Example of how to implement a Singleton pattern in JavaScript.

javascript-singleton.js

/*
 * The instance symbol should be defined in the scope 
 * in which you want the singleton to exist. This does 
 * not necessarily mean globally. Just within your 
 * execution context.
 */
let instance

const Singleton = (() => {
    const createInstance = () => {
        console.log('Calling createInstance')
        return true
    }

    return {
        getInstance: () => {
            console.log('Call getInstance');
            if (instance === undefined) {
                instance = createInstance()
            }
            return instance
        },
    }
})()

Singleton.getInstance()
Singleton.getInstance()
Singleton.getInstance()
Singleton.getInstance()

// Output
// Calling createInstance
// Call getInstance
// Call getInstance
// Call getInstance
// Call getInstance