How to define an instance in javascript

Question

Hi I see this line of code

CtvDfpAd.GetInstance = function () {
    if (!CtvDfpAd.__instance) {
        CtvDfpAd.__instance = new CtvDfpAd();
    }

    return CtvDfpAd.__instance;
};

is "instance" a keyword in javascript? i searched online, i don't believe it is, can someone explain?

Thanks

Jesus! I think I've found a new keyword! Lol. +1 for making me laugh ;) — user1636522
– user1636522, Commented Feb 27, 2014 at 19:02

Tmdean · Accepted Answer · 2014-02-27 18:36:35Z

4

No, __instance is not a keyword. The double-underscore prefix appears to be a convention to treat the member variable as private, since JavaScript has no concept of private variables.

This code is an implementation of the Singleton pattern in JavaScript. A singleton class means that it is designed to have only one instance throughout the lifetime of the program. This function checks if the instance exists - if it does, it returns it. Otherwise it creates one and returns the new instance.

answered Feb 27, 2014 at 18:36

Tmdean

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ktm5124 · Accepted Answer · 2014-02-27 18:35:37Z

1

instanceof is a keyword in JavaScript. Apart from that, it's a technical term in object-oriented programming. The code you posted uses a common pattern for instantiating objects

answered Feb 27, 2014 at 18:35

ktm5124

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Dan Prince · Accepted Answer · 2014-02-27 18:36:22Z

1

Looks like instance is just a protected property of that CtvDfpAd object. Looks like it might be a singleton based on the pattern exhibited here:

http://addyosmani.com/resources/essentialjsdesignpatterns/book/#singletonpatternjavascript

You'll almost certainly get a better idea of what that code is doing by reading that article, but in short, what this code is doing:

If there is already an instance of our singleton, return it.
Otherwise create a new instance of this object and return that.

answered Feb 27, 2014 at 18:36

Dan Prince

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