为什么在super()之前不允许这样做

构造函数默认返回'this'.根据oops概念,子类总是通过super()调用从父类继承'this'对象.因此,如果我们尝试在没有超级调用的子类中使用它,它将引发错误.如果我们从子类返回除"this"之外的任何内容,则不需要执行super()调用.我通过一些简单的例子解释过.

例1

class A {
  constructor() {
    this.a = 0;
  }
}

class B extends A {
    constructor() {
        console.log(this);
    }
}
const instanceA = new A();
console.log(instanceA) // A {a: 0}
const instanceB = new B();
console.log(instanceB) // Error: Must call super constructor in derived class before 
accessing 'this' or returning from derived constructor 

例2

class A {
  constructor() {
    this.a = 0;
  }
}

class B extends A {
    constructor() {
      return {b: 3}
    }
}
const instanceA = new A();
console.log(instanceA) // A {a: 0}
const instanceB = new B();
console.log(instanceB) // Object {b: 3}

例3

class A {
  constructor() {
    this.a = 0;
  }
}

class B extends A {
    constructor() {
      super()
    }
}

const instanceB = new B();
console.log(instanceB) // B {a: 0}