◆ JavaScript Patterns - Chapter 7. Design Patterns, Singleton

JavaScript Patterns

[Singleton]
JavaScript doesn't have classes, so the verbatim definition for singleton doesn't technically make sense.
// what we want is
var uni = new Universe();
var uni2 = new Universe();
uni === uni2; // true
Instance in a Static Property
function Universe() {
// do we have an existing instance?
if (typeof Universe.instance === "object") {
return Universe.instance;
}

// proceed as normal
this.start_time = 0;
this.bang = "Big";

// cache
Universe.instance = this;

// implicit return:
// return this;
}
The drawback of above code is that instance is public. Other code may change it by mistake.

Instance in a Closure
function Unvierse() {
// the cached instance
var instance = this;

// proceed as normal
this.start_time = 0;
this.bang = "Big";

// rewrite the constructor
Universe = function () {
return instance;
};
}

The drawback is property added to Universe between initial definition and redefinition will lose. Also the constructor of created instance is not the same as Universe.

Alternative implementation
function Universe() {
// the cached instance
var instance;

// rewrite the constructor
Universe = function Universe() {
return instance;
};

// carry over the prototype properties
Universe.prototype = this;

// the instance
instance = new Universe();

// reset the constructor pointer
instance.constructor = Universe;

// all the functionality
instance.start_time = 0;
instance.bang = "Big";

return instance;
}