Need help with Promise Chaining

JavaScript
1

Appreciate some help on understanding promise chaining.

const somePromise = new Promise((res, rej) => {
  res(100);
});

somePromise
  .then((result) => new Promise((res, rej) => res(99)))     //1
  .then((result) => console.log(result))    //2

// Output 99

From what I understand, the chaining works cos a promise is returned at //1, which enables the then() at //2 to work.

However, for the example below, when a promise is Not returned at //3, how does the chaining continue? Somehow it still works with output 10

somePromise
  .then((res) => 10)      //3
  .then((res) => console.log(res))

Thank you in advance!

2

The then method automatically returns the promise. You don’t need to manually return the promise so that you can call “then” as many times as you want.

3

Promise.then() returns Promise, so wrapping return value into Promise in step //1 wasn’t necessary. Couple notes:

  1. In scenario when you’re immediately resolve value, you can simply use static .resolve() method:
const somePromise = Promise.resolve(100);
  1. .then() method receives value as an argument, it’s quite confusing (+ creates shadow in your example //1) to use res as parameter name - I wouldn’t suggest that
 somePromise
  .then((v) => v * 2) // Resolved value was passed here
  .then((v) => console.log(v)); // 200
4

Thanks all for the explanation.

Just to clarify a few points.

If the returned value (non-Promise) of the callback function is automatically wrapped in a Promise (as in //3 above), I would imagine the very simplified representation of the then() method would be something like:

function then(fn) {
<some other codes>
return Promise.resolve(fn())       // fn() == 10 in this case
}

With this, the next then()'s res variable will have the value of 10.
Now if that is correct, when a Promise (instead of value like 10) is returned in //1, shouldn’t // 2 should have output of ‘Promise {}’ instead of 99, based on the assumption below:

function then() {
<some other codes>
return Promise.resolve(new Promise((res, rej) => res(99))
}

Thanks!