When to use one-time binding (::)

In AngularJS, one-time binding (denoted by ::) is a powerful feature that can be used to optimize performance, especially in applications with complex or large-scale data. One-time binding allows you to tell AngularJS to bind a value to the view only once and stop watching it for future changes. This can reduce the load on AngularJS’s digest cycle, improving performance in scenarios where the data doesn’t change frequently or needs to be updated only once.

When to Use One-Time Binding (::)

1. Static Data That Does Not Change After Initialization If you have data that is only initialized once and will not change over time, you can use one-time binding. For example, if a piece of data is used for display purposes only and does not need to be updated later, using :: can reduce the overhead of constantly watching that data. Example: <div>{{::user.name}}</div> In this example, user.name will be bound only once, and any further changes to user.name will not cause AngularJS to re-evaluate the expression or update the view. This is beneficial if user.name doesn’t change frequently.
2. Display Only Data When you need to display data to the user without needing any interaction (i.e., it does not require continuous updates), one-time binding is a good choice. This is commonly used for static content that doesn’t require any dynamic behavior after initialization, such as labels, headings, or other UI elements with fixed values. Example: <h1>{{::pageTitle}}</h1> Here, pageTitle is expected to remain constant, and there’s no need for AngularJS to keep track of it after the initial binding.
3. Large or Expensive Models That Don’t Change If you have large datasets or expensive models (like large arrays or objects) that don’t need constant updates, one-time binding can help by preventing AngularJS from watching each item for changes, which can be very expensive in terms of performance. Example: <ul> <li ng-repeat="item in ::items">{{item.name}}</li> </ul> This way, Angular will initialize the items array once and will not attempt to recheck the list in the future, significantly improving performance in cases with large datasets or complex models.
4. Non-Interactive Content or Read-Only Data If the content on the page is non-interactive and does not change after the page load (e.g., displaying static content from a configuration file or a database), you should use one-time binding. This ensures that Angular doesn’t need to constantly update the DOM or check for changes. Example: <p>{{::config.description}}</p> If config.description remains the same throughout the application lifecycle, there is no need for AngularJS to keep checking its value for changes. One-time binding eliminates unnecessary watcher calls.
5. Optimizing Large Applications or SPAs In Single Page Applications (SPAs) where you have a lot of dynamic content or complex components, using one-time binding for data that doesn’t change after initialization can significantly reduce the computational overhead on the digest cycle, improving performance and responsiveness. This becomes especially critical as your application grows in size and complexity.
6. Performance Optimization in Complex Views If you are building a view with multiple bindings (such as tables, charts, or other complex UI elements), using one-time binding can reduce the number of watchers in your application. Fewer watchers mean that AngularJS has to do less work during each digest cycle, improving overall performance. Example: <div>{{::user.email}}</div> <div>{{::user.address}}</div> In this case, once the user object is initialized, Angular will no longer track changes to email or address, making the view faster.

When NOT to Use One-Time Binding

1. Dynamic or User-Interactive Data If the data changes over time or is interactive (e.g., form inputs, dynamic lists, user actions), one-time binding is not suitable. Using :: in such cases will cause your view to not update when the data changes, which is the opposite of what you want. For instance, binding a user’s input to a variable should always use regular two-way binding (ng-model) instead of one-time binding: htmlCopyEdit<input ng-model="user.name">
2. When Data Needs to Be Reactive If your view needs to reflect updates to the model over time (such as live data updates, user input changes, or dynamic content), you should avoid one-time binding. One-time binding only updates the view once, and further changes to the model will not be reflected in the view.
3. When You Need to Watch for Changes After Initialization One-time binding is not suitable if the data can change after it’s initially set and you need the view to react to those changes. For example, if you are displaying a list of items that could be updated dynamically (such as with a real-time feed), one-time binding would not update the view with the new data.

How to Use One-Time Binding

One-time binding is done by prefixing the interpolation expression with ::. This tells AngularJS that the model should be bound only once.

Syntax:

{{::expression}}

Example 1: Basic One-Time Binding

<div>{{::user.name}}</div>

Here, the user.name will be bound to the view once, and any changes to user.name will not be reflected in the view after the initial binding.

Example 2: One-Time Binding in ng-repeat

<ul>
  <li ng-repeat="item in ::items">{{item.name}}</li>
</ul>

In this case, items will be watched only once, and any changes to the items array after the initial rendering will not trigger updates in the view.

Example 3: One-Time Binding in Directives

<custom-directive content="{{::content}}"></custom-directive>

Here, content is passed to the custom-directive as a one-time binding. The directive will only receive the value once and will not re-evaluate it.