Question

I’m reading a tutorial for Nodejs, but I can’t understand this snippet of code, please explain it to me.

function async(arg, callback) {
  console.log('do something with \''+arg+'\', return 1 sec later');
  setTimeout(function() { callback(arg * 2); }, 1000);
}
function final() { console.log('Done', results); }

var items = [ 1, 2, 3, 4, 5, 6 ];
var results = [];
var running = 0;
var limit = 2;

function launcher() {
  while(running < limit && items.length > 0) {
    var item = items.shift();
    async(item, function(result) {
      results.push(result);
      running--;
      if(items.length > 0) {
        launcher();
      } else if(running == 0) {
        final();
      }
    });
    running++;
  }
}

launcher();

This code produces run 2x then pause for a second then run 2x again until there is no item in the items array.

But when I removed the anonymous function in the setTimeout:

setTimeout(callback(arg*2), 1000);

Then the code runs with out stopping any second. Why?

Answer 1

Then the code runs with out stopping any second. Why?

Because instead of passing a function to setTimeout, you are passing the return value of a function call to it.

The function call executes immediately, and then setTimeout does nothing with the return value because it (presumably) isn’t a string or a function.

Don’t remove the anonymous function wrapper.