Promises

Promises are part of the ECMAScript 2015 standard (or ES6, which is why they’re also called ES6 promises) and have been natively available in Node.js since version 4. But the history of promises goes back a few years earlier, when there were dozens of implementations around, initially with different features and behaviors. Eventually, the majority of those implementations settled on a standard called Promises/A+.

Promises represent a big step ahead toward providing a robust alternative to continuation-passing style callbacks for propagating an asynchronous result. As we’ll see, the use of promises will make all the major asynchronous control flow constructs easier to read, less verbose, and more robust compared to their callback-based alternatives.

What’s a Promise?

A Promise is an object that embodies the eventual result (or error) of an asynchronous operation. In promises jargon, we say that a Promise is pending when the asynchronous operation is not yet complete, it’s fulfilled when the operation successfully completes and rejected when the operation terminates with an error. Once a Promise is either fulfilled or rejected, it’s considered settled.

To receive the fulfillment value or the error (reason) associated with the rejection, we can use the then() method of a Promise instance. The following is its signature: