Question

143

Asynchronous constructor

rated 0 times [ 145] [ 2] / answers: 1 / hits: 23677 / 12 Years ago, wed, august 8, 2012, 12:00:00

How can I best handle a situation like the following?

I have a constructor that takes a while to complete.

var Element = function Element(name){

   this.name = name;

   this.nucleus = {};



   this.load_nucleus(name); // This might take a second.

}



var oxygen = new Element('oxygen');

console.log(oxygen.nucleus); // Returns {}, because load_nucleus hasn't finished.

I see three options, each of which seem out of the ordinary.

One, add a callback to the constructor.

var Element = function Element(name, fn){

   this.name = name;

   this.nucleus = {};



   this.load_nucleus(name, function(){

      fn(); // Now continue.

   });

}



Element.prototype.load_nucleus(name, fn){

   fs.readFile(name+'.json', function(err, data) {

      this.nucleus = JSON.parse(data); 

      fn();

   });

}



var oxygen = new Element('oxygen', function(){  

   console.log(oxygen.nucleus);

});

Two, use EventEmitter to emit a 'loaded' event.

var Element = function Element(name){

   this.name = name;

   this.nucleus = {};



   this.load_nucleus(name); // This might take a second.

}



Element.prototype.load_nucleus(name){

   var self = this;

   fs.readFile(name+'.json', function(err, data) {

      self.nucleus = JSON.parse(data); 

      self.emit('loaded');

   });

}



util.inherits(Element, events.EventEmitter);



var oxygen = new Element('oxygen');

oxygen.once('loaded', function(){

   console.log(this.nucleus);

});

Or three, block the constructor.

var Element = function Element(name){

   this.name = name;

   this.nucleus = {};



   this.load_nucleus(name); // This might take a second.

}



Element.prototype.load_nucleus(name, fn){

   this.nucleus = JSON.parse(fs.readFileSync(name+'.json'));

}



var oxygen = new Element('oxygen');

console.log(oxygen.nucleus)

But I haven't seen any of this done before.

What other options do I have?

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kiley

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3 Years ago

answered 12 Years ago dominics · Accepted Answer

Given the necessity to avoid blocking in Node, the use of events or callbacks isn't so strange⁽¹⁾.

With a slight edit of Two, you could merge it with One:

var Element = function Element(name, fn){

    this.name = name;

    this.nucleus = {};



    if (fn) this.on('loaded', fn);



    this.load_nucleus(name); // This might take a second.

}



...

Though, like the fs.readFile in your example, the core Node APIs (at least) often follow the pattern of static functions that expose the instance when the data is ready:

var Element = function Element(name, nucleus) {

    this.name = name;

    this.nucleus = nucleus;

};



Element.create = function (name, fn) {

    fs.readFile(name+'.json', function(err, data) {

        var nucleus = err ? null : JSON.parse(data);

        fn(err, new Element(name, nucleus));

    });

};



Element.create('oxygen', function (err, elem) {

    if (!err) {

        console.log(elem.name, elem.nucleus);

    }

});

(1) It shouldn't take very long to read a JSON file. If it is, perhaps a change in storage system is in order for the data.