promisify

把callback形式的异步函数promisify化：

function promisify(fn) {
  return function(...args) {
    return new Promise((resolve, reject) => {
      fn.call(this, ...args, (err, content) => {
        if (err) {
          reject(err)
        }
        resolve(content)
      })
    })
  }
}

const {readFile} = require('fs')
let fn = promisify(readFile)

fn('./index.js', 'utf8')
.then(res => console.log(res))
.catch(err => console.log(err))