Simple Promise Implementation

class PromiseSimple {
  constructor(executionFunction) {
    this.promiseChain = [];
    this.handleError = () => {};

    this.onResolve = this.onResolve.bind(this);
    this.onReject = this.onReject.bind(this);

    executionFunction(this.onResolve, this.onReject);
  }

  then(handleSuccess) {
    this.promiseChain.push(handleSuccess);

    return this;
  }

  catch(handleError) {
    this.handleError = handleError;

    return this;
  }

  onResolve(value) {
    let storedValue = value;

    try {
      this.promiseChain.forEach((nextFunction) => {
        storedValue = nextFunction(storedValue);
      });
    } catch (error) {
      this.promiseChain = [];

      this.onReject(error);
    }
  }

  onReject(error) {
    this.handleError(error);
  }
}

fakeApiBackend = () => {
  const user = {
    username: 'treyhuffine',
    favoriteNumber: 42,
    profile: 'https://gitconnected.com/treyhuffine'
  };

  // Introduce a randomizer to simulate the possibility of encountering an error
  if (Math.random() > .05) {
    return {
      data: user,
      statusCode: 200,
    };
  } else {
    const error = {
      statusCode: 404,
      message: 'Could not find user',
      error: 'Not Found',
    };

    return error;
  }
};

// Assume this is your AJAX library.
// Almost all newer ones return a Promise object.
const makeApiCall = () => {
  return new PromiseSimple((resolve, reject) => {
    // Use a timeout to simulate the network delay waiting for the response.
    // This is THE reason you use a promise.
    // It waits for the API to respond, and after received, it executes code in the `then()` blocks in order.
    // If it executed this immediately, there would be no data.
    setTimeout(() => {
      const apiResponse = fakeApiBackend();

      if (apiResponse.statusCode >= 400) {
        reject(apiResponse);
      } else {
        resolve(apiResponse.data);
      }
    }, 5000);
  });
};

makeApiCall()
  .then((user) => {
    console.log('In the first .then()');

    return user;
  })
  .then((user) => {
    console.log(`User ${user.username}'s favorite number is ${user.favoriteNumber}`);

    return user;
  })
  .then((user) => {
    console.log('The previous .then() told you the favoriteNumber')

    return user.profile;
  })
  .then((profile) => {
    console.log(`The profile URL is ${profile}`);
  })
  .then(() => {
    console.log('This is the last then()');
  })
  .catch((error) => {
    console.log(error.message);
  });

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

    const resolve = (value) => {
      if (this.status === "pending") {
        this.status = "fulfilled";
        this.value = value;
        this.onFulfilledCallbacks.forEach((fn) => fn(value));
      }
    };

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

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

  then(onFulfilled, onRejected) {
    return new Promise((resolve, reject) => {
      if (this.status === "pending") {
        this.onFulfilledCallbacks.push(() => {
          try {
            const fulfilledFromLastPromise = onFulfilled(this.value);
            if (fulfilledFromLastPromise instanceof Promise) {
              fulfilledFromLastPromise.then(resolve, reject);
            } else {
              resolve(fulfilledFromLastPromise);
            }
          } catch (err) {
            reject(err);
          }
        });
        this.onRejectedCallbacks.push(() => {
          try {
            const rejectedFromLastPromise = onRejected(this.value);
            if (rejectedFromLastPromise instanceof Promise) {
              rejectedFromLastPromise.then(resolve, reject);
            } else {
              reject(rejectedFromLastPromise);
            }
          } catch (err) {
            reject(err);
          }
        });
      }

      if (this.status === "fulfilled") {
        try {
          const fulfilledFromLastPromise = onFulfilled(this.value);
          if (fulfilledFromLastPromise instanceof Promise) {
            fulfilledFromLastPromise.then(resolve, reject);
          } else {
            resolve(fulfilledFromLastPromise);
          }
        } catch (err) {
          reject(err);
        }
      }

      if (this.status === "rejected") {
        try {
          const rejectedFromLastPromise = onRejected(this.value);
          if (rejectedFromLastPromise instanceof Promise) {
            rejectedFromLastPromise.then(resolve, reject);
          } else {
            reject(rejectedFromLastPromise);
          }
        } catch (err) {
          reject(err);
        }
      }
    });
  }
}