JavaScript Object Creation: Patterns and Best Practices

In this article, I’m going to take you on a tour of the various styles of JavaScript object creation, and how each builds on the others in incremental steps.

JavaScript has a multitude of styles for creating objects, and newcomers and veterans alike can feel overwhelmed by the choices and unsure which they should use. But despite the variety and how different the syntax for each may look, they’re more similar than you probably realize.

Object Literals

The first stop on our tour is the absolute simplest method of JavaScript object creation — the object literal. JavaScript touts that objects can be created “ex nilo”, out of nothing — no class, no template, no prototype — just poof!, an object with methods and data:

var o = {
  x: 42,
  y: 3.14,
  f: function() {},
  g: function() {}
};

But there’s a drawback. If we need to create the same type of object in other places, then we’ll end up copy-pasting the object’s methods, data, and initialization. We need a way to create not just the one object, but a family of objects.

Factory Functions

The next stop on our JavaScript object creation tour is the factory function. This is the absolute simplest way to create a family of objects that share the same structure, interface, and implementation. Rather than creating an object literal directly, instead we return an object literal from a function. This way, if we need to create the same type of object multiple times or in multiple places, we only need to invoke a function:

function thing() {
  return {
    x: 42,
    y: 3.14,
    f: function() {},
    g: function() {}
  };
}

var o = thing();

But there’s a drawback. This approach of JavaScript object creation can cause memory bloat, because each object contains its own unique copy of each function. Ideally, we want every object to share just one copy of its functions.

Prototype Chains

JavaScript gives us a built-in mechanism to share data across objects, called the prototype chain. When we access a property on an object, it can fulfill that request by delegating to some other object. We can use that and change our factory function so that each object it creates contains only the data unique to that particular object, and delegate all other property requests to a single, shared object:

var thingPrototype = {
  f: function() {},
  g: function() {}
};

function thing() {
  var o = Object.create(thingPrototype);

  o.x = 42;
  o.y = 3.14;

  return o;
}

var o = thing();

In fact, this is such a common pattern that the language has built-in support for it. We don’t need to create our own shared object (the prototype object). Instead, a prototype object is created for us automatically alongside every function, and we can put our shared data there:

thing.prototype.f = function() {};
thing.prototype.g = function() {};

function thing() {
  var o = Object.create(thing.prototype);

  o.x = 42;
  o.y = 3.14;

  return o;
}

var o = thing();

But there’s a drawback. This is going to result in some repetition. The first and last lines of the thing function are going to be repeated almost verbatim in every such delegating-to-prototype factory function.

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by Jeff Mott via SitePoint