Difference between 'this' and 'var' when declaring variable, nodeJS

Question

I have the next piece of code:

function Server() {
    this.isStarted = false;
//  var isStarted = false;

    function status() {
        return isStarted;
    }

    console.log(status());
}

var a = new Server()

When I run it I get

ReferenceError: isStarted is not defined
    at status (/a/fr-05/vol/home/stud/yotamoo/workspace/ex4/text.js:7:10)
    at new Server (/a/fr-05/vol/home/stud/yotamoo/workspace/ex4/text.js:10:14)
    at Object.<anonymous> (/a/fr-05/vol/home/stud/yotamoo/workspace/ex4/text.js:

However if I change this.isStarted = false; to var isStarted = false; everything works fine.

Does anyone care to explain why?

Thanks

user1882585 · Accepted Answer · 2012-12-11 12:04:15Z

2

This referes to the owner of something. See this article about this. Where as var declares a local variable.

In your case, you want to refer to know if a server is started, so you need to add 'this' to your status function.

function status() {
    return this.isStarted;
}

answered Dec 11, 2012 at 12:04

user1882585

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Comments

Bogdan Kulynych · Accepted Answer · 2012-12-11 13:09:37Z

0

Long story short. Since isStarted, when defined as this.isStarted = true, is a property of the current object (JavaScript this keyword refers to an object where the function was called), in status function you will have to access it as this.isStarted.

Declaring it as variable (var) is different. Technically, isStatus will become a property of hidden lexical scope object. You have to access it as just isStatus in the whole Server function body and and all of the child functions.

edited Dec 11, 2012 at 13:09

answered Dec 11, 2012 at 12:03

Bogdan Kulynych

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