Struggles with handling asynchronous events in Angular

I'm currently working on a piece of code that iterates through an array containing 10 items. For each item, a request is made and the returned data is stored in another array. The entire process goes smoothly until reaching the line that contains $q.all.

details.fetchDetails = function(ids, pageNumber) {
  var page = ids[pageNumber],
      mainDeferred = $q.defer(),
      promises = [],
      combinedData = [];

  for(var i=0; i<page.length; i++){
    var deferred = $q.defer();
    ngGPlacesAPI.placeDetails({placeId: page[i][i]})
      .then(function(data){
        combinedData.push(data);
        console.log(combinedData); /// Outputting the objects being added to the array
        deferred.resolve();
      });

    promises.push(deferred.promise);
  }

  console.log(promises); /// Showing the 10 promises

  $q.all(promises).then(function() { /// Nothing executes after this point
    console.log('test', mainDeferred.promise); /// No log output here
    mainDeferred.resolve(combinedData);
  });
  return mainDeferred.promise;
};

Answer №1

Encountering errors related to async operations in JavaScript is quite common.

The crucial question to ask is: "When the resolver (the function passed to .then) references the variable deferred, which instance of the variable does it access?" The answer is that all references point to the very last instance of deferred that you declare.

The issue arises from declaring deferred outside of your resolve function. This leads to how JavaScript searches for variables:

  1. Is the variable (deferred in this case) accessible within the immediate function scope? (In this scenario, no)
  2. It traverses up through parent scopes until finding a variable with the given name.

By the time the resolver executes, you have redeclared deferred multiple times, each declaration overwriting the previous one. Consequently, every resolver triggers resolves the same deferred!

To solve this dilemma, enclose your deferred declaration within a closure:

for(var i=0; i<page.length; i++){
  (function(){
    var deferred = $q.defer();
    ngGPlacesAPI.placeDetails({placeId: page[i][i]})
      .then(function(data){
        combinedItems.push(data);
        console.log(combinedItems); /// This shows the objects being pushed into the array
        deferred.resolve();
      });
    promises.push(deferred.promise);
  })()
}

Alternatively, simplify your entire program and avoid using deferreds. Let me know if this approach resonates with you!:

details.getDetails = function(idSet, pageNum) {
  var page = idSet[pageNum];

  // generate an array from 0 to page.length
  var items = Array.apply(null, { length: page.length })

  var promises = items.map(function (i) {
    return ngGPlacesAPI.placeDetails({placeId: page[i][i]});
  })

  return $q.all(promises)
};

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