Q151- How can you create a prototype for a Promise?

In JavaScript, a Promise represents an asynchronous operation that can resolve or reject. You can create a custom Promise-like object using a constructor function and adding methods to its prototype.

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1️⃣ Basic Custom Promise Implementation

function MyPromise(executor) {
  let onResolve;
  let onReject;
  let fulfilled = false;
  let rejected = false;
  let value;

  function resolve(val) {
    fulfilled = true;
    value = val;
    if (onResolve) onResolve(val);
  }

  function reject(err) {
    rejected = true;
    value = err;
    if (onReject) onReject(err);
  }

  this.then = function(callback) {
    onResolve = callback;
    if (fulfilled) onResolve(value);
    return this; // for chaining
  };

  this.catch = function(callback) {
    onReject = callback;
    if (rejected) onReject(value);
    return this;
  };

  executor(resolve, reject);
}

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2️⃣ Usage Example

const promise = new MyPromise((resolve, reject) => {
  setTimeout(() => {
    resolve("Success!");
  }, 1000);
});

promise
  .then((res) => console.log(res))   // Logs "Success!" after 1 second
  .catch((err) => console.error(err));

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🔹 Explanation

1. Executor Function
   • Passed to MyPromise, gets resolve and reject functions.
2. Prototype Methods
   • then() → handles success callback.
   • catch() → handles error callback.
3. State Handling
   • Tracks if the promise is fulfilled or rejected to allow async resolution.

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💡 In Short:

You can create a custom Promise by defining a constructor function, managing its state, and adding then/catch methods to the prototype for chaining asynchronous operations.

This is a simplified prototype; real Promises handle chaining, multiple .then calls, and async resolution more robustly.