利用闭包与代理的特性，将目标函数转化为promise对象。

    1. function promisify(target, ctx = null) {
    2.   if (typeof target !== "function") {
    3.     throw TypeError(
    4.       `Expected target function. Received ${getType(target)}`
    5.     )
    6.   }
    8.   return function(...args) {
    9.     ctx || (ctx = this)
    11.     return new Promise((resolve, reject) => {
    12.       const fulfill = (err, res) => err ? reject(err) : resolve(res)
    14.       target.call(ctx, ...args, fulfill)
    15.     })
    16.   }
    17. }

    参考：https://github.com/octet-stream/promisify/blob/master/promisify.js