Building Your Own Promise: A Deep Dive into Asynchronous JavaScript

In modern JavaScript development, Promises provide an elegant solution for handling asynchronous operations. This article delves into manually implementing a Promise from scratch, covering essential features like state transitions, asynchronous execution, chaining, and additional methods such as catch, resolve, all, and race.

1. Understanding the Promise Skeleton

A Promise object must have:

• A constructor function accepting an executor callback.
• Three states: pending, fulfilled, and rejected.
• A .then() method to handle success and failure.

Basic Promise Structure

This implementation lacks state transitions and asynchronous behavior, which we will address next.

2. Handling State Changes

A Promise must:

• Transition from pending to fulfilled or rejected irreversibly.
• Execute onFulfilled only if the state is fulfilled, and onRejected if rejected.

Improved Version

3. Implementing Asynchronous Execution

To ensure .then() executes asynchronously, we use setTimeout:

4. Implementing Promise Chaining

To enable promise chaining, we introduce the resolvePromise function:

Now, we modify .then() to use resolvePromise:

5. Implementing Additional Methods

.catch()

Promise.resolve()

Promise.reject()

Promise.all()

Promise.race()

Conclusion

This implementation covers the fundamental concepts of Promises, including state management, asynchronous execution, chaining, and additional methods. With this foundation, you can now understand and debug Promise-based JavaScript more effectively.