How Do Promises Work Under the Hood

You are passing a callback that defines the specific behavior of your promise. A Promise is a container that gives us an API to manage and transform a value, and its specificity is that it lets us manage and transform values that are actually not already there yet.

How are promises implemented?

To be able to chain promises you need to generate a new defer for each call to then and, when the promise is resolved/rejected, resolve/reject the new promise with the result of the callback. So when the promise is done, if the callback returns a new promise it is bound to the promise returned with the then() .

What are the promises and how do they work in JS?

The Promise object represents the eventual completion (or failure) of an asynchronous operation and its resulting value. To learn about the way promises work and how you can use them, we advise you to read Using promises first.