fs.createWriteStream如何知道已经完成了呢？

Question

原谅我这个伸手党，完全是头大了。上代码：var qr_image = require('qr-image');  var images = require('images');var request = require('request');var fs = require('fs');var logo = request('http:/\/wx.qlogo.cn\/mmopen\/PiajxSqBRaEKqqa31ybC79PqVmErpI2QiapLA5yxT9ampXSa1WrDxcTc6AokhZpBBoOvDYFwTJ5aP6Gvtarcib70w\/64').pipe(fs.createWriteStream('logo.jpeg'));var qr = qr_image.imageSync('http://weixin.qq.com/q/027920JGB8eZ010000M07Q');var width = images(qr).width() /2 ;var height = images(qr).height() /2;var size = width/2;//images(qr).draw(images('logo.jpeg').size(size),width-size/2,height-size/2).save('t11est.jpeg',{quality:50});我想等logo写入完成后执行注释的代码。请问如何写？谢谢大神?