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

Before we can utilize the <mat-expansion-panel> component, we must integrate the corresponding module into our Angular environment. The expansion panel is part of the @angular/material suite, which requires a set of global configurations.

Step 1: Install Angular Material

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

ng add @angular/material

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

In 2026, most Angular developers have transitioned to standalone components. Here is how you include the module in a modern standalone component configuration:

import { MatExpansionModule } from '@angular/material/expansion';
import { BrowserAnimationsModule } from '@angular/platform-browser/animations';

@Component({
  standalone: true,
  imports: [
    MatExpansionModule,
    BrowserAnimationsModule // Required for open/close transitions
  ],
  // ...
})
export class ExpansionOverviewExample {}

The expansion panel is designed to be highly modular. At its simplest, it consists of a header and a body. The header is what the user clicks to toggle the visibility of the content beneath it.

<mat-expansion-panel>
  <mat-expansion-panel-header>
    <mat-panel-title>
      Personal Data
    </mat-panel-title>
    <mat-panel-description>
      Click to view user details
    </mat-panel-description>
  </mat-expansion-panel-header>
  <p>This content is only visible when the panel is open.</p>
</mat-expansion-panel>

In enterprise-grade applications, expansion panels often serve as containers for more granular data. Integrating mat-list components within a mat-expansion-panel is a common UX pattern for settings pages and navigation sidebars.

<mat-expansion-panel>
  <mat-expansion-panel-header>
    <mat-panel-title>Project Files</mat-panel-title>
  </mat-expansion-panel-header>
  <mat-list>
    <mat-list-item>
      <mat-icon mat-list-icon>folder</mat-icon>
      <div mat-line>src/components</div>
    </mat-list-item>
    <mat-list-item>
      <mat-icon mat-list-icon>description</mat-icon>
      <div mat-line>index.html</div>
    </mat-list-item>
  </mat-list>
</mat-expansion-panel>

When handling dynamic data for your expansion panels, you might need to structure your data using a typescript dictionary implementation. This ensures that your local state remains synchronized with the expanded panels.

Generating panels dynamically allows you to build data-driven UIs. By combining the mat-accordion (which acts as a manager for multiple panels) with the *ngFor directive, you can create interactive FAQs or dashboard widgets effortlessly.

<mat-accordion class="example-headers-align" multi>
  <mat-expansion-panel *ngFor="let panel of panels">
    <mat-expansion-panel-header>
      <mat-panel-title> {{ panel.title }} </mat-panel-title>
      <mat-panel-description> {{ panel.desc }} </mat-panel-description>
    </mat-expansion-panel-header>
    <div> {{ panel.content }} </div>
  </mat-expansion-panel>
</mat-accordion>

The multi attribute on the mat-accordion determines whether multiple panels can be open simultaneously. By default, it is set to false.

Setting the [expanded] Property

To force a panel to be open when the page loads, use the [expanded] binding. This is useful for emphasizing important information or restoring user session states.

Using Button Clicks to Toggle Panels

Sometimes you need to trigger a panel transition from a different part of the UI, such as an "Expand All" button in a toolbar.

// TypeScript Logic
import { ViewChild } from '@angular/core';
import { MatExpansionPanel } from '@angular/material/expansion';

@Component({...})
export class MyComponent {
  @ViewChild('adminPanel') adminPanel: MatExpansionPanel;

  toggleAdmin() {
    this.adminPanel.toggle();
  }
}

Handling Events: opened and closed

Angular Material provides two output events: (opened) and (closed). These are perfect for lazy loading data or triggering analytics tracking when a user interacts with a panel.