在构造函数中的任何内容之前运行链接方法？

Question

I have this simple class:

 class Foo { constructor() { this.init(); return this; } init() { this.onInit(); } onInit(callback) { this.onInit = () => callback(); return this; } } new Foo().onInit(() => console.log('baz')); 

It's obviously flawed, because it will call init before the onInit method is able to define the onInit property/callback.

How can I make this work without change the interface?

Answer 1

How can I make this work without change the interface?

You can't, the interface is inherently flawed. That's really the answer to your question.

---

Continuing, though, with "what can I do instead":

If you need to have a callback called during initialization, you need to pass it to the constructor, not separately to the onInit method.

class Foo {
  constructor(callback) {
    this.onInit = () => {
        callback();  // Call the callback
        return this; // Chaining seemed important in your code, so...
    };
    // Note: Constructors don't return anything
  }
}

new Foo(() => console.log('baz'));

---

In a comment you've said:

I see your point, the fact is that my library is new Something().onCreate().onUpdate()

It sounds like you might want to adopt the builder pattern instead:

class Foo {
    constructor(callbacks) {
        // ...use the callbacks here...
    }
    // ...
}
Foo.Builder = class {
    constructor() {
        this.callbacks = {};
    }
    onCreate(callback) {
        this.callbacks.onCreate = callback;
    }
    onUpdate(callback) {
        this.callbacks.onUpdate = callback;
    }
    // ...
    build() {
        // Validity checks here, do we have all necessary callbacks?
        // Then:
        return new Foo(this.callbacks);
    }
};

let f = new Foo.Builder().onCreate(() => { /*...*/}).onUpdate(() => { /*... */}).build();

...although to be fair, a lot of the advantages (though not all) of the builder pattern can be realized in JavaScript by just passing an object into constructor directly and doing your validation there, eg:

let f = new Foo({
   onCreate: () => { /*...*/},
   onUpdate: () => { /*...*/}
});

Answer 2

Assuming that onInit is supposed to be some sort of hook to be called synchronously whenever an object is instantiated, you can't solve this on the instance level.

You can make onInit a static function, like so:

 class Foo { constructor() { // whatever Foo.onInit(); } static onInit() {} // empty default } Foo.onInit = () => console.log('baz'); // Override default with your own function const f = new Foo(); const f2 = new Foo();