为什么 Node.js 中的文件系统 (fs) 模块需要返回

Question

I am learning node.js . I came across the below code in w3schools website.

    var http = require('http');
    var fs = require('fs');
    http.createServer(function (req, res) {
      fs.readFile('demofile1.html', function(err, data) {
        res.writeHead(200, {'Content-Type': 'text/html'});
        res.write(data);
        **return** res.end();
      });

}).listen(8080);

Can you please explain the significance of return in fs.readFile's callback function in the above code. I tried with using return for res.end() and it still works normally.

Answer 1

The return statement is not needed in this case, as res.end() is the last statement - it makes no difference. Consider this case however:

fs.readFile('demofile1.html', function(err, data) {
        if(err) {
          return res.status(500).end();
        }
        res.writeHead(200, {'Content-Type': 'text/html'});
        res.write(data);
        res.end();
});

Here, in case of an error, we use return res.status(500).end() to make sure we do not execute any more code in that function and run into an "Headers have already been sent"-error. You could of course use if/else and don't use return in case of an error, but I personally find the first option to be a bit cleaner.