Promises

Promises came about as a response to the callback hell problem described in the previous section. They have quite a long history, stretching back to the early 80s, when the legendary Barbara Liskov coined the term Promise. The idea of a Promise is to flatten out async code. A promise is said to have the following states:

• Pending: This means it has not yet been decided or that the data is not available yet
• Fulfilled: The data has come back
• Rejected: An error happened during the operation

Thenables

Something important to know is that a Promise returns straight away, but the result is not available straight away. Promises are also known as thenables, because you need to register a callback with its then() method once...