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Longevity is a choice. Teams make it with structure, tests, and steady upgrades. Fail to plan for churn and the code stalls. Plan well and it keeps shipping. The lessons below come from projects that crossed the five-year mark and stayed healthy under load, hires, and changing stacks.
Architecture That Ages Well
Start with seams. Keep domain logic away from UI. Expose a small public API per module. Guard it with contract tests. Hide breaking changes behind adapters. This lets the app swap renderers or data layers without a rewrite. Favor composition over inheritance. Map responsibilities: UI, state, side-effects, I/O. Draw the lines once, then defend them in code review.
Evolve in slices. Add TypeScript with strict flags per folder. Introduce ESLint, Biome, and Prettier from the trunk. Keep legacy JS behind thin facades until it can be retired. Move from Webpack to Vite or Turbopack only when the bundle map proves real wins. Document decisions with short ADRs. Version internal packages. Deprecate loudly, remove on schedule. When outside help is needed, partner with a javascript development company to audit boundaries, but keep ownership local. No big-bang migrations. Ship small, under flags.
Performance, Observability, and Risk Control
Set hard budgets. Bundle ≤ 250 KB gzip per route. p75 INP ≤ 200 ms on target devices. JS execution on interaction ≤ 50 ms. Fail the build if numbers drift. Track with Lighthouse CI, Bundlesize, and WebPageTest in CI. Split code by route and intent. Remove Moment for date-fns or Day.js. Drop lodash in favor of native. Prefer server rendering or RSC where it cuts JS on the wire. Cache well. Measure again.
See problems early. Wire Sentry for errors, OpenTelemetry for traces, and Real User Monitoring for Core Web Vitals. Alert on spikes, not noise. Keep error rates under 1% of sessions. Use synthetic checks for critical flows: sign-in, checkout, dashboard. Automate Playwright tests for “can’t break” paths. Roll out with canaries at 10% traffic. Watch logs. Roll back fast when signals turn red. Release trains keep cadence. Flags keep risk low.
Teams, Process, and Knowledge that Stick
Freeze standards in tooling, not policy docs. Enforce code owners per directory. Keep short PRs. Review for API boundaries, not taste. Require tests for public functions and critical reducers. For new features, draft a short spec. Define done with metrics. After incidents, run a clean postmortem and fix the guardrail that failed.
Teach the code. Keep an onboarding map: folders, flows, owners. Record a 20-minute walkthrough of the top five features. Maintain a living glossary of backend contracts. Use Renovate to update deps weekly with safety rules. Pin Node, pin browserslist, lock files. Scan supply chain with npm-audit, osv-scanner, and provenance checks. Sign artifacts. Cache builds. Keep CI under 15 minutes. Every quarter, budget time for debt: prune APIs, archive stale flags, delete dead code, and pay for the tests that were skipped in a rush.
Projects that survive five years do simple things well and repeat them. Draw clear boundaries. Measure what matters. Ship in safe steps. Write down why decisions were made, then update the doc when facts change. The stack will move. The team will change. The code will hold if the system around it holds too.