customPromise.sol.js

/* Polyfill of promise */

const STATE = {
    PENDING: 'pending',
    FULFILLED: 'fulfilled',
    REJECTED: 'rejected'
  };
  class MyPromise {
    #state = STATE.PENDING;
    #value = null;
    #fulfilledCallbacks = [];
    #rejectedCallbacks = [];
  
    constructor(executorFunc) {
      try {
        executorFunc(value => this.#resolve(value), value => this.#reject(value))
      }catch(error) {
        this.#reject(error)
      }
    }
  
    then(onFulfilled, onRejected) {
      return new MyPromise((resolve, reject) => {
        const fulfilledCallback = () => {
          if(!onFulfilled) {
            return resolve(this.#value);
          }
          queueMicrotask(() => {
            try {
              const value = onFulfilled(this.#value);
              resolve(value);           
            }catch(error) {
              reject(error)
            }
          })
        };
        const rejectCallback = () => {
                  if(!onRejected) {
            return reject(this.#value);
          }
          queueMicrotask(() => {
            try {
              const value = onRejected(this.#value);
              resolve(value);           
            }catch(error) {
              reject(error)
            }
          })
        }
              switch(this.#state) {
          case STATE.PENDING:
            this.#fulfilledCallbacks.push(fulfilledCallback);
            this.#rejectedCallbacks.push(rejectCallback);
            break;
          case STATE.FULFILLED:
                fulfilledCallback();
                break;
          case STATE.REJECTED:
                rejectCallback();
                break;
          default:
            throw new Error('Unexpected promise state')
        } 
      });   
    }
  
    catch(onRejected) {
      return this.then(null, onRejected)
    }
  
    get state() {
      return this.#state;
    }
  
    get value() {
      return this.#value;
    }
    #resolve(value) {
      this.#value = value;
      this.#state = STATE.FULFILLED;
      this.#fulfilledCallbacks.forEach(callback => callback());
    }
  
    #reject(value) {
      this.#value = value;
      this.#state = STATE.REJECTED;
      this.#rejectedCallbacks.forEach(callback => callback());
    }
  }
  
  new MyPromise((res,rej) => {
    res(10);
  }).then(value => {
    console.log(value * 2);
    return value * 2;
  }).then(value => {
    console.log(value * 3);
    return value * 3;
  })
  
  exports.MyPromise = MyPromise;