Callback not executing asynchronously in Node.js

Question

A callBack function:

function queryDemo(param,callBack){
    function sleep(milliSeconds) {
        var startTime = new Date().getTime();
        while (new Date().getTime() < startTime + milliSeconds);
    } 
    sleep(10000);
    callBack(param);
}

Express code:

app.get('/demo', function(req, res){
    console.log(1);
    queryDemo(JSON.stringify(req.query),function(result){
        console.log(2);
    });
    console.log(3);
});

Then browse http://127.0.0.1/demo, the output in console is

1
//wait 10 seconds here
2
3

I think the right output should be:

1
3
// 10 seconds later
2

hexacyanide · Accepted Answer · 2013-09-27 03:18:37Z

3

The callback will execute synchronously unless you queue it to run on the next iteration of the event loop. This is what process.nextTick() is used for.

function queryDemo(param, callback) {
  function sleep(milliseconds) {
    var startTime = new Date().getTime();
    while (new Date().getTime() < startTime + milliseconds);
  } 
  sleep(10000);
  process.nextTick(function() {
    callback(param);
  });
}

However, if you use this, you will still block the application and get this output:

1
// 10 second pause
3
2

To delay the execution of the function itself, then queue the calling of the function itself:

process.nextTick(function() {
  queryDemo(JSON.stringify(req.query), function(result) {
    console.log(2);
  });
});

Also do note that your sleep() function will still block the application, so you should be using setTimeout() instead.

edited Sep 27, 2013 at 3:18

answered Sep 27, 2013 at 3:10

hexacyanide

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1 Comment

leesei Over a year ago

Point to add: Even the OP is using callback, the calling is not asynchronous.

Jason · Accepted Answer · 2013-09-27 03:12:18Z

1

I think you are killing the thread with that tight loop. Why are you trying to write you own sleep function instead of the built in timers setTimeout or setInterval?

answered Sep 27, 2013 at 3:12

Jason

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