Singleton

About

Used when you want to maintain only one instance of something in the application. (Like DB instance, socket, etc)

It does this by making the constructor private, so that only ONE static method (like getInstance()) from the class can call it, ONLY ONCE to instantiate it.

This helps so that you do not need to pass these classes by parameters to dependency inject them.

It can become an anti-pattern in data-shared environments with multi thread, since it won't be clear who should be mutating the state.

Ex.: It can lead to race conditions.

To avoid multiple threads trying to create at the same time an instance, you must use a lock and recheck the instance === null inside the lock.

class MyClass {
    private someProp: any;
    static instance: MyClass;
    
    private constructor (){}
    
    doSomething() {
        return this.someProp;
    }
    
    static getIntance() {
        if (!MyClass.intance) MyClass.instance = new MyClass();
        return MyClass.instance;
    }
}

On the outside you just call the getInstance() before using the Class.