Define Properties in javaScript

Javascript is a powerful interpreted language that gives the developers a lot of flexibility. As the language has matured, it is fast enough that it can also be a compilation target. The tradeoff of speed is functionality - and object properties are a prime example.

The following code shows the fast way of making an object with a property, and the robust way of making an object with properties.

var fastObject = function () {
	var self = this;
	
	self.prop = 1;
}

var powerfulObject = function () {
	var self = this;
	
	Object.defineProperties(self, {
		"prop" : {
			"get": function() { 
				// _prop is not really private, but just pretend	
				return self._prop; 
			},
			"set": function(value) {
			   self._prop = value;			   
			}
		}
	});
};

// And if you really like closure notation
var powerfulClosureObject = (function () {

	function obj() {
		var self = this;
		
		Object.defineProperties(self, {
			"prop" : {
				"get": function() { 
					return self._prop; 
				},
				"set": function(value) {
				   self._prop = value;			   
				}
			}
		});
	}
	return obj;
}());

Now the following code can be written:

var q = new fastObject();
q.prop = 2;
var p = new powerfulObject();
p.prop = 2;

So far, it just seems like the second object is just extra work. However, it's pretty useful if you want to apply constraints on properties.

var powerfulObject = function () {
	var self = this;	

	Object.defineProperties(self, {
		"prop" : {
			"get": function() { 
				if (typeof(self._prop) != "number") {
					throw "object not initialized";
				}
				return self._prop; 
			},
		    "set": function(value) {
				if (typeof(value) != "number") {
					throw "prop must be a number";
				}
				self._prop = value;			   
			}
		}
	});
}

Using this approach...

var p = new powerfulObject();
console.log(p.prop); // fail "object not initialized"
p.prop = "test"; // fail "prop must be a number"
p.prop = 2; // success
console.log(p.prop); // displays 2