AngularJS Using the $q constructor to create promises


The $q constructor function is used to create promises from asynchronous APIs that use callbacks to return results.

$q(function(resolve, reject) {...})

The constructor function receives a function that is invoked with two arguments, resolve and reject that are functions which are used to either resolve or reject the promise.

Example 1:

function $timeout(fn, delay) {
    return = $q(function(resolve, reject) {
         setTimeout(function() {
              try {
                 let r = fn();
              catch (e) {
         }, delay);

The above example creates a promise from the WindowTimers.setTimeout API. The AngularJS framework provides a more elaborate version of this function. For usage, see the AngularJS $timeout Service API Reference.

Example 2:

$scope.divide = function(a, b) {
    return $q(function(resolve, reject) {
      if (b===0) {
        return reject("Cannot devide by 0")
      } else {
        return resolve(a/b);

The above code showing a promisified division function, it will return a promise with the result or reject with a reason if the calculation is impossible.

You can then call and use .then

$scope.divide(7, 2).then(function(result) {
    // will return 3.5
}, function(err) {
    // will not run

$scope.divide(2, 0).then(function(result) {
    // will not run as the calculation will fail on a divide by 0
}, function(err) {
    // will return the error string.