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Javascript deal with inheritance through prototypes; in alternative there are libraries like Klass and Selfish.js.

Basically you need to do two things:

• Hook up the prototype of the parent into the prototype of the child. This is done by calling Object.create(parentObj);
• Call the parent constructor. This is done with parentObj.call(params);

Here's an example:

	function Parent(age){
		this.age = age; // Ignored because overridden
		
		this.showAge = function(){ // Ignored because overridden
			document.write("Showing something");
		}
	}

	function Son(age){
		// Call the superclass constructor
		Parent.call(this, age);

		// Method created every time the constructor is called
		this.showAge = function(){
			document.write("The age is " + this.age + "<br/>");
		}
	}

	Son.prototype = Object.create(Parent.prototype);

	Son.prototype.name = "Bob";

	// Method created only once, but it can't see "age"
	Son.prototype.showSomething = function() {
		document.write("Name : " + this.name + "<br/>");
	}

	function Grandson(age){	
		// Call the superclass constructor
		Son.call(this, age); 
		
		this.age = age + 1;
	}

	// Inherit Son (makes showSomething() visible)
	Grandson.prototype = Object.create(Son.prototype);

	// Example of a Grandson instance
	var gs = new Grandson(14);
	gs.showAge();
	gs.showSomething();

You should also avoid bad practices like:

1) Son.prototype = Parent.prototype.
This would make every change in the child also visible into the parent and that's not what you want, isn't it?

2) Son.prototype = new Parent();
The main problem here is that you can't pass parameters to the constructor, because we are using Parent.call(params).

The correct solution is:
childObj.prototype = Object.create(parentObj);

In addition, you can also set the constructor:
Grandson.prototype.constructor = Grandson;

But this operation in itself doesn't do anything, because this property is not used internally. This might be useful if you need to create a library or you need for some reasons to read this property.