A/B test developer for hire (split test developer)
I build and ship the code behind client-side A/B tests: the variation JavaScript and CSS, the QA, and a clean revert. Most of the work has been on Screwfix's Next.js Pages Router site, where sitewide tests passed first-time engineering review. Split testing and experimentation across Optimizely, VWO, Convert and the rest, written to survive production: SPA-aware, flicker-free, with accessibility built in. Available for direct briefs and as overflow capacity for CRO and experimentation teams.
Arafat Islam is a freelance A/B test developer and CRO developer with 7+ years and 500+ shipped tests, working UK and US hours for retailers, DTC brands and CRO agencies. Company: Arafatcro Ltd, UK.
Hire an A/B test developer: available now
Direct briefs or overflow build capacity, with a reply within one UK business day.
What ships, and why it survives production
Plenty of A/B test code works in the editor preview and falls apart in production. SPA route changes never re-trigger the variant. Late-rendering elements miss it entirely. The page flickers as the control flashes before the variation paints, and the engineering team bounces the build before the test even starts. The difference is in how the variation code is written.
- SPA-aware activation that re-applies on route changes and tears the variant down on navigation away, rather than trusting the platform snippet to do it.
- Variation code for a hostile DOM, polling for late-rendering targets instead of guessing a timeout, so the variant lands every time.
- Anti-flicker that hides the element, not the page, so the control never flashes and Core Web Vitals stay inside budget.
- Accessibility built in, the focus order, names and live-region announcements the brief usually leaves out, because client engineering will check them.
- QA that passes first-time review, hand-written test code, a clean revert, and a handover the client team can read.
- A split you can check. Every goal goes through GA4, Tealium or your data layer, then gets checked: if the platform served 50:50, your analytics should count 50:50 too. When the two disagree, the build is the first thing to look at, before the statistics.
- Split URL tests that redirect cleanly, with query strings, UTM parameters and session state carried across the redirect and no flash of the original page first.
Three builds that show the difference
- Tracking · SRMSample Ratio Mismatch in SPA experiments: why your GA4 split never matches the platformWhy GA4 and your testing platform disagree on the split for SPA A/B tests.
- ReactWhy your A/B test variation won't stick on a React inputYou set the value and React wipes it, or onChange never fires.
- Screwfix · OptimizelyDesktop search immersive UIA Screwfix test repositioning the desktop search overlay to landscape with search-driven subcategories, built as a custom form replacement on a Next.js SPA.
Platforms I build A/B tests on
The variation code is platform-aware, but the engineering carries across all of them. Each platform has its own page on how the builds work:
A/B tests I've shipped
- Desktop search immersive UI (Screwfix, Optimizely)
- Bestsellers carousel (Avon, Dynamic Yield)
- Trade-price prominence (Travis Perkins, Monetate)
- Customer gallery homepage (Luxury Flooring, Convert.com)
Field notes on A/B testing
The specific failures that come up building client-side tests, each one with the fix and the code:
- A/B tests that die on SPA route changes
- When your A/B test can't find the element
- Stop A/B test flicker without tanking Core Web Vitals
- Accessible A/B test variants: focus, ARIA and screen readers
How pricing works
Submit a brief and you get an hour estimate back within one UK business day, with a total you approve before any work starts. A single test usually takes between 2 and 20 build hours: a copy or layout change sits near the bottom of that range, a new component or a multi step flow near the top. Regular volume earns a discounted rate, and there is no lock-in: one-off builds are as welcome as a weekly commitment.
Hire an A/B test developer
For A/B test and split-test builds, overflow capacity for CRO and experimentation teams, or to discuss a specific test idea. I respond within one UK business day.
Related roles
What a CRO developer does: the role, how it differs from a CRO specialist, and when to hire one.
CRO services for agencies: white-label build capacity, billed per brief with no retainer.
Freelance A/B test developers worth hiring in 2026: a fair shortlist, with me on it, if you want to compare before you hire.
Common questions
What does an A/B test developer do?
An A/B test developer writes and ships the code behind an experiment: the variation JavaScript and CSS, the targeting and QA, and a clean revert when the test ends. Not the hypothesis or the statistics engine, the engineering that makes a test run correctly in production and return data you can trust.
Is a split test developer the same as an A/B test developer?
Yes. Split test developer, A/B test developer and experimentation developer all describe the same work: building the client-side code for a controlled experiment. The labels track the tool and the team, the craft is identical.
How much does it cost to hire an A/B test developer?
Freelance A/B test developers usually charge by the hour or by the build. My pricing is on this page under "How pricing works": every brief gets an hour estimate within one UK business day, and you approve the total before any work starts. Regular volume earns a discounted rate, with no retainer or lock-in. The price covers the variation code, QA, tracking checks and a clean revert.
A/B test developer vs CRO agency: which should I hire?
An agency sells research, hypotheses, design, build and analysis as one retainer. An A/B test developer sells the build: variation code, QA, tracking and revert. If you already have a backlog of test ideas and a platform, a developer is the faster and cheaper route. If you need someone to decide what to test, start with an agency or a CRO consultant. Many agencies use freelance developers like me as their build capacity.
Which platforms do you build A/B tests on?
Optimizely, VWO, Convert.com, Adobe Target, Dynamic Yield, Monetate, Intelligems, Webtrends Optimize and AB Tasty. The variation code is platform-aware but the engineering, SPA-aware activation, anti-flicker and accessibility, carries across all of them.
Do you build A/B tests on React, Next.js and other single-page apps?
Yes. Most of the Screwfix work on this site ran on a Next.js Pages Router storefront. Single-page apps need activation that re-applies on route changes, polling for late-rendering elements, and tracking that survives client-side navigation. The guides on this site cover each of those.
Can you work as overflow capacity for a CRO or experimentation team?
Yes. I take direct briefs and act as overflow build capacity for CRO and experimentation teams running their own programmes, shipping variation code that passes their engineering review.
Are you available to hire?
Yes, available now for new work. Email hello@arafatcro.com or book a call and I reply within one UK business day.
Agencies contract and invoice Arafatcro Ltd, a UK limited company registered in England & Wales, on standard supplier terms. Company no. 17325504, verifiable on Companies House.