callback2promise.ts

const fs = require('fs')

function readFilePromise(path: string, flags: string): Promise<string> {
  return new Promise((resolve, reject) => {
    fs.readFile(path, flags, (err, data) => {
      // if the error exists, reject the promise
      // and pass the error, otherwise resolve
      // with our data
      err ? reject(err) : resolve(data)
    })
  })
}

(async function() {
  try {
    const data: string = await readFilePromise('/helloworld', 'r')
    console.log(data)
  }
  catch (err) {
    console.error('Oh no')
    console.error(err)
  }
  // or
  readFilePromise('/helloworld', 'r')
  .then((data: string) => console.log(data))
  .catch(err => console.error(err))
})()