Angular 17: What’s New and What Broke

May 11, 2024*

Angular 17 continues the framework’s evolution with exciting new features focused on hydration, deferrable views, and enhancements in control flow syntax — all aiming to improve performance, developer experience, and app maintainability. However, like any major release, Angular 17 also introduces some breaking changes that developers should know upfront to ensure smooth migrations.

Let’s dive into the highlights, changes, and what you need to watch out for when upgrading to Angular 17.

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1. Updated Hydration Support

Hydration — the process of turning server-rendered HTML into a fully interactive client-side app — is critical for Angular Universal and SSR scenarios.

Angular 17 introduces:

• Improved hydration APIs for faster and more memory-efficient bootstrapping.
• Support for partial hydration, allowing deferral of certain UI components until needed, reducing initial load times.
• Better integration with Angular Signals (introduced in Angular 16) during hydration, ensuring reactive state is preserved seamlessly.

Example: Using Deferrable Hydration

@Component({
  selector: 'app-heavy-widget',
  templateUrl: './heavy-widget.component.html',
  standalone: true,
  deferrable: true, // New Angular 17 feature to defer hydration
})
export class HeavyWidgetComponent {}

This tells Angular to skip hydrating this component initially and hydrate it only when required (e.g., when it comes into view), boosting perceived performance on SSR pages.

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2. Deferrable Views

Angular 17 extends the idea of deferrable components with deferrable views, a new way to delay rendering parts of templates or child views until necessary.

This is especially useful for complex or rarely used UI parts, improving Time to Interactive (TTI).

<div *ngIf="showDetails" @deferrable>
  <!-- Heavy sub-tree -->
</div>

By adding the @deferrable directive, Angular queues the rendering of that block, deferring it until a certain condition or user interaction triggers it.

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3. New Control Flow Changes

Angular 17 improves template syntax by introducing control flow directives that offer better readability and performance compared to the classic *ngIf and *ngFor.

For example, the new @if and @for syntax simplifies templates:

@if (user.isLoggedIn) {
  <p>Welcome back, {{ user.name }}!</p>
} else {
  <p>Please log in.</p>
}

@for (let item of items) {
  <li>{{ item.name }}</li>
}

This syntax reduces boilerplate, aligns Angular closer to modern templating paradigms, and enables more efficient change detection internally.

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4. Breaking Changes to Watch

While Angular 17 introduces cool features, some breaking changes require attention:

• Removal of legacy View Engine APIs: If you still have code relying on Angular’s old View Engine compiler, you must migrate to Ivy fully.
• Changes in lifecycle hooks: Some deprecated lifecycle hooks have been removed or replaced. For example, ngOnChanges handling has stricter rules regarding input changes.
• Strict typing on template context: Angular now enforces stronger typing inside templates, so certain implicit any types may cause errors during compilation.

Migration Tip: Use Angular’s official migration tool (ng update @angular/core) to help with these changes and catch potential issues early.

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5. Other Noteworthy Updates

• Improved DevTools support for debugging hydration and deferrable views.
• Enhanced type checking for reactive forms and signals inside templates.
• Updates to Angular CLI with better build speed and new flags for deferrable hydration.

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Summary

Angular 17 pushes the envelope with:

• Smarter hydration and deferrable components/views
• Cleaner control flow syntax with @if and @for
• Some breaking changes focused on modernizing the framework

Upgrading means better performance and developer ergonomics, but requires attention to deprecated APIs and strict typings.

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Ready to upgrade?

Try Angular 17 on a small project or branch, run the Angular update schematic, and pay close attention to compiler warnings. Embrace deferrable views and the new control flow for a smoother, faster app experience!

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