構築されたオブジェクト内のさまざまなものの動作を例で説明します。
// Defined as a variable from an anonymous function
// so that there is scope closure over variables
// shared across all instances and the prototype.
// If this isn't important, you don't need to close
// scope around it, so define directly
var ConstructedObject = (function constructorCreator () {
// Define any variables/methods to be shared across
// all instances but not polluting the namespace
var sharedVariable = 'foo';
// Next the actual constructor
function ConstructedObject () {
// Variables here are normally used to help
// each instance and will be kept in memory as
// long as the instance exists
var instanceVariable = 'bar';
// instance-specific properties get defined
// using the "this" keyword, these are the
// properties expected to be changed across
// each different instance
this.instanceProperty = true;
this.instanceMethod = function () { return instanceVariable; };
this.changeInstanceVar = function () { instanceVariable = 'foo'; };
// you do have access to the shared
// variables here if you need them.
}
// After the constructor, you set up the
// prototype, if any. This is an object of shared
// properties and methods to be inherited by every
// instance made by the constructor, and it also
// inherits the prototype's prototype, too.
// Lets use a literal object for simplicity.
ConstructedObject.prototype = {
// Accessing the instance to which a method
// applies is done using the "this" keyword,
// similar to in the constructor
sharedMethod : function () { return [sharedVariable, this.instanceMethod(),this.instanceProperty]; },
changeSharedVar : function () { sharedVariable = 'bar'; }
// properties may also be defined
};
// Finally, the constructor is returned so it
// can be kept alive outside of the anonymous
// function used to create it
return ConstructedObject;
// and the anonymous function is called to execute
// what we've done so far
})();
上記のコードを実行すると、インスタンス固有の変数と共有変数の両方を使用してオブジェクトを作成するコンストラクターができます。次に、2つのインスタンスを作成し、いくつかの変更の前後でそれらを比較することによって、それらがどのように動作するかを見てみましょう。
// First create the two instances
var myObjA = new ConstructedObject(),
myObjB = new ConstructedObject();
// Now compare them, the sharedMethod method we
// used in the prototype offers an easy way to
// do this
console.log( myObjA.sharedMethod(), myObjB.sharedMethod() );
// ["foo", "bar", true] ["foo", "bar", true]
// Next lets change the different variables in
// myObjB so we can see what happens, again the
// change* methods defined before let us do this
// easily
myObjB.changeInstanceVar();
myObjB.changeSharedVar();
// For completeness, lets also change the property
// on myObjB.
myObjB.instanceProperty = false;
// Now when we compare them again, we see that our
// changes to the myObjB instance have only changed
// the shared variables of myObjA
console.log( myObjA.sharedMethod(), myObjB.sharedMethod() );
// ["bar", "bar", true] ["bar", "foo", false]
見やすくするために、ログに記録された2つのステートメントをまとめます。
// myObjA myObjB
["foo", "bar", true] ["foo", "bar", true]
["bar", "bar", true] ["bar", "foo", false]