45. What are the main rules of promise?

Here are the main rules of Promises in JavaScript:

1. Immutable State Once Settled
   A promise can have one of three states:

   • Pending – Initial state, neither fulfilled nor rejected.
   • Fulfilled – The operation was successful.
   • Rejected – The operation failed.
     Once a promise is either fulfilled or rejected, its state cannot change.

2. Thenable Behavior
   A promise can be chained using .then():

   • .then(onFulfilled, onRejected) – Executes the appropriate callback when the promise settles.
   • If a .then() callback returns a value, the next .then() receives that value.
   • If a .then() callback throws an error or returns a rejected promise, the next .catch() is triggered.

3. Chaining
   A promise can be chained:

   js
   CopyEdit
   fetchData()
     .then(response => processData(response))
     .then(result => console.log(result))
     .catch(error => console.error(error));
   

4. Error Propagation

   • If any .then() callback throws an error, it is propagated down the chain to the nearest .catch().

5. Microtask Queue

   • Promises are executed in the microtask queue (after the current execution context and before the next event loop).

6. No Side Effects on Rejection Without Catch

   • If a promise is rejected and no .catch() or try...catch is attached, it results in an unhandled promise rejection.

7. Promise Resolution Rules

   • If a promise is resolved with another promise, it adopts the state of the inner promise.
   • If a promise is resolved with a non-promise value, it is fulfilled with that value.

Example:

js
CopyEdit
const promise = new Promise((resolve, reject) => {
  setTimeout(() => resolve('Success'), 1000);
});

promise
  .then(result => console.log(result))
  .catch(error => console.error(error));

In this example:

• The promise is resolved after 1 second.
• then receives the resolved value.
• If reject is called instead, catch will handle the error.