chaining nested promises in a loop

javascript, loops, nested, promise

Solution

You don't need the deferred, that's the deferred anti pattern you have there since promises chain.

Also, you have to return a promise from a `.then` handler if you want it to wait for it to resolve.

You can simply use a for loop:

function do(val) {

  var q = Q();
  for(var i = 0; i < val; i++){
    q = q.then(asyncCall.bind(null,i))
         .then(console.log.bind(console))
         .then(console.log.bind(console,"x"));
  }

  return q; // in case you want to chain
}

fiddle.

Note: Bind just fixates the value for the function call. In this case since the first param (the `this` value) is null it acts like `function(fn,arg){ return function(arg){ return fn(arg); }}` that is, it translates a function call to a "partial application" - for more info see the MDN docs.

Problem

I am kind of new to promises and are stuck on the following exercise. I have an array of values and I want to execute an async call on each one. In the callback, I want to execute another call on the outcome of the first call. Basically, my frustration is in the following: The order of execution should be '1x2x3x' but the order is '123xxx' In other words, the loop is already going to the next iteration when the sub/nested promise of the first promise is not fullfilled yet.. ``` var values = ["1", "2", "3"]; function do(val) { var deferred = Q.defer(); asyncCall(val) .then( function( response ) { console.log(val); asyncCall(response) .then( function ( response ) { console.log('x'); deferred.resolve(true) }); }); return deferred.promise; } var result = do(values[0]); values.forEach( function(f) { result = result.then(do(f)); } ``` There is probably an easy solution but I'm stuck on it.

Original source