Combining Promises in a foreach Loop
In order to consolidate and chain promises, values or promises are returned to the handler function within the .then method. To merge multiple promises, $q.all is used which itself creates a promise that can be chained.
function retrieveData(argument) {
var items = argument;
var assurances = items.map(function(item){
var subAssurances = [fetchImage(item), fetchUserData(item)];
return $q.all(subAssurances).then(function (result) {
//return for chaining
return {image: result[0], user: result[1]};
});
});
//consolidate promises
var finalAssurance = $q.all(assurances);
return finalAssurance;
};
By utilizing the .then method of a promise, new derived promises can be created to form a chain of promises. These chains of promises can vary in length and resolve with another promise, allowing for the postponement of resolution at any point within the sequence.1
The resulting promise will either be fulfilled with an array of users or rejected with the first encountered error. The final resolution can be accessed using the promise's .then and .catch methods.
retrieveData(arguments)
.then ( function onFulfilled(objectArray) {
$scope.users = objectArray.map(x => x.user);
return objectArray;
}).catch ( function onRejected(response) {
console.log("ERROR: ", response);
throw response
})
;
The Pitfalls of $q.defer
Using $q.defer() can introduce issues such as breaking the promise chain, losing error details, and potentially causing memory leaks if errors are not properly handled. For more insights on this, refer to AngularJS Is this a “Deferred Antipattern”?.