Frontend Engineering

Overview

Frontend engineering is full-stack engineering from the network edge to the pixel. Performance, accessibility, maintainability, and security are all first-class concerns — not features to add later.

When to Use

• Before implementing a new UI feature
• Before a frontend component is marked complete
• As part of the /review workflow for frontend PRs

Process

Step 1: Define the UI states

Every component has states: loading, empty, error, populated, and edge cases. Define and implement all of them before calling it done.

Step 2: Accessibility first

See accessibility-engineering skill. Apply before visual polish.

Step 3: Performance budget

Define before building:

• Largest Contentful Paint (LCP) target: < 2.5s
• Cumulative Layout Shift (CLS) target: < 0.1
• First Input Delay (FID) / INP target: < 200ms
• Bundle size budget for new routes

Measure with Lighthouse on a representative device.

Step 4: Component design

• Components are pure functions of their props/state
• No side effects in render
• Extract business logic from UI components
• Co-locate styles with components

Step 5: State management

Prefer: local state → lifted state → context → global store (in that order). Only reach for global state when local state genuinely doesn't work.

Step 6: Error boundaries

Wrap sections of the UI in error boundaries. A crash in one section should not take down the whole page.

Step 7: Testing

• Unit test business logic extracted from components
• Component tests for component behavior (not implementation)
• E2E tests for critical user journeys

Step 8: Security

• Sanitize all user-generated HTML rendered to the DOM
• No dangerouslySetInnerHTML or equivalent without sanitization
• Content Security Policy headers configured

Verification Requirements

• All UI states implemented (loading, empty, error, populated)
• Accessibility requirements met (see accessibility-engineering)
• Lighthouse performance score ≥ 90
• Error boundaries in place
• Critical journeys covered by E2E tests
• No XSS vulnerabilities in dynamic HTML rendering