Senior Full-Stack Engineer (Node.js, Typescript, React)

Looi Consulting LLC.; Polska; On-site; PLN 240,000 - 360,000; Full time

Job summary

Jobsoid Inc. is seeking a Senior Full-Stack Engineer to lead the transition of a mature SaaS app from legacy Microsoft tech to React and Node.js.

You will own design, migration planning, and delivery across UI, APIs, services, and data with an emphasis on data integrity and test coverage. Remote work is offered; ideal candidates will have modernized live applications, strong REST API design skills, and experience with AI-assisted coding tools.

Qualifications

• 7+ years of professional software development experience with full-stack focus.
• Strong React and TypeScript in production apps.
• Backend experience with Node.js and TypeScript.
• Designed REST APIs, service layers, and data-access components.
• Strong SQL and relational DB expertise.
• Experience modernizing live production systems.
• Knowledge of migration approaches minimizing downtime.
• Strong automated testing across unit and integration.
• Experience with Git, PRs, and CI/CD workflows.
• Experience using AI coding assistants in development.
• Ability to document requirements and architecture.
• Ownership from discovery to production validation.
• Strong written communication and collaboration.

Responsibilities

• Design and develop full-stack functionality using React, Node.js, and TypeScript.
• Shape target architecture, project organization, conventions, testing strategy, and delivery practices.
• Migrate complex workflows from legacy to modern stack.
• Build maintainable REST APIs and data-access components.
• Divide migration work into testable, deployable increments.
• Ensure legacy and modern components operate safely together.
• Analyze code and workflows to preserve business rules and data relationships.
• Write secure, parameterized SQL queries and optimize DB interactions.
• Develop unit, integration, and end-to-end tests.
• Produce modular, well-documented code and focused PRs.
• Apply migration safeguards including staged releases and rollback planning.
• Use AI-assisted tools to support coding and testing.
• Evaluate AI-generated code for correctness and security.
• Collaborate with leadership through documentation and reviews.
• Investigate and resolve deployment and validation issues.
• Establish sustainable engineering patterns for the team.

Skills

React

TypeScript

Node.js

REST APIs

SQL

Automated testing

Git

CI/CD

AI coding assistants

Code reviews

Tools

Playwright

AWS

SQL Server

CI/CD tooling

Job description

Role Overview

We are seeking a Senior Full-Stack Engineer to help transition a mature, business-critical SaaS application from a legacy Microsoft technology stack to a modern React and Node.js architecture.

This is a senior, hands-on role combining application development, technical design, and migration planning. You will work across the full stack—from user interfaces and APIs to business services and relational data—while helping the team replace existing functionality in controlled, production-safe increments.

The ideal candidate has successfully modernized a live application and understands that effective migration requires more than rewriting code. You must be able to uncover existing business behavior, preserve data integrity, establish meaningful test coverage, and make pragmatic decisions that balance delivery speed with long-term maintainability.

Location: Remote

Key Responsibilities

• Design and develop full-stack functionality using React, Node.js, and TypeScript.
• Help shape the target architecture, project organization, development conventions, testing strategy, and delivery practices.
• Transition complex business workflows from an established legacy application to the modern technology stack.
• Build maintainable REST APIs, service-layer components, and data-access functionality with clearly defined responsibilities.
• Break migration work into manageable, independently testable, and deployable increments.
• Enable legacy and modern components to operate safely alongside one another during the transition.
• Analyze existing code and workflows to identify and preserve important business rules, system behavior, and data relationships.
• Write secure, parameterized SQL queries and optimize database interactions supporting transactional workflows.
• Develop unit, integration, and end-to-end tests to confirm behavioral consistency and prevent regressions.
• Produce modular, well-documented code and focused pull requests that are easy to review and maintain.
• Apply appropriate migration safeguards, including staged releases, feature controls, monitoring, and rollback planning.
• Use AI-assisted development tools to support codebase analysis, implementation, refactoring, and test development.
• Evaluate AI-generated code carefully and take full responsibility for its correctness, security, and maintainability.
• Collaborate with engineering leadership and developers through technical documentation, design discussions, and code reviews.
• Investigate and resolve issues encountered during implementation, deployment, and production validation.
• Establish sustainable engineering patterns that the internal team can maintain after the engagement.

Required Qualifications

• 7+ years of professional software development experience, including substantial full-stack responsibility.
• Strong, recent experience developing production applications with React and TypeScript.
• Strong backend development experience with Node.js and TypeScript.
• Experience designing REST APIs, service layers, and maintainable data-access components.
• Solid SQL and relational database expertise, including query development, optimization, and safe parameterization.
• Hands‑on experience modernizing, re‑platforming, or incrementally replacing functionality within a live production system.
• Understanding of migration approaches that minimize downtime, regressions, and disruption to users.
• Strong automated‑testing practices across unit and integration levels.
• Experience with Git‑based development, pull requests, peer review, and CI/CD workflows.
• Practical experience using AI coding assistants as part of day‑to‑day software development.
• Sound judgment when assessing AI‑generated code for accuracy, security, maintainability, and unintended behavior.
• Ability to document technical requirements, implementation plans, and architectural decisions clearly.
• Demonstrated ownership from discovery and design through deployment and production verification.
• Strong written communication, collaboration, and problem‑solving skills.

Preferred Qualifications

• Ability to read and understand legacy applications built with .NET Framework and C#.
• Experience working with SQL Server or comparable relational database platforms.
• Familiarity with AWS, serverless computing, or other cloud application services.
• Experience with payment platforms or other transaction‑sensitive third‑party integrations.
• Experience with Playwright or a comparable browser‑automation framework.
• Familiarity with feature flags, phased cutovers, parallel operation, and strangler‑pattern migrations.
• Experience identifying undocumented behavior in legacy code and converting it into testable requirements.
• Experience modernizing mature SaaS applications while maintaining production stability.
• Familiarity with workflow‑intensive software involving customer records, financial transactions, scheduling, or operational processes.
• Experience establishing development standards that can be sustained by an internal engineering team.

Technology Environment

• Frontend: React, TypeScript
• Backend: Node.js, TypeScript, REST APIs
• Existing Platform: Microsoft‑based legacy application
• Data: SQL Server and relational databases
• Cloud: AWS and serverless services
• Testing: Unit, integration, and browser‑based automated testing
• Delivery: Git, pull requests, code review, and CI/CD
• Development Tools: AI‑assisted coding and code‑analysis tools
• Core Competencies: Application Modernization, Incremental Migration, Data Integrity, Automated Testing, Production Stability, and Technical Documentation

What We Offer

• Meaningful influence over the architecture and engineering practices of a major application‑modernization initiative.
• End‑to‑end ownership across planning, development, testing, deployment, and production validation.
• The opportunity to establish standards for a modern React and Node.js platform.
• Close collaboration with an established engineering team responsible for a mature, customer‑facing SaaS application.
• An engineering environment that supports thoughtful and responsible use of AI‑assisted development tools.
• Potential engagement extension based on performance and evolving business needs.

Interview Process

• Recruiter Pre‑Screen
• Technical and Culture‑Fit Interview with Engineering Leadership
• Technical Interview with the Engineering Team