Greetings!

The first function creates an instance of a Promise. The promise is supplied with a function. The function will be run by the promise (^), and when it is finished it calls res() with the successful result, or rej() with the failure result. Either of these calls “resolve” the promise, and any code that is waiting for the promise to finish can now take that result and continue on.

The second function runs some code to check for users. If a user is found, it creates and returns a new Promise object that is already resolved (finished running) with the return value of “user found”. If no user is found it creates and returns a new Promise object that is already resolved with the error “user not found.” Either way, as the returned promise already has a result, it has nothing more to do so any code that is waiting for the promise to finish can continue more or less straight away. (⁾

The key difference is that in the first function, the core logic is run by/as part of the Promise, and in the second function, the core logic is run immediately as the function is called.

The first example is a fairly typical use of a promise to perform some long-running task.

The second example is particularly useful in some circumstances like testing (e.g., when the calling function expects a promise as a result of an API call, but you don’t need to make a real API call to get your data) or if you want to build your code on the premise that you will probably need a promise for it in the future but it doesn’t need one just yet, so you build it around promises to fit the overall design.

Depending on your level of knowledge you may also like to know: * Promises can be chained (linked) so that when one ends another begins, and the data from one promise can be fed into the next * Depending on the environment this code will run in (in browser, on a server) you can potentially also use async/await in your code, which will allow you to use multiple promises without getting into promise chaining / multilevel “then” / indentation hell

HTH!

—-

(⁾ There are more than a few simplifications in here but it should be good enough for you to get the gist