How can you use the $interval service in Angular to execute a function repeatedly?

1 What is the $interval service?

The $interval service is a wrapper for the window.setInterval() function, which invokes a callback function after a specified delay, and repeats the invocation until it is canceled. This service has some advantages over the window.setInterval() function, such as returning a promise object to track the status and result of the timer, triggering the $digest cycle after each invocation to ensure data bindings are updated in the view, and being injectable and mockable in unit tests to make the code more testable and maintainable.

2 How to create a timer with the $interval service?

To create a timer with the $interval service, you need to inject it as a dependency in your controller or component, and then call it with the following syntax: $interval(callback, delay, [count], [invokeApply], [pass]); . The parameters are callback, which is the function to be executed repeatedly; delay, which is the number of milliseconds between each invocation; count, which is the number of times the function should be invoked; invokeApply, a boolean value that indicates whether to trigger the $digest cycle after each invocation (default value is true); and pass, an array of values that will be passed as arguments to the callback function. For example, you can create a timer that increments a counter every second by using the following code: var counter = 0; var timer = $interval(function() { counter++; }, 1000);

3 How to cancel a timer with the $interval service?

To cancel a timer with the $interval service, you need to call the $interval.cancel() method with the promise object returned by the $interval() call as the argument. For example, you can cancel the timer created in the previous section by using the following code: $interval.cancel(timer); You can also use the count parameter to specify how many times the function should be invoked before it is automatically canceled. For example, you can create a timer that increments a counter 10 times and then stops by using the following code:

var counter = 0;
var timer = $interval(function() {
  counter++;
}, 1000, 10);        

4 How to handle errors and results with the $interval service?

The promise object returned by the $interval service has two methods: then() and catch(). You can use these methods to handle the errors and results of the timer. The then() method takes two arguments: a success callback and an error callback. The success callback is invoked when the timer is successfully completed or canceled, and receives the final value of the counter as the argument. The error callback is invoked when the timer is rejected due to an exception, and receives the error object as the argument. For example, you can use the then() method to display the final value of the counter or the error message by using the following code:

timer.then(function(value) {
  alert("The final value of the counter is " + value);
}, function(error) {
  alert("An error occurred: " + error);
});        

The catch() method takes one argument: an error callback. It is equivalent to calling the then() method with only the error callback. For example, you can use the catch() method to display the error message by using the following code:

timer.catch(function(error) {
  alert("An error occurred: " + error);
});        

5 How to use the $interval service with Angular directives and components?

The $interval service can be used with Angular directives and components to create dynamic and interactive UI elements that depend on the timer. For example, you can use the $interval service with the ng-repeat directive to create a carousel that changes the images every second by using the following code:

<div class="carousel">
  <img ng-repeat="image in images" ng-src="{{image.src}}" ng-show="$index === currentIndex">
</div>        

$scope.images = [
  {src: "image1.jpg"},
  {src: "image2.jpg"},
  {src: "image3.jpg"}
];
$scope.currentIndex = 0;
var timer = $interval(function() {
  $scope.currentIndex = ($scope.currentIndex + 1) % $scope.images.length;
}, 1000);        

You can also use the $interval service with custom components to create reusable and configurable UI elements that use the timer. For example, you can create a countdown component that displays the remaining time until a deadline by using the following code: <countdown deadline="2021-01-01"></countdown>

angular.module("app").component("countdown", {
  bindings: {
    deadline: "@"
  },
  template: "<div class='countdown'>{{timeLeft}}</div>",
  controller: function($interval) {
    var ctrl = this;
    var deadline = new Date(ctrl.deadline);
    var timer = $interval(function() {
      var now = new Date();
      var diff = deadline - now;
      if (diff > 0) {
        var days = Math.floor(diff / (1000 * 60 * 60 * 24));
        var hours = Math.floor((diff % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
        var minutes = Math.floor((diff % (1000 * 60 * 60)) / (1000 * 60));
        var seconds = Math.floor((diff % (1000 * 60)) / 1000);
        ctrl.timeLeft = days + " days " + hours + " hours " + minutes + " minutes " + seconds + " seconds";
      } else {
        ctrl.timeLeft = "Time's up!";
        $interval.cancel(timer);
      }
    }, 1000);
  }
});        

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?