function (file, callback) {
    fs.readFile(file, (err, 'utf8', data) => {
        if (err) return callback(err);

        var lines = data.split('\n');

        fs.open(file, 'w', (err, fd) => {
            if (err) return callback(err)

            lines.forEach(line => {
                if (line === 'meet your condition') {
                    // do your write using fs.write(fd, )
                }
            })
            callback();
        })
    })
}

使用节点文件系统模块与文件系统的帮助你可以执行操作异步以及同步.下面是作为一个例子异步

function readWriteData(savPath, srcPath) {
    fs.readFile(srcPath, 'utf8', function (err, data) {
            if (err) throw err;
            //Do your processing, MD5, send a satellite to the moon or can add conditions , etc.
            fs.writeFile (savPath, data, function(err) {
                if (err) throw err;
                console.log('complete');
            });
        });
}

同步示例

function readFileContent(srcPath, callback) { 
    fs.readFile(srcPath, 'utf8', function (err, data) {
        if (err) throw err;
        callback(data);
        }
    );
}

function writeFileContent(savPath, srcPath) { 
    readFileContent(srcPath, function(data) {
        fs.writeFile (savPath, data, function(err) {
            if (err) throw err;
            console.log('complete');
        });
    });
}