CustomPromise.js

export function CustomPromise(executorFn) {
  const PossibleStates = {
    PENDING: 'PENDING',
    RESOLVED: 'RESOLVED',
    REJECTED: 'REJECTED',
  };
  let currentSuccessResult;
  let currentFailureResult;
  const listOfSuccessCallbacks = [];
  const listOfFailureCallbacks = [];

  let currentState = PossibleStates.PENDING;

  this.then = (callbackFunction) => {
    listOfSuccessCallbacks.push(callbackFunction);
    return this;
  };
  this.catch = (callbackFunction) => {
    listOfFailureCallbacks.push(callbackFunction);
    return this;
  };

  const resolve = (resolvedValue) => {
    if (currentState !== PossibleStates.PENDING) return;

    currentSuccessResult = resolvedValue;

    listOfSuccessCallbacks.forEach((callbackFunction) => {
      currentSuccessResult = callbackFunction(currentSuccessResult);
    });

    currentState = PossibleStates.RESOLVED;
  };
  const reject = (rejectedValue) => {
    if (currentState !== PossibleStates.PENDING) return;

    currentFailureResult = rejectedValue;

    listOfSuccessCallbacks.forEach((callbackFunction) => {
      currentFailureResult = callbackFunction(currentFailureResult);
    });

    currentState = PossibleStates.REJECTED;
  };

  executorFn(resolve, reject);
}