循环内的异步功能

Question

I'm newer in javascript and trying to execute async function inside loop. Anyone can explain why loop continuous executing after printing "all done" here is my code

 function asynchFunc(n, loop) { setTimeout(loop, n); } function processItems(items, cb) { (function loop (index) { if (index == items.length) return cb(); console.log(items[index]); asynchFunc(items[index], loop); loop(++index); }(0)); } processItems([1000,2000,3000,4000,5000], function(ret){ console.log('all done'); }); 

Answer 1

There is many problems in your code:

loop call asynchloop wich call loop after n millisecond (with index == undefined).

You're test case is if index == items.length. But when you call loop inside the setTimeout, you pass no parameters so when the setTimeout call loop, your test case failed every time (so the recursivity never ends).

If you want your code works you need to pass index in your asyncFunc function and stop calling loop at the end of the loop function, like this :

function asynchFunc(n, loop, index) {
    setTimeout(function() { loop(++index); }, n);
}

function processItems(items, cb) {
(function loop (index) {
  if (index == items.length) return cb();
  console.log(items[index]);
  asynchFunc(items[index], loop, index);
}(0));
}

processItems([1000,2000,3000,4000,5000], function(ret){
console.log('all done');
});

I hope this is clear

Answer 2

The lines:

  if (index == items.length) return cb(); 

and

  loop(++index); 

… are called synchronously. You don't want for asynchFunc to finish (and it doesn't accept a callback or return a promise, so unless you edit it, you can't).