it may be good to fix sth. for chapter04 promisify.js

[YMC-GitHub]

i run cmd with your code:

$ node spider.js http://frps.eflora.cn/frps/Rosales

then a err comes--the file content i write to is undefined{1} .

function dowload(){
//...
 writeFile(filename, body))//{1}
//...
}

i think it may be good to change sth in :

• add another _agruments to save the aguments for function promisified({1})
• use the var at ({2},{3}) ,not agruments
  since here a function too, it may have its own arguments ({0})

-or change (not testing) args.push(function(err, result) { ...} to args.push((err, result)=>{...}

function promisify (callbackBasedApi) {
  return function promisified() {
    const args = [].slice.call(arguments);
    const _arguments = arguments; //{1}
    return new Promise((resolve, reject) => {  
      args.push(function(err, result) {     //{0}
        if(err) {
          return reject(err);       
        }
        if(_arguments.length <= 1) {       //{2}
          resolve(result);
        } else {
          resolve([].slice.call(_arguments, 1)); //{3}
        }
      });
      callbackBasedApi.apply(null, args);    
    });
  }
};

it may lead to the result of request operation in download function:

function dowload(){
//...
return request(url)
    .then(response => {
      body = response.body;
      //console.log(JSON.stringify(response));
     //the response is an array with length 2 . But it expect to an object since `request(url)`
    })
//...
}

thanky you !