这个异步应该怎么返回?

import fs from ‘fs’; fs.readFile(‘index.html’,function(err,data){ var html = data.toString(‘utf-8’); var reg = /(<template[\s]src=["|’])([\w.]+)(["|’][\s]></template>)/ig; var results = reg.test(html); if(results){ var finalStr = html.replace(reg, function(){ var importFile = arguments[2]; if(fs.existsSync(importFile)){ //return fs.readFileSync(importFile).toString(‘utf-8’); fs.readFile(importFile,function(err,data){ var partHtml = data.toString(‘utf-8’); return partHtml; //怎么返回呢? }); }else{ return “hello”; } return ‘test’ }) console.log(finalStr) } })