Javascript : async constructor pattern

Question

I'm defining a class which instantiates several modules which depend on previous ones. The modules themselves may require an async operation before they are ready (i.e. establishing a mysql connection) so I've provided each constructor with a callback to be called once the module is ready. However I've run into a problem when instantiating classes which are ready immediately:

var async = require('async');

var child = function(parent, cb) {
    var self = this;
    this.ready = false;

    this.isReady = function() {
        return self.ready;
    }

    /* This does not work, throws error below stating c1.isReady is undefined*/
    cb(null, true);

    /* This works */
    setTimeout(function() {      
        self.ready = true;
        cb(null, true);
    }, 0);
}


var Parent = function(cb) {
    var self = this;
    async.series([
        function(callback){
            self.c1 = new child(self, callback);                       
        },
        function(callback){
            self.c2 = new child(self, callback);   
        }
    ],
    function(err, results){
        console.log(self.c1.isReady(), self.c2.isReady);
        console.log(err, results);
    });
}

var P = new Parent();

I'm guessing the issue is calling cb within the constructor means async proceeds to the next function before the constructor finishes. Is there a better approach to this? I considered using promises, but I find this approach easier to understand/follow.

related: [Is it bad practice to have a constructor function return a Promise?](http://stackoverflow.com/q/24398699/1048572) — Bergi, Sep 26 '15 at 14:51
Possible duplicate of [Asynchronous constructor](http://stackoverflow.com/questions/11856778/asynchronous-constructor) — Evan Carroll, May 31 '16 at 17:20

jfriend00 · Answer 1 · 2015-01-30T05:43:21.233

You will have to delay the call to the callback when everything is synchronous because any code in the callback won't be able to reference the object yet (as you've seen). Because the constructor hasn't finished executing, the assignment of it's return value also hasn't finished yet.

You could pass the object to the callback and force the callback to use that reference (which will exist and be fully formed), but you're requiring people to know that they can't use a normally accepted practice so it's much better to make sure the callback is only called asynchronously and people can then write their code in the normal ways they would expect to.

In node.js, it will be more efficient to use process.nextTick() instead of setTimeout() to accomplish your goal. See this article for more details.

var child = function(parent, cb) {
    var self = this;
    this.ready = false;

    this.isReady = function() {
        return self.ready;
    }

    /* This works */
    process.nextTick(function() {      
        self.ready = true;
        cb(null, true);
    }, 0);
}

Here's a general observation. Many people (myself included) think that it overcomplicates things to put any async operation in a constructor for reasons related to this. Instead, most objects that need to do async operations in order to get themselves set up will offer a .init() or .connect() method or something like that. Then, you construct the object like you normally would in a synchronous fashion and then separately initiate the async part of the initialization and pass it the callback. That gets you away from this issue entirely.

If/when you want to use promises for tracking your async operation (which is great feature direction to go), it's way, way easier to let the constructor return the object and the .init() operation to return a promise. It gets messy to try to return both the object and a promise from the constructor and even messier to code with that.

Then, using promises you can do this:

var o = new child(p);
o.init().then(function() {
    // object o is fully initialized now
    // put code in here to use the object
}, function(err) {
    // error initializing object o
});

score 1 · Answer 2 · answered Aug 10 '17 at 11:27

If you can't put the object into a Promise, put a promise into the object.

Give it a Ready property that is a promise.

If you have a class hierarchy (I'm talking about Typescript now) of (say) widgets, the base class can define the Ready property and assign it an already resolved Promise object. Doing this is a base class means it's always safe to write code like this

var foo = Foo();
foo.Ready.then(() => {
  // do stuff that needs foo to be ready
});

Derived classes can take control of promise resolution by replacing the value of Ready with a new promise object, and resolving it when the async code completes.

Here's a comprehensive workup of Asynchronous Constructor design pattern

Javascript : async constructor pattern

2 Answers2

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