Question

I have been using the following template to create objects in javascript.

function FileSpec(directory, filename){
  var object = {};
  object.full_path = function(){
    return directory + '/' + filename;
  }
  return object;
}

var filespec = FileSpec('tmp', 'index.html');

Are there any particular disadvantages in using the above implementation versus using prototype and new?

function FileSpec(directory, filename){
  this.directory = directory;
  this.filename = filename;
}

FileSpec.prototype.full_path = function(){
  return this.directory + '/' + this.filename
}

var filespec = new FileSpec('tmp', 'index.html');

Answer 1

1. You can’t use the instanceof operator, or the Object.isPrototypeOf() method effectively with 1, but you can with #2.

   filespace instanceof FileSpec; // false with 1, true with 2
   Filespec.isPrototypeOf(filespace); //false with 1, true with 2
   

2. When using prototype inheritance, prototype members (methods etc) are defined only once on the prototype. In #1, you’re defining a new group of members on each instance, which is more memory intensive.

   var ar = [];
   
   for (var i=0;i<10000000;i++) {
       ar.push(FileSpec('tmp', 'index.html'));
   }
   
   // check memory usage, cry.
   

   Compared to:

   var ar = [];
   
   for (var i=0;i<10000000;i++) {
       ar.push(new FileSpec('tmp', 'index.html'));
   }
   
   // check memory usage.