节点js中“切换案例”的执行顺序是什么

Question

I'm new to node.js, I met a strange issue when run a simple sample.

var http = require("http");
var url = require('url');
var fs = require('fs');

var server = http.createServer(function(request, response){ 
    console.log('Connection');
    var path = url.parse(request.url).pathname;
    console.log(1);
    switch(path){
        case '/':
            response.writeHead(200, {'Content-Type': 'text/html'}); 
            response.write('hello world');
            break;
        case '/socket.html':
            console.log(2);
            fs.readFile(__dirname + path, function(error, data){
                if (error){
                    response.writeHead(404);
                    response.write("opps this doesn't exist - 404");
                    console.log(3);
                }
                else{
                    response.writeHead(200, {"Content-Type": "text/html"});
                    response.write(data, "utf8");
                    console.log(4);
                }
            });
            break;
        default:
            response.writeHead(404);
            response.write("opps this doesn't exist - 404");
            break;
    }
    console.log(5);
    response.end(); 
}); 

server.listen(8001); 

when I visit

http://localhost:8001/socket.html

backend log output as following:

    Connection
    1
    2
    5
    4
    Connection
    1
    5

In the browser, there is no output, the source of html is empty. Notice the log, I guess this maybe the response has already closed after log print "5", before log print "4".

I can't understand why the log sequence is not "1245", could somebody explain this to me?

Answer 1

The fs.readFile method is asynchronous . When you call it, execution continues immediately after it (at the break statement), and then proceeds to the console.log(5) , and the response.end() calls.

Therefore, response.end() is being called before the fs.readFile callback gets a chance to execute. You need to move the response.end() call inside the callback.