问如何使用node.js中的文件流进行单元测试？

EN

var _ = require('lodash');
var fs = require('fs');
var csv = require('fast-csv');
var topPostStream = fs.createWriteStream('top_posts.csv');
var topOtherStream = fs.createWriteStream('other_posts.csv');
var topCsvStream = csv.createWriteStream();
var otherCsvStream = csv.createWriteStream();
var dailyTops = {};

var headerRow = [];

topCsvStream.pipe(topPostStream);
otherCsvStream.pipe(topOtherStream);

console.log(process.argv);

csv
 .fromPath('posts.csv')
 .on('data', function(rowData) {
   if(headerRow.length === 0) {
     // Save header row in CSV's
     headerRow = rowData;
     topCsvStream.write(headerRow);
     otherCsvStream.write(headerRow);
     return;
   }
   if(rowData[2] === 'public' && rowData[5] > 10 && rowData[4] > 9000 && rowData[1].length < 40) {
     // Save to the top_posts file
     topCsvStream.write(rowData);
   } else {
     // Save to the other_posts file
     otherCsvStream.write(rowData);
   }

   // Save to the daily tops
   var postDate = new Date(rowData[6]);
   postDate = new Date(postDate.getYear(), postDate.getMonth(), postDate.getDate());

   if(!dailyTops[postDate] || parseInt(rowData[3]) > parseInt(dailyTops[postDate][3])) {
     dailyTops[postDate] = rowData;
   }
 })
 .on('end', finishWrite);

 function finishWrite() {
   topCsvStream.end();
   otherCsvStream.end();
   writeDailyList(dailyTops, headerRow);
 }

 function writeDailyList(dailyTops, headerRow) {
   var daily = _.map(_.keys(dailyTops), function(key) {
     return dailyTops[key];
   });
   var daily = [headerRow].concat(daily);
   csv
   .writeToPath('daily_top_posts.csv', daily, {headers: true})
 }