Custom Promise using vanilla js

customPromise.js

class CustomPromise {
  // Takes the callback methods of `.then`
  thenQueue = [];

  // Catch callback
  catchCallBack = null;

  // Finally callback
  finallyCallback = null;

  // executableMethod takes the resolve, reject arguments as its parameters.
  constructor(exectableMethod) {
    if (typeof exectableMethod !== "function") {
      throw new Error("function is not provided in the arguments. exiting.");
    }

    function resolve(data) {
      this.state = "fulfilled";
      this.result = data;
      let value = data;
      this.thenQueue.forEach((method) => {
        if (typeof method === "function") {
          value = method.call(null, value);
        }
      });
      handleFinally.call(this);
    }
    function reject(err) {
      this.state = "fulfilled";
      if (typeof this.catchCallback === "function") {
        this.catchCallback.call(null, err);
      }
      handleFinally.call(this);
    }
    function handleFinally() {
      if (typeof this.finallyCallback === "function") {
        this.finallyCallback.call(null);
      }
    }

    // This is critical as we need to bind the `resolve`
    // and the `reject` methods to our internal methods
    // for us to be able to access whenever the promise
    // is either resolved or rejected from the code by the user.
    exectableMethod.call(null, resolve.bind(this), reject.bind(this));
  }

  then(thenCallback) {
    this.thenQueue.push(thenCallback);
    return this;
  }

  catch(catchCallback) {
    this.catchCallback = catchCallback;
    return this;
  }

  finally(finallyCallback) {
    this.finallyCallback = finallyCallback;
  }
}

// Setting some defaults.
CustomPromise.prototype.state = "pending";
CustomPromise.prototype.result = undefined;

// const customPromise = new CustomPromise((res, rej) => {
//   setTimeout(() => {
//     res("Hey");
//   }, 2000);
// });
// customPromise
//   .then((data) => {
//     console.log("First then", data);
//   })
//   .then((data) => {
//     console.log("Second then", data);
//   })
//   .then((data) => {
//     console.log("Third then", data);
//   })
//   .catch((err) => {
//     console.log("Catch", err);
//   })
//   .finally(() => {
//     console.log("Finally");
//   });

export default CustomPromise;