Javascript。 挂钩 new MyClass()

Question

I would like to do some stuff after before or after new has run.

function F() {

    this.init = function  () { alert(0) }

}

F.prototype.init = function () { alert(1) }

new F().init(); // Will not run mine, because on new F(), init is reassigned within the class.

I understand I can create my own method function Create() { new F().init() }

But I was wondering if there is a way to hook into the new Function call ?

I hope you understand what I mean.

Answer 1

new is not a function call, F() is. You could do something like this, replacing F with your own delegate.

function F() {
    this.init = function  () { alert(0) }
}

var oldF = F;
F = function() {
    oldF.apply(this, arguments);
    this.init = function() { alert(1); };
};

new F().init();

If you want a utility function to do this kind of thing:

function wrap(constructor, config) {
    return function() {
        constructor.apply(this, arguments);
        for (var key in config) {
            this[key] = config[key];
        }
    }
}

F = wrap(F, {init: function() { alert(1); }});

or use one of many frameworks/libraries (ExtJS, jQuery, Prototype) that provides this stuff.

Following discussion

This can start you off on what you're trying to do, but I don't guarantee it works in all situations or implementations (only tested on V8). You could pass the context in which F exists as an additional parameter, or make sure you apply/bind/call extend with it.

function extend(constructor, config) {
    this[constructor.name] = function() {
        constructor.apply(this, arguments);
        for (var key in config) {
            this[key] = config[key];
        }
    }
}

extend(F, {init: function() { alert(1); }});

Answer 2

You can use handler.construct() of Proxy to hook the new Operator. Here I alter the arguments passed to the new WebSocket() call:

let WebSocket = require('ws');
// let's hook the new WebSocket(...)
WebSocket = new Proxy(WebSocket, {
  construct: function(target, args, newTarget) {
    // https://github.com/websockets/ws/blob/8.4.0/lib/websocket.js#L51
    let [address, options] = args;  // the original constroctor actually has tree arguments, here we only use 2, to keep it simple

    const extraOptions = createWsOptions(address);

    options = {
      ...options,
      ...extraOptions
    };
    console.log(`options of new Websocket(): ${JSON.stringify(options, null, 2)}`);

    return new target(...[address, options]);
  }
});
ws = new WebSocket(aServer, {
  protocol: 'binary'
});

Answer 3

Constructors in javascript are just like any other methods, what you're really looking for is AOP in javascript.

See this SO question for some good AOP libraries for javascript.