For educational purpose, here we have a custom made promise handler.

custom-promise.js

/**
This is for educational purpose only and for fun.
*/

class customPromise {
  constructor(functionToResolve) {
    this.status = "pending";
    this.onFulfilledCallbacks = [];
    this.onRejectedCallbacks = [];

    const resolve = (value) => {
      console.log("resolve"); // console logs are for learinng purposes
      if (this.status === "pending") {
        this.status = "fulfilled";
        this.value = value;
        this.onFulfilledCallbacks.forEach((fn) => fn(value));
      }
    };

    const reject = (value) => {
      console.log("reject");
      if (this.status === "pending") {
        this.status = "rejected";
        this.value = value;
        this.onRejectedCallbacks.forEach((fn) => fn(value));
      }
    };

    try {
      functionToResolve(resolve, reject);
    } catch (err) {
      reject(err);
    }
  }

  then(onFulfilled, onRejected) {
    return new customPromise((resolve, reject) => {
      if (this.status === "pending") {
        console.log(
          "push functions to array before executing them!",
          onFulfilled,
          onRejected
        );
        this.onFulfilledCallbacks.push(() => {
          try {
            const fulfilledLastPromise = onFulfilled(this.value);
            resolve(fulfilledLastPromise);
          } catch (err) {
            reject(err);
          }
        });

        this.onRejectedCallbacks.push(() => {
          try {
            const rejectedLastPromise = onRejected(this.value);
            reject(rejectedLastPromise);
          } catch (err) {
            reject(err);
          }
        });
      }

      if (this.status === "fulfilled") {
        try {
          const fulfilledLastPromise = onFulfilled(this.value);
          resolve(fulfilledLastPromise);
        } catch (err) {
          reject(err);
        }
      }

      if (this.status === "rejected") {
        try {
          const rejectedLastPromise = onRejected(this.value);
          resolve(rejectedLastPromise);
        } catch (err) {
          reject(err);
        }
      }
    });
  }
}