javascript - setTimeout in Node.js loop

Question

I'm a bit confused as to how setTimeout works. I'm trying to have a setTimeout in a loop, so that the loop iterations are, say, 1s apart. Each loop iteration makes an HTTP request and it seems like the server on the other end can't handle that many requests in such a short time span.

for (var i = 1; i<=2000 && ok; i++) {
    var options = {
        host:'www.host.com',
        path:'/path/'+i
    };

    setTimeout(makeRequest(options, i), 1000);
};

Why does this not work and how can I achieve this?

Thank you

Answer 1

setTimeout is non blocking, it is asynchronous. You give it a callback and when the delay is over, your callback is called.

Here are some implementations:

Using recursion

You can use a recursive call in the setTimeout callback.

function waitAndDo(times) {
  if(times < 1) {
    return;
  }

  setTimeout(function() {

    // Do something here
    console.log('Doing a request');

    waitAndDo(times-1);
  }, 1000);
}

Here is how to use your function:

waitAndDo(2000); // Do it 2000 times

About stack overflow errors: setTimeout clear the call stack (see this question) so you don't have to worry about stack overflow on setTimeout recursive calls.

Using generators (io.js, ES6)

If you are already using io.js (the "next" Node.js that uses ES6) you can solve your problem without recursion with an elegant solution:

function* waitAndDo(times) {
  for(var i=0; i<times; i++) {

    // Sleep
    yield function(callback) {
      setTimeout(callback, 1000);
    }    

    // Do something here
    console.log('Doing a request');
  }
}

Here is how to use your function (with co):

var co = require('co');

co(function* () {
  yield waitAndDo(10);
});

BTW: This is really using a loop ;)

Generator functions documentation.