同步转换异步进程

Question

I'm playing with javascript and node.js, so I was trying naively to make an asynchronous call synchronous (I know fs lib has readFileSync, this is just for playing)

So here's the code, it does a loop until the callback changes the flag value, and the loop should break and the flow should continue, but it hangs...

var fs = require("fs");

//read a file, just something asynchronous
var readfilesync = function(filename){

  var getout  = false,
      result  = null,
      error   = null;

  fs.readFile(filename, "utf8", function(err, data){
    if (err) {
      error = err;
    }
    else {
      result = data;
    }
    getout = true;
  });
  while (!getout) {
    console.log('---waiting');
  }
  return (error)? error : result;
};

console.log("========== redis.conf: ",
    readfilesync("/var/log/system.log"));

Can anybody help me on a lecture about this ? regards.

Answer 1

while (!getout) is blocking, so the callback of fs.readFile never executes and getout never changes.

Remember, JS is single threaded. The callback can only be executed if resources are available, but they never become available because the while loop never terminates.

Maybe this helps to understand the execution model of JavaScript better: MDN - EventLoop .