Initially, it's important to grasp the functioning of a nodeFunction. A nodeFunction takes a callback function as its final argument. This callback requires two arguments: error as the first one and data as the second. Take for instance the require("fs").readFile example:
// Here is the callback
function callback (error, data) {
if (error) {
console.error('An error occurred', error)
} else {
console.log('This is the data', data)
}
}
require('fs').readFile('my-file.txt', callback)
Note that this convention is not enforced by JavaScript itself.
Now let's shift our focus to Promise.promisify. This function takes a nodeFunction and converts it into a promisified version. The process can be outlined as follows:
function promisifiedReadFile (filePath) {
return new Promise(function (fulfill, reject) {
require('fs').readFile(path, function (error, data) {
if (error) { reject(error) } else { fulfill(data) }
})
})
}
Although slightly verbose, you now have a revamped version of readFile that returns a promise instead of requiring a callback function. Naturally, this specific example pertains to readFile, yet Promise.promisify is suitable for any nodeFunction:
const promisifiedReadFile = Promise.promisify(require('fs').readFile)
Both instances of promisifiedReadFile essentially function in the same manner.
Lastly, Promise.promisifyAll operates on an object, iterates through it, identifies all methods, and subsequently applies Promise.promisify to each method.
By executing
Promise.promisifyAll(require('fs'))
, you will receive a modified version of the
fs module where all methods return promises rather than necessitating callbacks.
In reference to your provided example, I am uncertain about your objective, however, it is worth noting that the defined methods do not qualify as nodeFunctions and therefore cannot undergo promisification.