I'm Todd, a Developer Advocate @Telerik. Creator of the Angular 2 migration guide, founder of Voux. JavaScript, Angular, React, conference speaker. Developer Expert at Google.

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Exploring the Angular 1.5 .component() method

Angular 1.5 introduced the .component() helper method, which is much simpler than the .directive() definition and advocates best practices and common default behaviours. Using .component() will allow developers to write in an Angular 2 style as well, which will in turn make upgrading to Angular 2 an easier feat.

Let’s compare the differences in syntax and the super neat abstraction that .component() gives us over using the .directive() method.

Update: use component() now in Angular 1.3+

I’ve back-ported the Angular 1.5 .component() functionality to Angular 1.3 and above! Read the article and grab the latest source code on GitHub.

.directive() to .component()

The syntax change is very simple:

// before
module.directive(name, fn);

// after
module.component(name, options);

The name argument is what we want to define our Component as, the options argument is a definition Object passed into the component, rather than a function that we know so well in versions 1.4 and below.

I’ve prebuilt a simple counter component for the purposes of this exercise in Angular 1.4.x which we’ll refactor into a version v1.5.0 build to use .component().

.directive('counter', function counter() {
  return {
    scope: {},
    bindToController: {
      count: '='
    },
    controller: function () {
      function increment() {
        this.count++;
      }
      function decrement() {
        this.count--;
      }
      this.increment = increment;
      this.decrement = decrement;
    },
    controllerAs: 'counter',
    template: `
      <div class="todo">
        <input type="text" ng-model="$ctrl.count">
        <button type="button" ng-click="$ctrl.decrement();">-</button>
        <button type="button" ng-click="$ctrl.increment();">+</button>
      </div>
    `
  };
});

A live embed of the 1.4.x Directive:

We’ll continue building this alongside how we’d build the Angular 1.4 version to compare differences.

Function to Object, method name change

Let’s start from the top and refactor the function argument to become an Object, and change the name from .directive() to .component():

// before
.directive('counter', function counter() {
  return {

  };
});

// after
.component('counter', {

});

Nice and simple. Essentially the return {}; statement inside the .directive() becomes the Object definition inside .component() - easy!

“scope” and “bindToController”, to “bindings”

In a .directive(), the scope property allows us to define whether we want to isolate the $scope or inherit it, this has now become a sensible default to (usually) always make our Directives have isolate scope. So repeating ourselves each time just creates excess boilerplate. With the introduction of bindToController, we can explicitly define which properties we want to pass into our isolate scope and bind directly to the Controller.

With the bindings property on .component() we can remove this boilerplate and simply define what we want to pass down to the component, under the assumption that the component will have isolate scope.

// before
.directive('counter', function counter() {
  return {
    scope: {},
    bindToController: {
      count: '='
    }
  };
});

// after
.component('counter', {
  bindings: {
    count: '='
  }
});

Controller and controllerAs changes

Nothing has changed in the way we declare controller, however it’s now a little smarter and has a default controllerAs value of $ctrl.

If we’re using a controller local to the component, we’ll do this:

// 1.4
{
  ...
  controller: function () {}
  ...
}

If we’re using another Controller defined elsewhere, we’ll do this:

// 1.4
{
  ...
  controller: 'SomeCtrl'
  ...
}

If we want to define controllerAs at this stage (which will over-ride the default $ctrl value), we’ll need to create a new property and define the instance alias:

// 1.4
{
  ...
  controller: 'SomeCtrl',
  controllerAs: 'something'
  ...
}

This then allows us to use something.prop inside our template to talk to the instance of the Controller.

Now, there are some changes in .component() that make sensible assumptions and automatically create a controllerAs property under the hood for us, and automatically assign a name based on three possibilities:

// inside angular.js
controllerAs: identifierForController(options.controller) || options.controllerAs || '$ctrl',

Possibility one uses this aptly named identifierForController function that looks like so:

// inside angular.js
var CNTRL_REG = /^(\S+)(\s+as\s+(\w+))?$/;
function identifierForController(controller, ident) {
  if (ident && isString(ident)) return ident;
  if (isString(controller)) {
    var match = CNTRL_REG.exec(controller);
    if (match) return match[3];
  }
}

This allows us to do the following inside .component():

// 1.5
{
  ...
  controller: 'SomeCtrl as something'
  ...
}

This saves adding the controllerAs property… however

We can add the controllerAs property to maintain backwards compatibility or keep it if that’s within your style for writing Directives/Components.

The third option, and better yet, completely removes all need to think about controllerAs, and Angular automatically uses the name $ctrl. For instance:

.component('test', {
  controller: function () {
    this.testing = 123;
  }
});

The would-be controllerAs definition automatically defaults to $ctrl, so we can use $ctrl.testing in our template which would give us the value of 123.

Based on this information, we add our controller, and refactor our Directive into a Component by dropping the controllerAs property:

// before
.directive('counter', function counter() {
  return {
    scope: {},
    bindToController: {
      count: '='
    },
    controller: function () {
      function increment() {
        this.count++;
      }
      function decrement() {
        this.count--;
      }
      this.increment = increment;
      this.decrement = decrement;
    },
    controllerAs: 'counter'
  };
});

// after
.component('counter', {
  bindings: {
    count: '='
  },
  controller: function () {
    function increment() {
      this.count++;
    }
    function decrement() {
      this.count--;
    }
    this.increment = increment;
    this.decrement = decrement;
  }
});

Things are becoming much simpler to use and define with this change.

Template

There’s a subtle difference in the template property worth noting. Let’s add the template property to finish off our rework and then take a look.

.component('counter', {
  bindings: {
    count: '='
  },
  controller: function () {
    function increment() {
      this.count++;
    }
    function decrement() {
      this.count--;
    }
    this.increment = increment;
    this.decrement = decrement;
  },
  template: `
    <div class="todo">
      <input type="text" ng-model="$ctrl.count">
      <button type="button" ng-click="$ctrl.decrement();">-</button>
      <button type="button" ng-click="$ctrl.increment();">+</button>
    </div>
  `
});

The template property can be defined as a function that is now injected with $element and $attrs locals. If the template property is a function then it needs to return an String representing the HTML to compile:

{
  ...
  template: function ($element, $attrs) {
    // access to $element and $attrs
    return `
      <div class="todo">
        <input type="text" ng-model="$ctrl.count">
        <button type="button" ng-click="$ctrl.decrement();">-</button>
        <button type="button" ng-click="$ctrl.increment();">+</button>
      </div>
    `
  }
  ...
}

That’s it for our Directive to Component refactor, however there are a few other changes worth exploring before we finish.

Inheriting behaviour using “require”

If you’re not familiar with “require”, check my article on using require.

{
  ...
  require: {
    parent: '^^parentComponent'
  },
  controller: function () {
    // use this.parent to access required Objects
    this.parent.foo();
  }
  ...
}

Inherited Directive or Component methods will be bound to the this.parent property in the Controller.

One-way bindings

A new syntax expression for isolate scope values, for example:

{
  ...
  bindings: {
    oneWay: '<',
    twoWay: '='
  },
  ...
}

Read my full write-up about one-way bindings.

Lifecycle hooks

Each component has a well-defined set of lifecycle hooks, read the full article here.

Disabling isolate scope

Components are always created with isolate scope. Here’s the relevant part from the source code:

{
  ...
  scope: {},
  ...
}

Stateless components

There’s now the ability to create “stateless” components, read my in-depth article on stateless components in the .component() method.

Essentially we can just use a template and bindings:

var NameComponent = {
  bindings: {
    name: '<',
    age: '<'
  },
  template: `
    <div>
      <p>Name: {{ $ctrl.name }}</p>
      <p>Age: {{ $ctrl.age }}</p>
    </div>
  `
};

angular
  .module('app', [])
  .component('nameComponent', NameComponent);

Sourcecode for comparison

Throughout the article I’ve referred to some Angular source code snippets to cross reference against. Here’s the source code below:

this.component = function registerComponent(name, options) {
  var controller = options.controller || function() {};

  function factory($injector) {
    function makeInjectable(fn) {
      if (isFunction(fn) || isArray(fn)) {
        return function(tElement, tAttrs) {
          return $injector.invoke(fn, this, {$element: tElement, $attrs: tAttrs});
        };
      } else {
        return fn;
      }
    }

    var template = (!options.template && !options.templateUrl ? '' : options.template);
    var ddo = {
      controller: controller,
      controllerAs: identifierForController(options.controller) || options.controllerAs || '$ctrl',
      template: makeInjectable(template),
      templateUrl: makeInjectable(options.templateUrl),
      transclude: options.transclude,
      scope: {},
      bindToController: options.bindings || {},
      restrict: 'E',
      require: options.require
    };

    // Copy annotations (starting with $) over to the DDO
    forEach(options, function(val, key) {
      if (key.charAt(0) === '$') ddo[key] = val;
    });

    return ddo;
  }

  // TODO(pete) remove the following `forEach` before we release 1.6.0
  // The [email protected] looks for the annotations on the controller constructor
  // Nothing in Angular looks for annotations on the factory function but we can't remove
  // it from 1.5.x yet.

  // Copy any annotation properties (starting with $) over to the factory and controller constructor functions
  // These could be used by libraries such as the new component router
  forEach(options, function(val, key) {
    if (key.charAt(0) === '$') {
      factory[key] = val;
      // Don't try to copy over annotations to named controller
      if (isFunction(controller)) controller[key] = val;
    }
  });

  factory.$inject = ['$injector'];

  return this.directive(name, factory);
};

Again, please note that Angular 1.5 isn’t released just yet, so this article uses an API that may be subject to slight change.

Upgrading to Angular 2

Writing components in this style will allow you to upgrade your Components using .component() into Angular 2 very easily, it’d look something like this in ECMAScript 5 and new template syntax:

import {Component} from [email protected]/core';

@Component({
  selector: 'counter',
  template: `
    <div class="todo">
      <input type="text" [(ng-model)]="count">
      <button type="button" (click)="decrement();">-</button>
      <button type="button" (click)="increment();">+</button>
    </div>
  `
})
export default class CounterComponent {
  constructor() {
    
  }
  increment() {
    this.count++;
  }
  decrement() {
    this.count--;
  }
}
Todd Motto

Todd Motto

I'm a Developer Advocate at Telerik. Founder of Voux. Interested in advanced JavaScript, Angular, React. Regular conference speaker and open source developer. Developer Expert at Google.