What are the pros and cons of using Promises instead of callbacks?

Pros

• Avoid callback hell which can be unreadable.
• Makes it easy to write sequential asynchronous code that is readable with .then().
• Makes it easy to write parallel asynchronous code with Promise.all().
• With promises, these scenarios which are present in callbacks-only coding, will not happen:
  • Call the callback too early
  • Call the callback too late (or never)
  • Call the callback too few or too many times
  • Fail to pass along any necessary environment/parameters
  • Swallow any errors/exceptions that may happen

Cons

• Slightly more complex code (debatable).
• In older browsers where ES2015 is not supported, you need to load a polyfill in order to use it.