Angular directives are one of the core features of AngularJS and later Angular frameworks. They are special markers on a DOM element that tell Angular’s HTML compiler ($compile) to attach a specified behavior to that DOM element or even transform the DOM element and its children. Directives help extend the functionality of HTML by creating custom elements, attributes, and even classes. They are prefixed by ng- which stands for Angular.
Directives can be broadly classified into three categories: component directives, structural directives, and attribute directives. AngularJS uses a declarative programming style and relies heavily on directives for data binding, DOM manipulation, conditionally rendering elements, repeating elements, and more. These directives encapsulate behavior in a clean and reusable way.
In AngularJS, directives are predefined but you can also create your custom directives for better modularity and code reuse. In this part, we will explore the fundamentals of AngularJS directives, understand their syntax, use cases, and see working examples to clarify their practical applications.
Understanding the ng-app Directive
The ng-app directive is the starting point of every AngularJS application. It tells AngularJS that the enclosed part of the HTML should be treated as an AngularJS application. When AngularJS finds this directive, it bootstraps the application and initializes it. This directive is typically placed in the <html> or <body> tag, but it can be added to any container element.
Here is how it looks in usage:
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<div ng-app=””>
…
</div>
This simple line tells Angular to compile everything within the div tag using AngularJS. If you omit the ng-app directive, Angular will not run and none of the other Angular directives like ng-model or ng-repeat will function. Essentially, the ng-app directive creates the root element for the AngularJS application and auto-initializes it when the web page loads.
There is also an option to define the name of a module within the ng-app attribute. If you are building a modular AngularJS application, it’s best to create a module and then initialize it with ng-app=”yourModuleName”. This adds structure and makes it easier to manage dependencies and configurations.
Initializing Data with ng-init
The ng-init directive is used to initialize data within the application. It assigns values to variables used by Angular expressions. This directive is often used for demonstration, learning, or testing purposes because in real-world applications, you usually initialize data in controllers or services.
Here is a basic example of using ng-init:
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<div ng-app=”” ng-init=”name=[‘abc’,’xyz’]”>
…
</div>
In this case, name is initialized with an array containing two strings. This allows you to reference name in expressions within the same container. Though useful, ng-init is not a preferred method for larger applications as it does not scale well. For production-level applications, managing state and data using controllers and services is recommended because that promotes better separation of concerns and code maintainability.
However, for small scripts and examples, ng-init can quickly set up data for use in the view. It’s a handy tool for prototypes, educational purposes, and small self-contained applications.
Two-way Data Binding with ng-model
The ng-model directive binds the value of HTML form controls to application data. This is a powerful feature of AngularJS as it enables two-way data binding. When you enter data into an input field, the model updates automatically, and if the model data changes from elsewhere, the input field reflects that change.
Here’s a simple implementation of ng-model:
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<p>Enter Text:<input type=”text” ng-model=”name”></p>
This directive binds the input field to a variable called name. As the user types into the input box, the name variable is updated in real-time. Similarly, if some logic changes the value of name, the input box updates immediately. This two-way binding is what made AngularJS stand out when it was first introduced because it reduced the need for writing extra code to keep the view and the model in sync.
The ng-model directive works with most HTML input elements including text inputs, checkboxes, radio buttons, text areas, and select boxes. This makes it highly flexible for building dynamic forms and real-time updates in the UI.
It also provides validation features. For instance, if you use type=”email” in your input field, AngularJS automatically validates the field using HTML5 and adds CSS classes based on the validation state. This allows developers to add feedback messages or styles conditionally without additional JavaScript logic.
Repeating Elements with ng-repeat
The ng-repeat directive is used to instantiate a template once per item in a collection. This is AngularJS’s way of handling loops or iterations in the DOM. It is similar to a for loop in traditional programming but written declaratively.
Here’s a basic example:
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<ul>
<li ng-repeat=”n in names”>
{{ n }}
</li>
</ul>
In this example, ng-repeat will iterate over the names array and render each item as a list item. This is a powerful feature that allows developers to dynamically create lists, tables, and other collections based on data in the model. The directive creates a new scope for each iteration which allows each element to have access to its version of the data.
ng-repeat can also be used with complex data structures such as arrays of objects. You can access properties of each object and display them accordingly. For instance, if each object in the array had a name and age property, you could write:
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<div ng-repeat=”person in people”>
{{ person.name }} – {{ person.age }}
</div>
This would render each person’s name and age from the people array. The directive is also smart about tracking which items are added, removed, or moved in the collection, minimizing the number of DOM manipulations.
Putting It All Together: A Complete Example
To illustrate how all these directives work together, consider the following example. This combines ng-app, ng-init, ng-model, and ng-repeat in a single functional application.
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<html>
<head>
<script src=”http://ajax.googleapis.com/ajax/libs/angularjs/1.3.14/angular.min.js”></script>
<title>Directive Example</title>
</head>
<body>
<h1>Example of Directive</h1>
<div ng-app=”” ng-init=”names=[‘abc’,’xyz’]”>
<p>Enter Text:<input type=”text” ng-model=”name”></p>
<ul>
<li ng-repeat=”n in names”>
{{ n }}
</li>
</ul>
</div>
</body>
</html>
This HTML page sets up a basic AngularJS application. The ng-app directive initializes the application, ng-init provides an initial array of names, ng-model binds the input to a variable, and ng-repeat loops through the array to display each item. As you expand this application, you could use ng-click to add names dynamically or connect to a controller to manage the logic more effectively.
Advanced AngularJS Directives
AngularJS provides several powerful built-in directives beyond the basic ones. These directives offer more control over form validation, conditional rendering, event handling, and more. In this part, we will explore advanced directives such as ng-if, ng-show, ng-hide, ng-switch, and ng-click. These are essential tools for controlling the UI dynamically based on user actions or data states.
Conditional Rendering with ng-if
The ng-if directive conditionally includes or removes an HTML element based on the truthiness of the expression provided. If the expression evaluates to true, the element is added to the DOM; if false, it is removed entirely.
Example usage:
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<div ng-if=”isVisible”>
This content is conditionally visible.
</div>
In this case, the div is only present in the DOM when the isVisible variable is true. If isVisible becomes false, AngularJS removes the element and all its child elements from the DOM. This differs from ng-show, which only toggles the CSS display property. ng-if is ideal when you want to avoid rendering unnecessary DOM elements altogether, which can help improve performance in large-scale applications.
Toggling Visibility with ng-show and ng-hide
The ng-show and ng-hide directives control the visibility of an element by toggling the CSS display property. Unlike ng-if, the element always exists in the DOM but is hidden or shown based on the expression.
Using ng-show:
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<p ng-show=”isLoggedIn”>Welcome back, user!</p>
Using ng-hide:
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<p ng-hide=”isLoggedIn”>Please log in to continue.</p>
In both cases, the p tag will either appear or disappear depending on the boolean value of isLoggedIn. These directives are useful when the presence of the element should remain constant for layout reasons, or when hiding and showing elements frequently based on user interactions.
Handling Multiple Conditions with ng-switch
The ng-switch directive lets you conditionally display one element from a set of alternatives. It behaves like a switch-case statement in traditional programming languages.
Example:
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<div ng-switch=”selectedOption”>
<div ng-switch-when=”A”>You chose option A</div>
<div ng-switch-when=”B”>You chose option B</div>
<div ng-switch-default>Choose an option</div>
</div>
The ng-switch directive watches the selectedOption variable. Depending on its value, AngularJS displays the matching case defined by ng-switch-when. If no match is found, the content inside ng-switch-default is displayed. This directive is especially useful when designing tabbed content, menus, or dynamic layouts that depend on a single variable.
Handling Events with ng-click
The ng-click directive is used to handle click events. It executes the expression provided when the element is clicked. This allows AngularJS applications to respond to user actions without writing custom event listeners in JavaScript.
Example:
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<button ng-click=”incrementCount()”>Click Me</button>
<p>You clicked {{ count }} times.</p>
In this example, clicking the button triggers the incrementCount() function, which updates the count variable. The paragraph element then reflects the new count automatically thanks to AngularJS’s data binding. You can define the function inside a controller or directly within the scope using ng-init.
ng-click works on any clickable HTML element, including buttons, links, and even custom elements. It is one of the most frequently used directives in interactive AngularJS applications.
Full Example Combining Advanced Directives
To illustrate how these directives work together, consider this example:
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<html>
<head>
<script src=”http://ajax.googleapis.com/ajax/libs/angularjs/1.3.14/angular.min.js”></script>
<title>Advanced Directives</title>
</head>
<body>
<div ng-app=”” ng-init=”count=0; isLoggedIn=false; selectedOption=”; incrementCount=function(){ count=count+1; }”>
<button ng-click=”incrementCount()”>Click Me</button>
<p ng-show=”count > 0″>Button clicked {{ count }} times</p>
<button ng-click=”isLoggedIn=true”>Log In</button>
<button ng-click=”isLoggedIn=false”>Log Out</button>
<p ng-if=”isLoggedIn”>Welcome, user!</p>
<p ng-if=”!isLoggedIn”>You are logged out.</p>
<select ng-model=”selectedOption”>
<option value=””>Select Option</option>
<option value=”A”>Option A</option>
<option value=”B”>Option B</option>
</select>
<div ng-switch=”selectedOption”>
<div ng-switch-when=”A”>Option A Selected</div>
<div ng-switch-when=”B”>Option B Selected</div>
<div ng-switch-default>Please select an option</div>
</div>
</div>
</body>
</html>
This example demonstrates multiple directives in action. ng-click triggers events. ng-show, ng-if, and ng-switch manage the display of elements. Together, they create an interactive and dynamic UI that responds immediately to user input.
Introduction to Custom Directives
While AngularJS offers many built-in directives, custom directives allow developers to encapsulate behavior and presentation into reusable components. These custom directives can extend HTML with new elements or attributes, making the code more organized and maintainable.
Custom directives are particularly useful when you find yourself repeating the same HTML structure or behavior in multiple places. By creating a directive, you can isolate that logic into a single definition and simply reference it in your views.
Custom directives are defined using the directive function of an AngularJS module. You provide a directive name and a factory function that returns an object specifying how the directive behaves.
Creating a Simple Custom Directive
To create a basic custom directive, you first define an AngularJS module and then call its directive method. The name of the directive is usually written in camelCase in JavaScript but used as kebab-case in HTML.
Here’s an example:
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<html>
<head>
<script src=”http://ajax.googleapis.com/ajax/libs/angularjs/1.3.14/angular.min.js”></script>
<script>
var app = angular.module(‘myApp’, []);
app.directive(‘customMessage’, function() {
return {
template: ‘This is a custom directive message.’
};
});
</script>
</head>
<body ng-app=”myApp”>
<custom-message></custom-message>
</body>
</html>
In this example, the directive is called customMessage in JavaScript, but used as <custom-message> in HTML. The template property defines the HTML content that will be rendered wherever the directive is used.
Understanding Directive Configuration Options
Custom directives can be configured with several properties that determine how they behave. The most commonly used options are restrict, template, templateUrl, scope, link, and controller.
The restrict option specifies how the directive can be used. It accepts the following values:
- E for Element name
- A for Attribute
- C for Class
- M for Comment
For example, if you want the directive to be used only as an attribute, you would write:
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restrict: ‘A’
If you want it usable as both element and attribute:
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restrict: ‘EA’
The template option defines the HTML that should replace the element. Alternatively, you can use templateUrl to point to an external HTML file. This is especially useful for larger templates.
The scope option defines whether the directive should inherit the scope from its parent or create an isolated scope. Isolated scopes are ideal when building reusable components.
Linking Functions to Directives
The link function is used to add behavior to the directive once it has been compiled and linked to the DOM. This is where you can manipulate the DOM, add event listeners, or watch scope variables.
Here’s an example with a link function:
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app.directive(‘hoverEffect’, function() {
return {
restrict: ‘A’,
link: function(scope, element, attrs) {
element.on(‘mouseenter’, function() {
element.css(‘background-color’, ‘yellow’);
});
element.on(‘mouseleave’, function() {
element.css(‘background-color’, ”);
});
}
};
});
Used in HTML:
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<div hover-effect>
Hover over this text to see the effect.
</div>
This directive adds interactivity by changing the background color when the user hovers over the element.
Using Isolated Scope in Custom Directives
To build truly reusable components, custom directives often use isolated scope. This allows the directive to have its scope, separate from the parent, and accept data via attributes.
Here is an example:
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app.directive(‘userCard’, function() {
return {
restrict: ‘E’,
scope: {
name: ‘@’,
age: ‘=’
},
template: ‘<div><h3>{{ name }}</h3><p>Age: {{ age }}</p></div>’
};
});
Used in HTML:
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<user-card name=”John” age=”userAge”></user-card>
In this example, @ binds a string from the attribute, and = creates a two-way binding to the parent scope variable userAge.
Example Combining Configuration and Behavior
Here’s a complete custom directive that uses a template, isolated scope, and a link function:
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<html>
<head>
<script src=”http://ajax.googleapis.com/ajax/libs/angularjs/1.3.14/angular.min.js”></script>
<script>
var app = angular.module(‘myApp’, []);
app.directive(‘userProfile’, function() {
return {
restrict: ‘E’,
scope: {
username: ‘@’,
age: ‘=’
},
template: ‘<div><h2>{{ username }}</h2><p>Age: {{ age }}</p></div>’,
link: function(scope, element, attrs) {
element.css({
border: ‘1px solid #ccc’,
padding: ’10px’,
‘border-radius’: ‘5px’
});
}
};
});
</script>
</head>
<body ng-app=”myApp” ng-init=”userAge=28″>
<user-profile username=”Alice” age=”userAge”></user-profile>
</body>
</html>
This directive renders a user profile card with styled borders, a dynamic name, and age from the parent scope. It encapsulates both appearance and behavior, demonstrating how custom directives can be both powerful and reusable.
Best Practices for Using AngularJS Directives
When working with directives in AngularJS, following best practices ensures your code is maintainable, scalable, and performs efficiently. One key principle is to isolate responsibilities. A directive should do one thing well—either handle display logic, behavior, or data interaction, but not all of them together. Avoid placing business logic inside directives. Instead, keep business logic inside services or controllers and use directives purely for UI behavior and structure.
Use isolated scopes when creating reusable components. This prevents unintended changes to or from the parent scope. Also, prefer using controller and controllerAs syntax instead of relying on $scope, as this promotes cleaner, more readable code. Keep your directive names consistent and descriptive. Use camelCase in JavaScript and kebab-case in HTML to match AngularJS’s parsing expectations.
Avoid manipulating the DOM directly inside controllers or services. All DOM interaction should be encapsulated within directives. This separation improves testability and reusability.
Performance Considerations with Directives
AngularJS relies on digest cycles to track changes in the application. Excessive use of directives like ng-repeat, ng-if, or ng-show with large data sets can lead to performance issues, especially when used inside nested scopes. When possible, use track by with ng-repeat to reduce the overhead of DOM diffing.
Avoid deeply nested directives that cause many scope watches. Each directive instance adds watchers to the digest cycle, which can slow down your application if not managed properly. Use one-time binding syntax (::) for static content that doesn’t change. This tells AngularJS not to watch that expression for future changes, improving performance.
Minimize the number of DOM manipulations in the link function. If a directive requires heavy DOM updates, consider using requestAnimationFrame or setTimeout to defer changes and allow the browser to batch updates efficiently.
Communication Between Directives
In complex applications, directives may need to communicate with each other. AngularJS provides several techniques for directive-to-directive communication. One method is using shared services. Services are singletons and can be injected into multiple directives, providing a shared communication channel.
Another method is using require to access a parent directive’s controller. This allows a child directive to call functions or access properties defined in the parent directive.
Here’s an example using require:
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app.directive(‘parentDir’, function() {
return {
controller: function($scope) {
this.sayHello = function() {
return ‘Hello from parent directive!’;
};
}
};
});
app.directive(‘childDir’, function() {
return {
require: ‘^parentDir’,
link: function(scope, element, attrs, parentCtrl) {
var message = parentCtrl.sayHello();
element.text(message);
}
};
});
Used in HTML:
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<div parent-dir>
<div child-dir></div>
</div>
In this example, the child directive accesses a function from the parent directive’s controller. This pattern allows for tight coordination between related directives while maintaining encapsulation.
Comparing Directives in AngularJS and Angular (2+)
While AngularJS introduced the directive system, newer versions of Angular (starting from Angular 2) evolved this concept. In Angular (2+), components are the primary building blocks and are essentially a specialized form of directive with their templates and encapsulated logic.
Angular also supports attribute directives (for behavior) and structural directives (like *ngIf and *ngFor). These are similar in concept to AngularJS directives but are implemented using TypeScript classes and decorators like @Directive or @Component.
In Angular, communication between components is handled using input and output bindings, rather than scope inheritance or require. This makes component relationships clearer and more predictable.
Unlike AngularJS, Angular uses unidirectional data flow by default, improving performance and state consistency. Also, Angular avoids digest cycles and instead uses a more efficient change detection system built around observables and zones.
If you are working with AngularJS now and planning to migrate, understanding these differences will help you design your directives in a future-proof way, making them easier to refactor into Angular components later.
Final Thoughts
AngularJS directives are one of the most powerful features in the framework, allowing developers to create rich, dynamic, and maintainable user interfaces. By mastering both built-in and custom directives, you gain full control over the structure and behavior of your web applications. Directives encourage clean separation of concerns—UI behavior is encapsulated within directives, while business logic stays in services and controllers.
As you build larger and more complex AngularJS applications, applying best practices like isolating scope, reusing components, minimizing watchers, and avoiding direct DOM manipulation becomes increasingly important. These practices not only improve code clarity and performance but also prepare your application for potential migration to newer frameworks like Angular (2+), where component-driven development is the norm.
Understanding the lifecycle of a directive—its template rendering, scope behavior, linking phase, and communication patterns—enables you to build reusable, testable, and high-performing components that scale with your application needs.
With the knowledge from all four parts of this guide, you now have a complete foundation for using AngularJS directives effectively in real-world projects. Whether maintaining legacy apps or enhancing existing codebases, directives remain a cornerstone of AngularJS development.