14. Promise Constructor Polyfill

Implement a Promise constructor from scratch that follows the Promise/A+ specification. This is the foundation for all Promise methods.

Create a MyPromise class that behaves like the native Promise constructor, supporting:

• Promise states: pending, fulfilled, rejected
• then method for chaining
• catch method for error handling
• finally method for cleanup
• Proper microtask scheduling

1. Basic Promise Behavior

• Accept an executor function (resolve, reject) => {}
• Support three states: pending, fulfilled, rejected
• State transitions are one-way (pending → fulfilled/rejected)
• Store resolved/rejected value

2. Then Method

• Accept onFulfilled and onRejected callbacks
• Return a new Promise for chaining
• Handle missing callbacks (value/error propagation)
• Support promise chaining and value transformation

3. Error Handling

• Unhandled rejections should be caught
• Errors in callbacks should be caught and rejected
• Support catch and finally methods

4. Microtask Scheduling

• Callbacks should execute asynchronously (microtasks)
• Use queueMicrotask or setTimeout as fallback

Examples

// Basic promise
const p1 = new MyPromise((resolve, reject) => {
  setTimeout(() => resolve('success'), 100);
});

p1.then(value => console.log(value)); // 'success'

// Error handling
const p2 = new MyPromise((resolve, reject) => {
  reject('error');
});

p2.catch(err => console.log(err)); // 'error'

// Chaining
new MyPromise(resolve => resolve(1))
  .then(x => x + 1)
  .then(x => x * 2)
  .then(console.log); // 4