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Node came out before await/async and promises, and a lot of its internal tools use callbacks like this:
const fs = require('fs')
function run() {
fs.readFile('./hello.txt', (err, data) => {
if(err) throw new Error('something went wrong');
console.log("Success!", data)
})
}
run()
We can easily turn this into using promises by promisifying it. There are multiple libraries out there, but we can easily do it by hand.
Here is a small example. You just have to use the callback (err, data) => {...} to call the resolve() or reject() promise argument.
const fs = require('fs')
function asyncReadFile(filename) {
return new Promise((res, rej) => {
fs.readFile(filename, (err, data) => {
if(err) rej(err)
res(data)
})
})
}
async function run() {
try{
const data = await asyncReadFile(filename);
console.log("Success!", data)
}
catch(error){
throw new Error('something went wrong');
}
}
run()
Use the new built in promises
You can also use the built in promises API (since node v10)
const fsPromises = require('fs').promises
async function run() {
const data = await fsPromises.readFile(filename)
console.log("Success", data)
}
Read more about the promises API at the official docs