Senior Frontend Developer

Immersum; Greater London; Hybrid; GBP 90,000 - 120,000; Full time

Job summary

Immersum is hiring a Senior Frontend Engineer to design and build a modern React single-page application with ownership from first commit to production. You’ll impact onboarding, accounts and KYC/KYB flows in a fast, autonomous fintech engineering culture.

You’ll shape frontend architecture, collaborate with teammates, and ensure high quality through testing, accessibility, and secure, observable code. Hybrid London-based role with real ownership and impact.

Qualifications

• Strong commercial experience building modern React and TypeScript SPAs in production.
• Experience with complex, form-heavy apps with multi-step onboarding and branching validation.
• Deep understanding of multi-step flows and frontend state management.
• Experience integrating with third-party services and APIs (identity verification, document upload, or generated API clients).
• Strong knowledge of roles and permissions in frontend, and accessibility (WCAG 2.1 AA).
• Solid testing mindset using Vitest, RTL, and MSW; familiar with component systems like Storybook.
• Ownership from idea/design through production with documentation and defense of decisions.
• Excellent communication with technical and non-technical stakeholders; willing to challenge ideas.

Responsibilities

• Design and build new functionality in a modern React/TypeScript SPA.
• Contribute across client onboarding, accounts, and KYC/KYB flows.
• Own features end-to-end from design to production.
• Address complex, multi-step forms and state management across reloads.
• Architectural decisions within your domain and communicate them to the team.
• Review pull requests and raise engineering standards for the squad.

Skills

Frontend architecture

React

TypeScript

Performance optimization

Testing mindset

Tools

React 18

TypeScript

Vite

TanStack Query

Form

Router

Tailwind CSS

Vitest

React Testing Library

MSW

Storybook

Job description

Location: London, hybrid (2–3 days per week in office)

Immersum are partnering with a rapidly growing Fintech business operating in the FX and payments space who are looking for a Senior Frontend Engineer to join an autonomous engineering team.

This is a role for someone who wants to build and own products, rather than simply adding features to an established platform. You will have ownership from the initial design and first commit through to production, with the freedom to make architectural decisions and be accountable for what you build.

The Company:

This global Fintech business is transforming how businesses and financial institutions move money internationally. Operating across FX, payments and banking infrastructure, they are building a high-volume platform where reliability, scalability and correctness are critical.

The engineering environment is fast-paced and highly autonomous, with short paths between making a decision and getting it into production. Engineers are trusted to build, deploy and run the software they own.

The Role:

You will be responsible for designing and building new functionality within a modern React single-page application, working across areas including client onboarding, accounts and KYC/KYB.

This isn't a role where you'll spend your time working on an established product and incrementally adding features. They're looking for a builder who enjoys taking ownership of a problem, designing the solution, building it and getting it into production.

You'll be working on complex, form-heavy applications used by real customers, including multi-step onboarding journeys, branching validation, cross-field dependencies and workflows where clients may leave and return later. You'll need to be comfortable reasoning about where state should live, how it behaves across reloads and how the frontend orchestrates complex interactions with backend APIs.

You'll also have genuine influence over how the frontend is built. You'll make architectural decisions within your domain, document and present those decisions to the wider tribe, challenge ideas when necessary and commit once a decision has been made.

The team places a strong emphasis on quality rather than simply shipping code. Thorough testing, accessibility, security, reliability and observability are expected to be built into the software from the start.

You'll also be expected to take ownership beyond your own code — reviewing pull requests, raising engineering standards and helping less experienced engineers develop.

Technology:

• React 18 / TypeScript (strict mode)
• Vite
• TanStack Query / Form / Router
• Tailwind CSS
• Vitest / React Testing Library / MSW
• Storybook

What they're looking for:

• Strong commercial experience building modern React and TypeScript single-page applications that are running in production with real users
• Experience building complex, form-heavy applications with branching validation and cross-field dependencies
• Strong understanding of multi-step application flows and frontend state management
• Deep experience with TanStack Query, including query-key design, cache invalidation, stale policies and prefetching
• Experience integrating with third-party services and APIs, ideally including identity verification, document upload or generated API clients
• Strong understanding of roles and permissions, including enforcing them appropriately within the frontend
• A strong testing mindset, with experience using tools such as Vitest, React Testing Library and MSW
• Experience building accessible applications, ideally against WCAG 2.1 AA
• Experience with component systems such as Radix/shadcn and/or Storybook
• A genuine builder mentality — someone who enjoys taking ownership from idea/design through to production
• Comfortable making architectural decisions, explaining and defending them, and taking accountability for the resulting implementation
• Strong communication skills with both technical and non-technical stakeholders, including the confidence to disagree constructively
• Experience using AI development tools in practice, with the judgement to understand when they add value and when they don't
• Strong engineering standards around security, reliability, observability and testing

Useful, but not essential:

• Auth0 or a similar identity provider
• Sentry or similar error tracking/observability tools
• Azure Static Web Apps and Azure DevOps
• Vite build and bundle optimisation, including chunk splitting, bundle analysis and source maps
• Experience within regulated financial services, KYC/KYB or client onboarding
• Storybook or another documented component library
• Real-time updates using WebSockets or SignalR

Experience within payments, FX, banking, KYC/KYB, client onboarding or other financial systems would be useful, but isn't essential.

If you're a Senior Frontend Engineer who wants genuine ownership, enjoys building products rather than simply maintaining them, and wants to work in an environment where architectural decisions and new software can move quickly into production, this could be a great opportunity to learn more.