CustomPromise.js

const CustomPromiseState = {
  PENDING: "PENDING",		
  RESOLVED: "RESOLVED",
  REJECTED: "REJECTED",
};
class CustomPromise {
  constructor(fn) {
    this.CustomPromiseState = CustomPromiseState.PENDING;
    this.resolver = this.resolver.bind(this);
    this.rejector = this.rejector.bind(this);
    this.thenFns = [];
    this.catchFns = [];
    this.finallyFn = null;
    this.resolvedData = null;
    this.rejectedData = null;
    fn(this.resolver, this.rejector);
  }
  resolver(resolvedData) {
    if (this.CustomPromiseState !== CustomPromiseState.PENDING) {
      return;
    }
    this.CustomPromiseState = CustomPromiseState.RESOLVED;
    while (this.thenFns.length) {
      const thenFn = this.thenFns.shift();
      this.resolvedData = thenFn(this.resolvedData || resolvedData);
    }
    this.finallyFn && this.finallyFn(this.resolvedData || resolvedData);
  }
  rejector(rejectedData) {
    if (this.CustomPromiseState !== CustomPromiseState.PENDING) {
      return;
    }
    this.CustomPromiseState = CustomPromiseState.REJECTED;
    while (this.catchFns.length) {
      const catchFn = this.catchFns.shift();
      this.rejectedData = catchFn(this.rejectedData || rejectedData);
    }
    this.finallyFn && this.finallyFn(this.rejectedData || rejectedData);
  }
  then(thenFn) {
    this.thenFns.push(thenFn);
    return this;
  }
  catch(catchFn) {
    this.catchFns.push(catchFn);
    return this;
  }
  finally(fn) {
    this.finallyFn = fn;
  }
}
const pr = () =>
  new CustomPromise((resolve, reject) => {
    setTimeout(() => {
      reject("rejected");
      resolve("resolved");
    }, 0);
  });

pr()
  .then((r) => console.log(r)) //resolved
  .then(() => "Success")
  .then((r) => console.log(r)) // Success
  .catch((e) => console.log(e)) // rejected
  .catch((e) => "Error")
  .catch((e) => console.log(e)) // Error
  .catch(() => "catch")
  .catch((e) => console.log(e)) // catch
  .finally((f) => {
    console.log(f); // resolved OR rejected
  });