Promise Polyfill

Promise.js

/* 
Author: Pawan Kumar
Date : 04/28/2023
Location: Bangalore, India
*/

class MyPromise {
  constructor(executor) {
    let resolved = false;
    let rejected = false;
    let resolveValue;
    let rejectValue;
    let resolveCallbacks = [];
    let rejectCallbacks = [];

    function resolve(value) {
      if (resolved || rejected) return;
      resolved = true;
      resolveValue = value;
      resolveCallbacks.forEach((fn) => fn(value));
    }

    function reject(reason) {
      if (resolved || rejected) return;
      rejected = true;
      rejectValue = reason;
      rejectCallbacks.forEach((fn) => fn(reason));
    }

    this.then = function (onResolve, onReject) {
      if (resolved) {
        onResolve(resolveValue);
      } else if (rejected) {
        onReject(rejectValue);
      } else {
        resolveCallbacks.push(onResolve);
        rejectCallbacks.push(onReject);
      }
      return this;
    };

    this.catch = function (onReject) {
      if (rejected) {
        onReject(rejectValue);
      } else {
        rejectCallbacks.push(onReject);
      }
      return this;
    };
    
    this.finally = function (cb) {
      return this.then(
        (value) => {
          cb();
          return value;
        },
        (reason) => {
          cb();
          throw reason;
        }
      );
    };

    executor(resolve, reject);
  }
}

const promise = new MyPromise((resolve, reject) => {
  setTimeout(() => {
    resolve("foo");
  }, 300);
});

promise
  .then((data) => console.log("data-->", data))
  .catch((e) => console.log("error-->", e));
//data--> foo