JavaScript Promises "Promisifying" functions with callbacks
Example
Given a function that accepts a Node-style callback,
fooFn(options, function callback(err, result) { ... });
you can promisify it (convert it to a promise-based function) like this:
function promiseFooFn(options) {
return new Promise((resolve, reject) =>
fooFn(options, (err, result) =>
// If there's an error, reject; otherwise resolve
err ? reject(err) : resolve(result)
)
);
}
This function can then be used as follows:
promiseFooFn(options).then(result => {
// success!
}).catch(err => {
// error!
});
In a more generic way, here's how to promisify any given callback-style function:
function promisify(func) {
return function(...args) {
return new Promise((resolve, reject) => {
func(...args, (err, result) => err ? reject(err) : resolve(result));
});
}
}
This can be used like this:
const fs = require('fs');
const promisedStat = promisify(fs.stat.bind(fs));
promisedStat('/foo/bar')
.then(stat => console.log('STATE', stat))
.catch(err => console.log('ERROR', err));
