有这么一个需求：想把input里面的内容写到outInput里面去，但是上面的方法都是把文档里面的内容重置了，只想添加，而且保留原内容怎么办呢？

可以在可读流创建完毕的回调函数里面进行操作，看代码：

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let fs = require('fs');
let data = '';
let data2 = '你的小青蛙是真的可爱';

//1.读取流
let readStream = fs.createReadStream("input.txt"); //创建可读流
readStream.setEncoding('UTF8'); //设置utf-8编码

//处理流事件
readStream.on('data', chunk => data += chunk);
readStream.on('end', () => writeS(data));
readStream.on("error", err => console.log(err.strck));
console.log("程序1执行完毕");

//2.写入流
let writeS = dataS =>{  //创建可写流
    let writeStream = fs.createWriteStream("outInput.txt");
    writeStream.write(data2+dataS, "UTF8"); //使用utf-8写入流   
    writeStream.end(); //标记文件末尾
    
    //处理事件流
    writeStream.on("finish", () => console.log("写入完成"));
    writeStream.on("error", err => console.log(err.stack));
    console.log("程序2执行完毕");
}

如此，便可以啦！

如上面可能存在覆盖状态,可以设置写入流的追加参数来解决:

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var fs = require('fs');
var read = fs.createReadStream('input.txt');
var write = fs.createWriteStream('out.txt', { 'flags': 'a' }); //设置第二个参数append
read.pipe(write); //管道流读写操作