Usage of prototype in Javascript

Question

I'm confused where should I use prototype to declare a method ? What I read is, if I create a method that is declared by prototype, all instances are using same reference so is it static or something different ? Because I can reach instance properties in a prototype method ? But in c#, you cannot reach class variables(not static) in static methods?

An Example:

function Calculator()
{
     if(this === window){
          return new Calculator();
     }

     this.Bar = "Instance Variable";
}

Calculator.prototype.SaySomething = function(thing){
     return thing + " " + Bar;
}


Calculator().SaySomething("Test"); // Test Instance Variable

JavaScript has an object system very different from that of other object oriented languages. For a introduction to JS and inheritance, read: phrogz.net/JS/classes/OOPinJS2.html Edit: Also read: developer.mozilla.org/en/JavaScript/Guide/… developer.mozilla.org/en-US/docs/JavaScript/Guide/… developer.mozilla.org/en-US/docs/JavaScript/Guide/… — rx80
– rx80, Commented Aug 20, 2012 at 0:18
in your prototype function u have a slight mistake (this.Bar) instead of Bar — unloco
– unloco, Commented Aug 20, 2012 at 0:21

dana · Accepted Answer · 2012-08-20 00:21:31Z

2

prototype's work in conjunction with the new keyword. Take the following example:

function Calculator(bar) {
     this.Bar = bar;
}

Calculator.prototype.SaySomething = function(thing){
     return thing + " " + this.Bar;
}

var calInstance = new Calendar("Instance Variable");
calInstance.SaySomething("Test");

answered Aug 20, 2012 at 0:21

dana

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Eudis Duran · Accepted Answer · 2012-08-20 00:22:30Z

1

I would suggest you read JS The Good Parts by Douglas Crockford. It will give you a better understanding of JS's prototypal object model.

answered Aug 20, 2012 at 0:22

Eudis Duran

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Comments

Polyov · Accepted Answer · 2012-08-20 00:22:29Z

1

You are correctly declaring the prototyped method, but incorrectly calling it. Calculator is not a static object, just a class, and so you can only call it's methods when you've created an instance of the object.

var calc = new Calculator();
calc.SaySomething('thing');
//this would return "thing Instance Variable"

In short, Javascript does not use class and instance methods, just instance methods.

answered Aug 20, 2012 at 0:22

Polyov

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