Lazy Promises

LazyPromiseComposition.js

//we don't need to inherit from Promise, await works fine with just a "thenable"
//notice in the second part of the code that Promise.resolve of a thenable returns a new Promise rather than the thenable
class LazyPromise {

    constructor(creationFn){
        console.log("creating LazyPromise");
        this.initialized = false;
        this.creationFn = creationFn;
    }

    then(resFn, rejFn){
        if (!this.initialized){
            console.log("creating Real Promise");
            this.initialized = true;
            this.internalPromise = new Promise(this.creationFn);
        }

        return this.internalPromise.then(resFn, rejFn);

    }

}

function sleep(ms){
    let resolveFn;
    let pr = new Promise(res => resolveFn = res);
    setTimeout(() => resolveFn(), ms);
    return pr;
}



(async () => {
    let pr1 = new LazyPromise(res => {
        console.log("starting query");
        setTimeout(() => {
            console.log("finishing query");
            res("hi");
        }, 2000);
    });
    
    console.log("before sleep");
    await sleep(5000);
    console.log("after sleep");
    let result = await pr1;
    console.log("promise returns: " + result);

    
    
    let prA = new Promise(() => {});
    let prB = Promise.resolve(prA);
    console.log(prA === prB); //true

    //LazyPromise is a thenable
    prA = new LazyPromise(() => {});
    prB = Promise.resolve(prA);
    console.log(prA === prB); //false

    let prC = new LazyPromise(res => {
        console.log("starting query");
        setTimeout(() => {
            console.log("finishing query");
            res("Bye");
        }, 2000);
    });

    let prD = Promise.resolve(prC);
    result = await prD;
    console.log(result); //Bye
    //as explained in MDN, it wraps the thenable in a new Promise, but this new promise is not resolved until the thenable is resolved
    //the returned promise will "follow" that thenable, adopting its eventual state;
    
    
})();

LazyPromiseInheritance.js

class LazyPromise extends Promise{

    constructor(creationFn){
        //compiler forces me to do a super call
        super(() => {});
        console.log("creating LazyPromise");
        this.initialized = false;
        this.creationFn = creationFn;
    }

    then(resFn, rejFn){
        if (!this.initialized){
            console.log("creating Real Promise");
            this.initialized = true;
            this.internalPromise = new Promise(this.creationFn);
        }

        return this.internalPromise.then(resFn, rejFn);

    }

}

function sleep(ms){
    let resolveFn;
    let pr = new Promise(res => resolveFn = res);
    setTimeout(() => resolveFn(), ms);
    return pr;
}



(async () => {
    let pr1 = new LazyPromise(res => {
        console.log("starting query");
        setTimeout(() => {
            console.log("finishing query");
            res("hi");
        }, 2000);
    });
    
    console.log("before sleep");
    await sleep(5000);
    console.log("after sleep");
    let result = await pr1;
    console.log("promise returns: " + result);
    
})();