Node | Promise

A Promise is an object that represents the eventual result (success or failure) of an asynchronous operation.

Think of it as a placeholder for a value that will be available later.

 Ordering food online:

• You place an order → Promise created
• Food arrives → Resolved
• Order cancelled → Rejected

Promise States

A Promise has 3 states:

Pending   → initial state
Fulfilled → operation successful
Rejected  → operation failed

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


Callback Hell Problem

Before Promise:

doSomething(() => {
  doSomethingElse(() => {
    doAnotherThing(() => {
      // messy
    });
  });
});

After Promise: Clean & readable
doSomething()
  .then(doSomethingElse)
  .then(doAnotherThing)
  .catch(console.error);


How Promises Work Internally (Important)
When resolved/rejected:
They don’t execute immediately
They go to Microtask Queue
Event loop executes them:
After process.nextTick
Before timers / I/O


Async/Await (Modern Way)
Built on top of Promises
async function run() {
  try {
    const result = await promise;
    console.log(result);
  } catch (err) {
    console.log(err);
  }
}


Chaining 

fetchData()
  .then(data => process(data))
  .then(result => save(result));



Error Handling 

promise
  .then(data => { throw new Error("fail"); })
  .catch(err => console.log(err));


Event Loop Context

setTimeout(() => console.log("timeout"), 0);
Promise.resolve().then(() => console.log("promise"));


Static Methods
Promise.all() → parallel execution (fails fast)
Promise.allSettled() → waits for all
Promise.race() → first result wins
Promise.any() → first success

A Promise in JavaScript is an object that represents the eventual completion 
or failure of an asynchronous operation. It allows handling async results 
using .then, .catch, or async/await, and internally it uses the microtask queue 
for execution.