Senior Full-Stack Developer

#vacancy #English #hot #job #Poland #Fullstack #EU #Ract #Node

Senior Full-Stack Developer - React / Next.js / Node.js at Elinext

We are looking for a **Senior Full-Stack Developer** to take ownership of an existing production SaaS application.
The ideal candidate has strong experience with **React, Next.js, TypeScript, and Node.js**, as well as a proven background in building and supporting **multi-tenant SaaS platforms**.

This is a practical development role rather than a purely architectural position. You will be expected to understand an existing codebase, implement new functionality, deploy changes, troubleshoot production issues, and gradually take responsibility for the application. Experience working with partially documented production systems is essential.

Location: EU (remote)

Responsibilities
- Take ownership of an existing production SaaS application
- Review and understand an established, partially documented codebase
- Develop and improve frontend functionality using React, Next.js, and TypeScript
- Build and maintain backend services and REST APIs using Node.js and TypeScript
- Work with multi-tenant architecture, including tenant isolation and tenant-specific access control
- Implement and maintain secure authentication and authorization flows
- Work with OAuth, OIDC, RBAC, and identity-management mechanisms
- Develop and maintain integrations with MongoDB, Cosmos DB, and PostgreSQL
- Deploy and troubleshoot applications in cloud-based environments
- Build and maintain CI/CD pipelines and Docker-based deployments
- Implement automated unit, integration, API, and end-to-end tests
- Follow secure-development practices, including OWASP recommendations and secrets management
- Investigate and resolve production issues independently
- Maintain clear technical documentation
- Support knowledge transfer and application handover activities

Requirements
- 5–10 years of professional software-development experience
- Strong commercial experience with React, Next.js, and TypeScript
- Strong backend-development experience with Node.js and TypeScript
- Solid experience developing and integrating REST APIs
- Proven production experience with SaaS applications
- Strong hands-on experience with multi-tenant SaaS environments
- Good understanding of tenant isolation and secure data separation
- Strong knowledge of authentication and authorization concepts
- Experience with OAuth, OIDC, and RBAC
- Experience with MongoDB or Cosmos DB
- Experience with PostgreSQL
- Practical experience with Git, CI/CD, Docker, and cloud deployments
- Ability to independently understand, build, deploy, and troubleshoot an existing codebase
- Experience with unit, integration, API, and end-to-end testing
- Knowledge of secure-development practices, OWASP, and secrets management
- Experience working with partially documented production systems
- Strong documentation and knowledge-transfer discipline
- Good English communication skills

Nice to Have
- Experience integrating OpenAI or Azure AI services
- Experience integrating Python-based AI services
- Experience with Supabase
- Experience with cloud-native European providers
- Experience with Sentry
- Experience with PostHog
- Strong knowledge of software and solution architecture