How does Angular 21 optimize JavaScript bundles?

Question

Anubhav Kumar · Answer

Angular 21 optimizes JavaScript bundles through a
combination of build-time, compile-time, and runtime strategies. Many of these were introduced gradually (Angular 15–20) and are now
default, mature, and production-grade in Angular 21.
Below is a clear, system-level breakdown of how it works.
1. ESBuild-based Application Builder (Major Upgrade)
Angular 21 uses the new Application Builder by default, powered by
esbuild instead of Webpack.
What this gives you
Much faster buildsSmaller bundlesBetter tree-shakingSmarter dependency graph analysis
Impact
Removes unused code more aggressivelyProduces fewer, cleaner chunksOptimizes both dev and prod builds
2. Advanced Tree Shaking
Angular 21 eliminates unused code at multiple levels:
a) Framework-level tree shaking
Unused Angular features (e.g., animations, forms) are removed if not importedStandalone components help reduce NgModule overhead
b) Library-level tree shaking
Uses ES modules (import/export)Honors sideEffects: false in package.json
c) Dead code elimination
Unreachable code paths are removed during minification
3. Standalone Components = Smaller Bundles
Standalone APIs reduce bundle size by:
Eliminating 
NgModule metadataEnabling more granular dependency inclusionAllowing per-component lazy loading
@Component({
  standalone: true,
  imports: [CommonModule],
})
export class DashboardComponent {}
Result
Less framework glue codeBetter tree-shakingSmaller initial bundles
4. Fine-Grained Lazy Loading
Angular 21 supports lazy loading at multiple levels:
a) Route-based lazy loading
loadComponent: () =andgt;
  import('./profile.component').then(m =andgt; m.ProfileComponent)
b) Component-level lazy loading
Signals + deferrable views allow runtime loading
c) Deferred blocks (@defer)
@defer {
  andlt;heavy-chart /andgt;
}
Result
Critical JS loads firstNon-essential JS loads later
5. Deferrable Views (@defer) – Runtime Optimization
Angular 21 can split bundles based on template usage:
@defer (on viewport) {
  andlt;video-player /andgt;
}
This:
Creates separate JS chunksLoads them only when neededReduces Time to Interactive (TTI)
6. Build Optimizer (Still Relevant)
Angular’s build optimizer:
Removes Angular decorators at runtimeConverts metadata into static fieldsEnables better minification
Works together with:
esbuildTerser-like minification
7. Smarter Code Splitting
Angular 21 automatically splits bundles into:
Bundle Type
Purpose
main
App bootstrap
vendor
Framework and libraries
polyfills
Browser compatibility
Lazy chunks
Routes / defer blocks
This avoids:
Monolithic JS filesRe-downloading unchanged code
8. Modern JavaScript Targeting
Angular 21 outputs modern JS by default:
ES2022+No legacy polyfills unless requiredDifferential loading removed (simpler output)
Result
Smaller syntaxFaster executionFewer helper functions
9. Signals Reduce Runtime Overhead
Signals improve bundle efficiency indirectly:
Less RxJS boilerplateFewer subscriptionsSmaller change detection code paths
This leads to:
Less framework runtime codeBetter tree shaking of unused reactive logic
10. Library Optimization and Partial Compilation
Angular libraries are shipped in partial compilation format:
Compiled at app build timeNot pre-expanded into runtime codeEnables better final bundle optimization
11. CSS and Asset Optimization (Indirect JS Impact)
Angular 21 also:
Removes unused CSSInlines critical CSSEliminates unused component styles
This reduces:
JS required for style managementRuntime overhead
Summary: How Angular 21 Keeps Bundles Small
Angular 21 optimizes bundles by default using:
esbuild-based Application BuilderAggressive tree shakingStandalone APIsLazy loading +
@deferModern JS outputSignals-based reactivityPartial compilation for libraries
Result:
Smaller initial load, faster runtime, and scalable bundle growth.
and

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How does Angular 21 optimize JavaScript bundles?

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1 Answers

1. ESBuild-based Application Builder (Major Upgrade)

What this gives you

2. Advanced Tree Shaking

a) Framework-level tree shaking

b) Library-level tree shaking

c) Dead code elimination

3. Standalone Components = Smaller Bundles

4. Fine-Grained Lazy Loading

a) Route-based lazy loading

b) Component-level lazy loading

c) Deferred blocks (`@defer`)

5. Deferrable Views (`@defer`) – Runtime Optimization

6. Build Optimizer (Still Relevant)

7. Smarter Code Splitting

8. Modern JavaScript Targeting

9. Signals Reduce Runtime Overhead

10. Library Optimization & Partial Compilation

11. CSS & Asset Optimization (Indirect JS Impact)

Summary: How Angular 21 Keeps Bundles Small

Liked By

Bundle Type	Purpose
`main`	App bootstrap
`vendor`	Framework & libraries
`polyfills`	Browser compatibility
Lazy chunks	Routes / defer blocks