// Conceptual-article //

Singleton Design Pattern in JavaScript

Published on September 21, 2020
Default avatar
By Devan Patel
Developer and author at DigitalOcean.
Singleton Design Pattern in JavaScript

While we believe that this content benefits our community, we have not yet thoroughly reviewed it. If you have any suggestions for improvements, please let us know by clicking the “report an issue“ button at the bottom of the tutorial.

A Singleton only allows for a single instantiation, but many instances of the same object. The Singleton restricts clients from creating multiple objects, after the first object created, it will return instances of itself.

Finding use cases for Singletons is difficult for most who have not yet used it prior. One example is using an office printer. If there are ten people in an office, and they all use one printer, ten computers share one printer (instance). By sharing one printer, they share the same resources.

var printer = (function () {

    var printerInstance;

    function create () {

    function print() {
        // underlying printer mechanics
    }

    function turnOn() {
        // warm up
        // check for paper
    }

    return {
        // public + private states and behaviors
        print: print,
        turnOn: turnOn
    };
    }

    return {
    getInstance: function() {
        if(!printerInstance) {
        printerInstance = create();
        }
        return printerInstance;
    }
    };

    function Singleton () {
    if(!printerInstance) {
        printerInstance = intialize();
    }
    };

})();

The create method is private because we do not want the client to access this, however, notice that the getInstance method is public. Each officer worker can generate a printer instance by interacting with the getInstance method, like so:

var officePrinter = printer.getInstance();

In AngularJS, Singletons are prevalent, the most notable being services, factories, and providers. Since they maintain state and provides resource accessing, creating two instances defeats the point of a shared service/factory/provider.

Race conditions occur in multi-threaded applications when more than one thread tries to access the same resource. Singletons are susceptible to race conditions, such that if no instance were initialized first, two threads could then create two objects instead of returning and instance. This defeats the purpose of a singleton. Therefore, developers must be privy to synchronization when implementing singletons in multithreaded applications.

If you’ve enjoyed this tutorial and our broader community, consider checking out our DigitalOcean products which can also help you achieve your development goals.

Learn more here


Tutorial Series: JavaScript Design Patterns

Every developer strives to write maintainable, readable, and reusable code. Code structuring becomes more important as applications become larger. Design patterns prove crucial to solving this challenge - providing an organization structure for common issues in a particular circumstance.

The design pattern below is only one of many useful patterns that can help you level up as a JavaScript developer. For the full set, see JavaScript Design Patterns.

About the authors
Default avatar
Developer and author at DigitalOcean.

Still looking for an answer?

Was this helpful?
Leave a comment

This textbox defaults to using Markdown to format your answer.

You can type !ref in this text area to quickly search our full set of tutorials, documentation & marketplace offerings and insert the link!