Sofia, Bulgaria

Aleksandar Kostov

Full-Stack TypeScript (React, Next.js, Node, React Native)

I build products end to end — typed, maintainable, and production-ready — using Claude and AI-assisted workflows to ship faster without cutting corners.

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Aleksandar Kostov

About

I'm a full-stack developer based in Sofia, Bulgaria, with over six years of experience building products end to end — from WordPress and PHP early on to TypeScript-heavy React/Next.js frontends and Node.js services today. I've worked across fintech, crypto, and iGaming, shipping data-dense dashboards, enterprise tables, and API gateways used by quantitative and engineering teams.

These days I'm especially energized by AI-assisted engineering. I'm a committed Claude enthusiast, and I build LLM-powered applications and automation that ship into real products — not just demos. I care about typed, maintainable code and software that holds up in production.

Experience

  1. Software Engineer

    90K Capital

    Sep 2025 – Present
    Sofia, Bulgaria · Hybrid

    • Built complex UIs with React, Next.js, TypeScript, TailwindCSS, shadcn/ui, and Redux for custom HFT (high-frequency trading) platforms, ensuring scalability and maintainability.
    • Developed a client-facing B2B platform enabling institutional clients to execute orders directly through the system.
    • API Integration: Built Node.js/Express API gateways connecting front-end applications with backend services.
    • UX & Component Design: Designed reusable UI components, data-heavy dashboards, and complex AG Grid Enterprise tables and charts to support live market data workflows.
    • Code Quality & Practices: Applied strongly typed TypeScript, latest React/Next.js patterns, and internal UI toolkit packages for consistency across projects.
    • Maintained internal shared component libraries with Storybook, ensuring design and behavior consistency across products.
    • Cross-team Collaboration: Worked closely with quantitative engineers and backend developers to translate requirements into reliable user-facing solutions in an Agile workflow.

  2. Software Engineer

    Nexo

    Mar 2024 – Aug 2025
    Sofia, Bulgaria · Hybrid

    • Built complex UIs with React, Next.js, TypeScript, TailwindCSS, shadcn/ui, and Redux, ensuring scalability and maintainability.
    • Developed Node.js/Express API gateways connecting front-end applications to backend services.
    • Designed reusable UI components, data-heavy dashboards, and complex AG Grid enterprise tables and charts to improve user workflows.
    • Collaborated with quantitative engineers and backend developers to translate requirements into reliable user-facing solutions in an Agile workflow.

  3. Back-end Node.js / TypeScript Developer

    Pateplay

    Nov 2022 – Nov 2023
    Sofia, Bulgaria · On-site

    • Developed internal platforms, iGaming products, and features.
    • Built and maintained server-side APIs with TypeScript, Node.js, and MySQL.
    • Assisted with React.js front-end development, wrote unit tests with Jest, and fixed bugs reported in Jira.
    • Followed the release and deployment process using AWS CodeCommit, WebStorm, Jira, Confluence, and Slack.

  4. Back-end Developer

    DevriX

    Jun 2021 – Oct 2022
    Sofia, Bulgaria · Full-time

    • PHP web development for WordPress: theme and plugin modifications, plugin development, and WordPress Core contribution.
    • Built deployment pipelines via Buddy; daily workflow with Linux, Git, Asana, and Slack.
    • Worked across Gutenberg, WooCommerce, WP Bakery, Classic Editor, and WP Engine using JavaScript, jQuery, and MySQL.

Skills

Frontend
ReactNext.jsTypeScriptReact NativeTailwindCSSshadcn/uiMaterial UIReduxZustandAG GridTanStack QueryZodVite
Backend
Node.jsExpressPHPREST APIs
Data
PostgreSQLMySQLMongoDBDrizzle ORMPrismaSQL & NoSQL
DevOps & Cloud
AWSDockerVercel
Testing
VitestPlaywright
CMS
WordPress
AI
LLM integrationsClaudeCodexAnthropic & OpenAI APIsPrompt engineeringAI-powered automation

Projects

WealthLens

A personal investment portfolio tracker for ETFs, stocks, options, gold, and multi-currency cash. Holdings, cost basis, and P&L are derived from an immutable transaction ledger; money is handled with exact decimals; multi-currency via a pluggable FX provider; options-wheel tracking and money-weighted IRR. 164 tests with CI.

  • Next.js
  • React
  • TypeScript
  • Drizzle ORM
  • PostgreSQL
  • Auth.js
  • shadcn/ui
  • AG Grid
  • Recharts
  • Vitest
GitHub

References

“Working with Aleksandar at Nexo and 90K Capital was genuinely one of the highlights of those years. He's one of those rare people who never says “that's not my job” — every task, regardless of scope, gets his full effort. He learns fast, thinks independently, and is always open to feedback or a good technical debate. What impressed me most was his consistency — he showed up the same way every single day, and the results spoke for themselves. I'd work with him again without hesitation.”
Antoan Popov · Engineering Manager & Full-Stack TypeScript Engineer
Managed Aleksandar directly · May 2026

Contact

Looking for my next front-end or full-stack role — bonus points if there's some AI in the mix. Email is the fastest way to reach me.

alex89773@gmail.comLinkedInGitHub