Promises VS Callbacks

Promises have some similarities to old-style callbacks. They are essentially a returned object to which you attach callback functions, rather than having to pass callbacks into a function.

However, promises are specifically made for handling async operations, and have many advantages over old-style callbacks:

• You can chain multiple async operations together using multiple .then() operations, passing the result of one into the next one as an input. This is much harder to do with callbacks, which often ends up with a messy "pyramid of doom" (also known as callback hell (opens new window)).
• Promise callbacks are always called in the strict order they are placed in the event queue.
• Error handling is much better — all errors are handled by a single .catch() block at the end of the block, rather than being individually handled in each level of the "pyramid".
• Promises avoid inversion of control, unlike old-style callbacks, which lose full control of how the function will be executed when passing a callback to a third-party library.