Mastering Angular Material Expansion Panel (mat-expansion-panel)

Getting Started: Importing MatExpansionModule

The mat-expansion-panel is a fundamental component of the Angular Material library, designed to provide developers with a clean, standardized way to display collapsible content. Before we can leverage its power, we must ensure our environment is correctly configured for the latest 2026 standards, which prioritize standalone components and signal-based reactivity.

Step 1: Install Angular Material

If you haven't already, add Angular Material to your project using the official CLI command. This will set up the necessary global styles and dependencies:

Step 2: Update app.module.ts (or Component Imports)

In modern Angular applications (v17+), you'll likely be working with standalone components. You need to import MatExpansionModule directly into the imports array of your component. If you are still using a module-based architecture, update your app.module.ts as follows:

Creating a Basic mat-expansion-panel Component

At its core, an expansion panel consists of a header and a content body. The header is always visible, acting as the trigger, while the content is revealed only when the panel is expanded. This pattern is essential for reducing cognitive load in feature-rich dashboards.

Here is the standard syntax for a single, manually controlled panel:

Notice the use of mat-panel-title and mat-panel-description. These two structural directives allow you to create a professional, multi-column header that automatically aligns itself across different screen sizes.

Advanced Layouts: Adding Material Lists to Panels

One of the most common real-world use cases is nesting a mat-list or a mat-nav-list inside an expansion panel. This is particularly useful for sidebar navigations or complex property inspectors where objects might contain internal arrays of items.

Combining these elements requires careful attention to padding and hover states. When building dynamic expansion panels, you might need to manage your data using a TypeScript dictionary mapping for better performance and easier lookups.

Building a Dynamic Angular Material Expansion Panel

In enterprise-level 2026 applications, content is rarely static. We often fetch arrays of objects from an API and need to render them as an accordion. The mat-accordion component acts as a wrapper that manages the expansion state of multiple panels simultaneously, ensuring that only one panel is open at a time (if multi="false").

Using *ngFor with mat-expansion-panel

By iterating over a data source, we can generate panels on the fly. This approach is highly scalable:

In the example above, items() refers to a modern Angular Signal. Signals provide a fine-grained reactivity model that ensures the DOM is only updated when precisely necessary, significantly boosting performance for large lists.

Controlling Panel States: Open by Default and Programmatic Toggles

Controlling panel state synchronously is a common requirement. You can set a panel to be open by default using the [expanded] input property.

How to set default opened state

Simply bind the expanded property to true or a boolean variable in your component:

Triggering toggle via button click

You can also reference the panel in your template and call its toggle(), open(), or close() methods directly:

Frequently Asked Questions about Material Expansion Panels