How do promises program flow work?

[shivajikobardan]

Promises syntax that I learnt:

let p = new Promise(function (resolve, reject) {
  let x = 20, y = 20;
  if (x == y) {
    resolve();

  }
  else {
    reject();
  }
})

p.then(function () {
  console.log("that's correct");
})

  .catch(function () {
    console.log("that's not correct");
  })

I don't even understand what happens here, the flow control of here. My mild guess is when resolve is called .then part happens, and when reject is called .catch part happens.


I've like read countless articles, banged my head against the wall but it's still not getting inside my head. I feel so dumb for asynchronous javascript.

Please help me understand the program flow step by step.

[Dormilich]

then() is called, when the promise gets fulfilled (resolved or rejected), i.e. the callback does not throw an error. The difference between resolve() and reject() is that resolve() will trigger the first callback in then() and reject() will trigger the second callback (if one was defined).

Any errors you throw inside a callback will trigger the callback in catch().

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/then

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/catch