Role overview 

We are looking for a highly experienced, hands-on Lead CRO Developer to build and deliver A/B and multivariate experiments across our client accounts. This is fundamentally a development role. The majority of the role is hands-on coding, implementation, QA and troubleshooting of experiments, not CRO strategy, programme management, or roadmap ownership. 

You will take test designs and specifications and turn them into robust, production-ready experiments across enterprise experimentation platforms. As the lead developer, you will also act as the technical escalation point for complex builds and help maintain high development and QA standards, while remaining deeply hands-on day to day. 

What you will be doing 

• Build A/B and multivariate experiments from supplied concepts, designs and test specifications. 

• Write clean, robust JavaScript (ES6+), HTML5 and CSS3 to create experiment variations and functionality. 

• Develop experiments within Optimizely and other enterprise testing platforms including VWO, AB Tasty, Convert, Monetate and similar tools. 

• Build experiments within modern React and Next.js environments, including single-page applications and dynamically rendered experiences. 

• Handle technically complex tests involving DOM manipulation, application state, targeting, customer journeys and dynamic content. 

• Where appropriate, develop or support full-stack / server-side and feature experiments using SDKs, APIs and feature flags. 

• Implement and validate experiment tracking, events and data-layer requirements using tools such as GTM and Tealium. 

• Debug failed or inconsistent experiments, resolve conflicts with client codebases and troubleshoot platform or implementation issues. 

• Identify performance issues such as flicker, race conditions or unnecessary page impact and implement appropriate solutions. 

• Work directly with client developers where access to the underlying application, APIs, SDKs or data layer is required. 

• Review particularly complex experiment builds and provide technical support to other CRO developers when needed. 

Requirements

Essential technical skills 

JavaScript: Advanced JavaScript (ES6+) skills. You must be comfortable writing substantial custom code for experiments rather than relying on visual editors. 

HTML & CSS: Strong HTML5 and CSS3 development skills, including responsive implementation and accurate translation of supplied designs. 

Optimizely: Strong hands-on experience building experiments in Optimizely Web. Experience with Optimizely Full Stack / Feature Experimentation is highly valuable. 

Experimentation development: Extensive experience personally building and shipping A/B and multivariate tests in live ecommerce or enterprise environments. 

React / Next.js: Strong experience developing and debugging experiments on React and Next.js applications, including SPAs. 

QA & debugging: Excellent browser developer-tools skills, troubleshooting ability and rigorous cross-browser/device experiment QA. 

Tracking & tag management: Experience with GTM and/or Tealium, event tracking and data layers as they relate to experimentation. 

Cross-platform delivery: Ability to work confidently across different CMSs, ecommerce platforms, and proprietary client technology stacks. 

Experimentation platforms 

We are particularly interested in developers with hands-on build experience across several of the following. Depth of practical development experience is more important than simply having used the platform. 

• Optimizely Web 

• Optimizely Full Stack / Feature Experimentation 

• VWO 

• AB Tasty 

• Convert 

• Monetate 

• Webtrends Optimize or equivalent enterprise experimentation platforms 

Highly desirable 

• TypeScript. 

• Node.js. 

• Server-side / full-stack experimentation. 

• Experimentation SDK and API implementation. 

• Feature flags and server-side decisioning. 

• Experience resolving flicker / FOOC and experiment performance issues. 

• GA4 implementation knowledge. 

• High-traffic retail and ecommerce experience. 

• Agency experience working across several client codebases and experiment pipelines at the same time. 

 What 'Lead' means in this role 

You will not be expected to own the CRO roadmap, hypothesis strategy, or wider department. 'Lead' reflects the level of technical capability we need: someone who can take on the most complex experiment builds, make sound implementation decisions, solve difficult development problems, review code where required and act as the technical point of escalation for CRO development. 

The person we are looking for:

• A genuinely hands-on developer who enjoys building experiments and wants to remain close to the code. 

• Senior enough to work independently and take ownership of technically difficult builds. 

• Experienced in high-volume experimentation environments and able to move quickly without compromising quality. 

• Comfortable receiving a design/specification and determining how best to implement it technically. 

• Able to explain technical constraints and solutions clearly to UX, CRO, account and client development teams. 

• Pragmatic, detail-oriented, and strong at debugging. 

• Comfortable working across multiple client accounts and technology stacks. 

• Experience of working with multi-disciplinary teams and communicating with them daily to update on progress  

• Advanced knowledge in Vanilla Javascript 

• Experience in A/B testing and A/B testing tools 

• Advanced knowledge of HTML/CSS 

• Advanced knowledge of CSS-Preprocessors (e.g. SCSS) 

• Experience in cross-browser/device testing and ensuring consistency across them all 

• Understand REST APIs and AJAX 

• Understand of GIT version control 

Nice to have, but not essential 

• Experience with JIRA 

• Node.js basics would be nice to have 

Why Biglight?

Biglight is an experience design agency focused on helping leading B2C and B2B brands create remarkable customer experiences that drive commercial impact. 

We combine UX, design, insight, and experimentation to unlock new value for our clients — including organisations like The North Face, George @ ASDA, M&S and Booker Cash N Carry. 

We are a people business, powered by talented specialists who want to make a difference. We hire the best people, regardless of background, and create an environment where they can grow while delivering exceptional work for our clients. 

Read more about us at https://biglight.co.uk/  

Benefits

• Competitive salary 
• Enhanced maternity/paternity pay 
• Access to Perkbox employee benefits platform 
• Hybrid office/home working (office Tuesday and Thursdays)
• Regular business updates and company socials 
• Opportunities for professional growth and career development 
• 25 days annual leave 
• Being part of a dynamic, fast-paced agency environment 
• Working with a team of incredibly talented professional colleagues 

We are committed to fostering a diverse and inclusive workplace where everyone feels respected, valued, and empowered to contribute their authentic selves.