什么是 Node.js 中的事件驱动非阻塞 IO 模型？

Question

I don't understand what is the real difference between the codes:

 const fs = require('fs'); fs.readFile('/file.md', (err, data) => { if (err) throw err; });

 const fs = require('fs'); const data = fs.readFileSync('/file.md');

Please somebody tell me what is going on here in simplified way.

Answer 1

In few words the difference is that the first snippet is asynchronous.

The real difference is when the code after the snippets get executed.

So if you try to execute:

const fs = require('fs');
console.log('preparing...');
fs.readFile('/file.md', (err, data) => {
  if (err) throw err;
  console.log('I am sure that the data is read!');
  console.log(data);
});

console.log('Not sure if the data is here... ');

you'll see (if the file is big enough):

preparing...
Not sure if the data is here...
I am sure that the data is read!
$data

In the other case (the readFileSync ), the data will be there (unless of errors).

Answer 2

Take look at this example

var express = require('express');
var fs = require('fs');

var app = express.createServer(express.logger());

app.get('/readFile', function(request, response) {
    fs.readFile('data.txt', function(err, data){
        response.send(data);
    });
});


app.get('/readFileSync', function(request, response) {
   let data =  fs.readFileSync('data.txt');
});

response.send(data);

fs.readFile takes a call back which calls response.send. If you simply replace that with fs.readFileSync, you need to be aware it does not take a callback so your callback which calls response.send will never get called and therefore the response will never end and it will timeout.

You need to show your readFileSync code if you're not simply replacing readFile with readFileSync.

Also, just so you're aware, you should never call readFileSync in a node express/webserver since it will tie up the single thread loop while I/O is performed. You want the node loop to process other requests until the I/O completes and your callback handling code can run. Though you can use the promise to handle this.

And from v10.0.0 The callback parameter is no longer optional for readFile. Not passing it will throw a TypeError at runtime.