Node.js Web Development - Fourth Edition

Conversion to async functions and the Promise paradigm

In the previous section we discussed the util.promisify and its ability to convert a callback-oriented function into one that returns a Promise. The latter play well within async functions and therefore it is preferable for functions to return a Promise.

To be more precise, util.promisify is to be given a function that uses the error-first-callback paradigm. The last argument of such functions is a callback function whose first argument is interpreted as an error indicator, hence the phrase error-first-callback. What util.promisify returns is another function that returns a Promise. 

The Promise serves the same purpose as the error-first-callback. If an error is indicated, the Promise ...