wrap nodejs fs#readFile to be a promise.

promise-wrap.js

let fs = {
  readFile: function(filename, cb){
    setTimeout(()=>{
      cb('hello')
    }, 100)
  }
}
let readFilePro = function(filename){
  return new Promise(function(resolve, reject) {
    fs.readFile(filename, (data, err) => {
      if(err) reject(err);
      resolve(data)
    })
  })
}
readFilePro('file').then(x => {
  console.log(x)
})

you have a problem there at line 10: the 'err' argument should go first in the callback function.