Constructor code not reachable

问题

I am passing a class method as a parameter to a new class instantiation like this:

class Abc {
    constructor() {
        this.a = () => { };
    }
    b = new Def(this.a);
}

I get 'cannot read property a of undefined' in browser console. Why is a undefined inside b = new Def(this.a)? On debugging, I found that browser throws the error and the constructor code is never reached. Why is this happening?

Note: I am using babel, so I can use class fields and hence b = new Def() is a valid syntax here.

回答1:

That's how class fields work, they are evaluated before constructor body (but after super()). Line 1 is evaluated before line 2, and the order in which constructor and b field are ordered doesn't matter:

constructor() {
    this.a = () => { }; // 2
}
b = new Def(this.a); // 1

Since class fields are already in use, in order to maintain proper execution order it should be:

a = () => { }; // 1
b = new Def(this.a); // 2

constructor() {}

来源：https://stackoverflow.com/questions/49599528/constructor-code-not-reachable