the code is as follows:

var cheerio     = require("cheerio");
var superagent1 = require("superagent");
var eventproxy  = require("eventproxy");
var async       = require("async");
var utils       = require("./utils");
var install     = require("superagent-charset");
var fs             = require("fs");
var superagent  = install(superagent1);
var current_book = {};
var error_list = [];
var n=1;
superagent.get("http://www.biquku.com/0/330/").charset("gbk")
    .end(function (err, sres) {
      // 
      if (err) {
        console.log(err);
      }
      // sres.text  html  cheerio.load 
      //  jquery  `$`
      //  jquery 
      var $ = cheerio.load(sres.text);
      var urls = $("-sharplist a");  

      current_book.title = $("-sharpmaininfo h1").text()
      current_book.author = $("-sharpinfo p").eq(0).text()
      current_book.update_time = $("-sharpinfo p").eq(2).text()
      current_book.latest_chapter = $("-sharpinfo p").eq(3).html()
      current_book.intro = $("-sharpintro").html()
      current_book.chapters = [];

      for(var i = 0; i< urls.length; iPP){
        var url = urls[i]
        var _url = $(url).attr("href")+"";
        var num = _url.replace(".html","");
        var title = $(url).text();
        current_book.chapters.push({
          num: num,
          title: title,
          url: _url
        })
      }
      console.log(current_book.chapters.length);

      async.mapLimit(current_book.chapters, 200, function (url, callback) {
          fetchUrl(url, callback);
        }, function (err, result) {
          console.log("final:");
          console.log(error_list);
        });
    });

function fetchUrl(url, callback){
    console.log(nPP);
    if(n==1637){
        console.log(error_list)
    }
    superagent.get("http://www.biquku.com/0/330/" + url.num + ".html").charset("gbk").end(function(err,res){
        var timer = setTimeout(function(){
            callback(null);
        },500);
        if(!err){
            clearTimeout(timer)
            if(res){    
            if(res.text){
                var $ = cheerio.load(res.text);
                var content = $("-sharpcontent").html();
                fs.writeFile("dist/0/330/" + url.num + ".html", content, function (err) {
                    if (err) throw err;
                    callback(null);
                  });

                }
            }else{
                error_list.push(url);
                callback(null);
            }
        }else{
            clearTimeout(timer)
            callback(null);
        }
        
    })
}

the final callback of async.mapLimit, which does not execute the program, is in a state of suspended animation; it is displayed on the command line as follows:

ask the great gods to analyze why this is so;