callback改成promise

let promisify = (fn, receiver) => {
  return (...args) => {
    return new Promise((resolve, reject) => {
      fn.apply(receiver, [...args, (err, res) => {
        return err ? reject(err) : resolve(res);
      }]);
    });
  };
};

var fs = require("fs");
var readFilePromise = promisify(fs.readFile, fs); //包装为 Promise 接口
readFilePromise("foo.json", "utf8").then(function(content){
    //正常情况
}).catch(function(err){
    //异常情况
})