Challenge
Typing in a search box re-renders the whole page.
Agnotic approach
State moves down to the component that needs it, and long lists render only what is visible.
ReactJS Consulting
ReactJS consulting for audits and upgrades
An existing React app gets slower and harder to change one dependency at a time. Agnotic Technologies audits your React codebase for rendering performance, Core Web Vitals, state management, accessibility and dependency health, then plans and delivers the upgrade, whether that is React 19 or leaving Create React App.
Codebase auditReact 19 upgradeOff Create React AppCore Web Vitals and WCAG 2.2
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ReactJS consulting
What does ReactJS consulting involve?
ReactJS consulting is an expert review and improvement plan for a React application that already exists. Agnotic Technologies audits rendering, bundle size, state and data fetching, accessibility, dependencies and tests, measures Core Web Vitals on your real pages, then upgrades or fixes the app with your team, including the move to React 19.
New frontend builds are covered by our frontend development services, and adding React developers to your team is our React developers service. This page is about an app you already run.
Most findings are not exotic: repeated renders from unstable props, one global store for everything, effects that fetch data on every keystroke and libraries nobody has updated since launch.
Audit checklist
What does a React app audit check?
The areas we review, the tool or metric for each, and the fix we most often recommend.
| Area | What we check | Tool or metric | Usual fix |
|---|---|---|---|
| Rendering | Components that re-render with no change in output | React DevTools Profiler | Stable props and keys, memoization only where the profiler shows a cost, state moved closer to where it is used |
| Bundle | JavaScript sent for each route | Bundle analyzer, network panel | Route-level code splitting and removal of unused or duplicate libraries |
| Loading | Largest Contentful Paint and layout shift on key pages | Field data and Lighthouse | Server rendering or prerendering for content pages, sized images, space reserved for late content |
| Interactivity | Slow responses to clicks and typing | Interaction to Next Paint | Long tasks broken up, and non-urgent updates deferred with transitions |
| State | Global stores that mix server data with local UI state | Code review | Server state in a data fetching library, local state kept local |
| Data fetching | Request waterfalls and effects that refetch too often | Network panel, code review | Parallel requests, caching and cancellation |
| Accessibility | Keyboard use, focus, labels, contrast and target size | axe checks plus manual screen reader tests | Semantic elements, labelled controls and visible focus that nothing covers |
| Dependencies | Outdated, unmaintained or duplicated packages and deprecated React APIs | Package audit, React 18.3 warnings | An upgrade plan ordered by risk |
| Tests | What is covered and how long the suite takes | Coverage report, CI timings | Component tests on key flows and end-to-end tests on the paths that earn money |
The audit report lists every finding with its evidence, its effect on users and the effort to fix it.
Failure points
What slows down an existing React app?
The findings we see most often in audits, and the fix for each.
Challenge
The first load downloads code for every route.
Agnotic approach
Each route loads its own code on demand.
Challenge
A global store holds API responses, so data is stale or refetched by hand.
Agnotic approach
Server data moves to a data fetching library with caching, and the store keeps only UI state.
Challenge
Late images and banners push content down the page.
Agnotic approach
Space is reserved for every late element, so Cumulative Layout Shift stays low.
Challenge
A sticky header hides the focused element, or keyboard users get stuck in a dialog.
Agnotic approach
Focus is managed on open and close, and focused elements are never covered, as WCAG 2.2 asks.
Upgrade path
How do we upgrade a React app to React 19?
Five steps, each releasable on its own.
Move to React 18.3
React 18.3 warns about everything React 19 removes, so we upgrade there first and collect the warnings.
Replace removed APIs
ReactDOM.render becomes createRoot, string refs become callback or object refs, and propTypes move to TypeScript types or runtime validation.
Run the codemods, then review
The React team's codemods make the mechanical changes; engineers review every diff and fix what they miss.
Update libraries
Router, form, UI and testing libraries move to versions that support React 19.
Release behind checks
The upgrade ships once the test suite, visual checks and a Core Web Vitals comparison with the old version pass.
Leaving Create React App
Where should a Create React App project move?
Create React App is deprecated for new apps. The right target depends on what your app needs from a server.
A framework
- Pages that need server rendering or static generation for speed and search
- Routing and data loading handled by the framework
- Next.js or React Router in framework mode are common choices
A build tool for a single-page app
- Apps behind a login where search indexing does not matter
- Vite, Parcel or Rsbuild in place of react-scripts
- Environment variables, dev proxies and test setup moved across
What moves in either case
- Webpack-specific imports and loaders
- Jest configuration, or a move to Vitest
- Absolute imports and path aliases
- CI scripts and the deploy step
Who it is for
Who asks for ReactJS consulting?
Teams with a React app in production that has become hard to change.
Teams stuck on Create React App
Apps on react-scripts that need a supported build tool or a framework.
Products with poor Core Web Vitals
Pages that miss LCP, INP or CLS targets for real users.
Apps facing accessibility requirements
Products that must meet WCAG 2.2 AA for customers, contracts or law.
Teams that inherited a React codebase
New owners who want to know what they have before they change it.
Companies planning a redesign
An audit first, so the redesign fixes structure as well as looks.
Standards and metrics
Which standards and metrics do we audit against?
The references behind the audit checklist.
WCAG 2.2 AACore Web VitalsReact 19WAI-ARIATypeScript
Facts and sources
Facts behind React audits and upgrades
React release notes, web performance guidance and W3C standards, each linked to its primary source.
The React team published React 18.3, identical to 18.2 but with warnings for deprecated APIs and other changes needed for React 19, and recommends upgrading to 18.3 first.
Source: React 19 Upgrade Guide (react.dev)React 19 removes ReactDOM.render, which must be replaced with createRoot, removes string refs, and drops propTypes checks, which are now silently ignored.
Source: React 19 Upgrade Guide (react.dev)On 14 February 2025 the React team deprecated Create React App for new apps and encouraged existing apps to migrate to a framework or to a build tool such as Vite, Parcel or Rsbuild; Create React App continues in maintenance mode.
Source: react.dev blog: Sunsetting Create React AppGoogle's Web Vitals guidance sets good thresholds of 2.5 seconds or less for Largest Contentful Paint, 200 milliseconds or less for Interaction to Next Paint and 0.1 or less for Cumulative Layout Shift, measured at the 75th percentile of page loads.
Source: web.dev: Web VitalsWCAG 2.2 adds new Level AA success criteria, including 2.4.11 Focus Not Obscured (Minimum), 2.5.7 Dragging Movements, 2.5.8 Target Size (Minimum) and 3.3.8 Accessible Authentication (Minimum).
Source: W3C, Web Content Accessibility Guidelines 2.2
Scope and cost
What drives the size of a React engagement?
React work is priced after the scoping call, once we know where your app stands on each of these.
Codebase size
How many routes, components and shared packages the app has.
Version gap
Moving from React 16 or 17 is a bigger step than moving from 18.
Build tool
Leaving Create React App or a custom webpack setup adds migration work.
Library estate
How many UI, form, state and routing libraries need major upgrades.
Test coverage
Whether tests already guard key flows, or some need writing before the upgrade.
Accessibility findings
How much remediation the WCAG 2.2 checks turn up.
Frequently Asked Questions
We do not publish prices. The audit is fixed work sized by the app; upgrade and fix work depends on the version gap, the build tool, the library estate and test coverage. The audit quote and an outline of the follow-on work arrive in writing after the scoping call.
Not by a fixed date, but the longer you wait, the more libraries drop support for older versions. Moving to React 18.3 first is low risk and shows how much work React 19 will be.
Existing apps still run, and Create React App continues in maintenance mode. It is deprecated for new apps, so build tool and dependency updates get harder over time. We plan the move to fit your release schedule.
Yes. We test key flows with a keyboard and a screen reader and run automated checks against WCAG 2.2 AA. Our accessibility team covers wider work, such as design system audits.
Usually. Most gains come from code splitting, image sizing, reserving space for late content and breaking up long tasks. A rewrite is rarely the first answer.
Both. The report stands on its own, and our engineers can carry out the upgrade and fixes with your team through the pull request workflow you already use.
ReactJS consulting for startups is short: a review of the first production app, the riskiest dependencies and the build setup, so early shortcuts do not harden into the architecture.