成，則會丟擲錯誤）。解決辦法是從派生類建構函式返回一個物件，在這種情況下，返回的物件將用作構造物件而不是 this，從而允許你不呼叫 super()。不過，這種情況很少見。

示例

無效案例

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
  }
}

有效情況

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

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

另見

• 類
• super