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Singleton

Singleton in TypeScript

Singleton is a creational design pattern, which ensures that only one object of its kind exists and provides a single point of access to it for any other code.

Singleton has almost the same pros and cons as global variables. Although they’re super-handy, they break the modularity of your code.

You can’t just use a class that depends on a Singleton in some other context, without carrying over the Singleton to the other context. Most of the time, this limitation comes up during the creation of unit tests.

Complexity:

Popularity:

Usage examples: A lot of developers consider the Singleton pattern an antipattern. That’s why its usage is on the decline in TypeScript code.

Identification: Singleton can be recognized by a static creation method, which returns the same cached object.

Conceptual Example

This example illustrates the structure of the Singleton design pattern and focuses on the following questions:

  • What classes does it consist of?
  • What roles do these classes play?
  • In what way the elements of the pattern are related?

index.ts: Conceptual example

/**
 * The Singleton class defines the `getInstance` method that lets clients access
 * the unique singleton instance.
 */
class Singleton {
    private static instance: Singleton;

    /**
     * The Singleton's constructor should always be private to prevent direct
     * construction calls with the `new` operator.
     */
    private constructor() { }

    /**
     * The static method that controls the access to the singleton instance.
     *
     * This implementation let you subclass the Singleton class while keeping
     * just one instance of each subclass around.
     */
    public static getInstance(): Singleton {
        if (!Singleton.instance) {
            Singleton.instance = new Singleton();
        }

        return Singleton.instance;
    }

    /**
     * Finally, any singleton should define some business logic, which can be
     * executed on its instance.
     */
    public someBusinessLogic() {
        // ...
    }
}

/**
 * The client code.
 */
function clientCode() {
    const s1 = Singleton.getInstance();
    const s2 = Singleton.getInstance();

    if (s1 === s2) {
        console.log('Singleton works, both variables contain the same instance.');
    } else {
        console.log('Singleton failed, variables contain different instances.');
    }
}

clientCode();

Output.txt: Execution result

Singleton works, both variables contain the same instance.

Singleton in Other Languages

Singleton in C# Singleton in C++ Singleton in Go Singleton in Java Singleton in PHP Singleton in Python Singleton in Ruby Singleton in Rust Singleton in Swift