利用Stream模式进行文件拷贝

const fs = require('fs');
const file = fs.createReadStream("readfile.js");
const outputFile = fs.createWriteStream("outputfile.js");
file.on("data", function(data) {
    outputFile.write(data);
});
outputFile.on("open", function(fd) {
    console.log("需要被写入的文件已被打开。");
});
file.on("end", function() {
    outputFile.end("The end.", function() {
        console.log("文件全部写入完毕。");
        console.log("共写入%d字节数据。", outputFile.bytesWritten);
    });
});