Nikola Pejčinović, Developer in Belgrade, Serbia
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Nikola Pejčinović

Front-end Developer

Belgrade, Serbia

Toptal member since April 12, 2022

Expertise
Front EndJavaScriptReact.jsCSSAgile DevelopmentHTML5TypeScriptWeb DevelopmentSassREST APIGit
Bio

Nikola is an experienced front-end developer specializing in React technology and its ecosystem. To date, his professional experience includes work on various projects across different industries, from large-scale SaaS applications to small-scale landing pages. Scalability, reusability, and flexibility of software architecture are the main focus of Nikola's programming.

Portfolio

2.am
React, React Hook Form, TanStack Table, React Query, React Router, Chakra UI...
Cline Bot Inc.
React, Next.js, TypeScript, Git, JavaScript, SWR, HeroUI, React Hook Form, Zod...
FPFX Technologies
JavaScript, Next.js, React, Tailwind CSS, Radix UI, Recharts, Zustand, Zod...

Experience

  • CSS - 5 years
  • HTML - 5 years
  • JavaScript - 5 years
  • React - 5 years
  • Context API - 4 years
  • Redux - 4 years
  • TypeScript - 3 years
  • Next.js - 2 years

Preferred Environment

Windows, MacOS, Linux, Visual Studio Code (VS Code), GitHub, JavaScript, Pixel Perfect, Agile

The most amazing...

...project I've built is a reusable UI library used worldwide, which improved the scalability and development of a client company's large applications and systems.

Work Experience

Front-end Developer

2025 - PRESENT
2.am
  • Led front‑end development for roster upload and candidate management workflows, implementing robust progress tracking (percentage and ETA), exponential backoff polling, and clear error handling for bulk file uploads and manual candidate entry.
  • Built and refined key user flows for the Jurisdiction and Candidate Document Centers, including viewing, searching, and downloading single and multiple documents, with strong attention to edge cases, permissions, and failure states.
  • Standardized UI components and patterns across portals by extracting shared pieces (e.g., accordions, cards, dropdowns, banners, toasts) into a design-system-driven library, aligning implementations with Figma specs and accessibility guidelines.
  • Improved data discoverability through advanced table experiences: multi‑select, filtering, sorting, and free‑text search for candidates, documents, and locations, including UI and API integration for back-end-driven sorting and filtering.
  • Delivered end‑to‑end interfaces for exam locations and room management, enabling jurisdictions to configure locations, manage capacities and accommodations, and bulk‑assign candidates with accurate real‑time counts and validations.
  • Enhanced application feedback mechanisms by implementing a consistent notification system (banners, toasts, and inline errors) that surfaces system events and user actions clearly while reducing noise and ambiguity.
  • Resolved numerous production UI and functional issues across multiple portals, improving reliability and polish by fixing layout glitches, navigation problems, inconsistent states, and back-end/front-end integration bugs.
  • Collaborated with designers and back-end engineers to refine APIs, error contracts, and UX flows, ensuring complex domain logic (exam administration, accommodations, document governance) is presented in a simple, intuitive interface.
Technologies: React, React Hook Form, TanStack Table, React Query, React Router, Chakra UI, Zod, Axios, Vite

Senior Front-end React Developer

2025 - 2025
Cline Bot Inc.
  • Developed, maintained, and improved the Cline Dashboard, the core application for managing organizations, billing, usage, and agent configuration for the Cline AI IDE agent.
  • Implemented Remote Config Admin Dashboard—a full-fledged admin module enabling real-time synchronization of configuration settings between the dashboard and the Cline AI agent.
  • Built and optimized multiple organization management features, including support for users belonging to multiple organizations, organization deletion, and organization renaming.
  • Improved invitation and membership workflows, resolving race conditions, adding missing UI flows, enabling deletion of invites and members under complex role rules, adding pagination, and removing UI inconsistencies.
  • Fixed major UX and state-management bugs: layout inconsistencies, toggle and input sync issues, unclear disabled states, missing error handling, and visual bugs caused by optimistic updates in modals.
  • Refactored the error-handling architecture across the dashboard to consistently rely on custom error interface implementation, enforcing a clean pattern for back-end thrown exceptions, timeout handling, and cross-component error normalization.
  • Improved the invitation acceptance flow by fixing a hydration and state synchronization issue requiring page refresh due to stale data.
  • Built a UI for user profile updates, including support for changing display names.
  • Added UX improvements for card and billing flows, including clearer disabled states and error toasts for failed invite operations.
  • Delivered a POC for metrics and charts using Apache ECharts, demonstrating high-performance and scalable visualization patterns for usage, team data, and future analytics modules.
Technologies: React, Next.js, TypeScript, Git, JavaScript, SWR, HeroUI, React Hook Form, Zod, Stripe, OpenAPI, Axios

Front-end Developer

2023 - 2025
FPFX Technologies
  • Contributed to developing and maintaining cross-project reusable front-end solutions, drastically improving development scalability, decreasing development time, and significantly saving company and clients' budgets.
  • Improved application UI using Tailwind CSS with other business-specific 3rd-party tools and libraries (Recharts, Radix UI, React Hook Form, and more).
  • Developed and maintained reusable UI solutions for multi-project mono repositories across different applications.
  • Improved application performance by employing SSR rendering strategies of the latest Next.js versions (13+).
  • Collaborated closely with management, operations, and delivery teams to estimate tasks and workload effectively.
  • Engineered complex fintech-related calculations, performing data remapping on the front end and creating dynamic data visualizations using the Recharts charting library.
  • Resolved critical bug fixes and incidents, ensuring smooth functionality and uptime.
  • Took full ownership of the code production lifecycle, from implementation to deployment across multiple environments (staging, development, and production).
  • Handled on-call responsibilities as the first responder for critical incidents and was the on-call engineer responsible for production deployments across environments and applications.
  • Conducted minor C#.NET development tasks in coordination with the back-end development team, enhancing and updating existing endpoints, services, controllers, and queries to align with evolving front-end requirements.
Technologies: JavaScript, Next.js, React, Tailwind CSS, Radix UI, Recharts, Zustand, Zod, React Hook Form, FontAwesome, Turborepo, TypeScript, Server-side Rendering (SSR), C#.NET

Front-end Developer

2023 - 2023
Kalshi Inc
  • Refactored portions of React CSR codebase by employing various Next.js SSR strategies.
  • Resolved authentication/authorization issues at the front-end level.
  • Improved application UI using Material UI library with other various business-specific 3rd-party tools and libraries.
Technologies: Front-end, React, Next.js, TypeScript, MobX, Server-side Rendering (SSR), Axios, Material UI, TradingView, Responsive Web Design (RWD)

React Developer

2023 - 2023
Stubben Edge Group Limited
  • Contributed to the development of a complete front-end architecture from the ground up.
  • Maintained and improved legacy code by constantly proposing fresh solutions and implementing new features according to industry standards and best practices.
  • Resolved time-sensitive critical bug fixes across multiple React-based projects.
Technologies: JavaScript, React, HTML, CSS, API Integration, APIs, Application State Management, Tailwind CSS, User Experience (UX), User Interface (UI), Front-end Design, Front-end Development, Responsive Web Design (RWD)

Senior Front-end Developer

2022 - 2023
iptiQ Life S.A. UK
  • Developed a reusable analytics tool that monitors customer usage across the whole application using Tealium tag management.
  • Created a significant knowledge base for other team members for further development by delivering a proof-of-concept payment solution.
  • Contributed to the development of a complete front-end architecture from the ground up.
  • Maintained and improved a client's legacy third-party solution integration according to industry standards and best practices.
  • Created customized components according to design specifications using the MUI library.
Technologies: React, CSS, HTML, JavaScript, Redux, Redux-Saga, Formik, Yup, Material UI, Storybook, React Router, GraphQL, Tealium, Tealium iQ Tag Management, Unit Testing, Front-end, APIs, REST APIs, React Redux, Application State Management, Reusable Libraries, Agile, User Experience (UX), User Interface (UI), Responsive Design, ECMAScript (ES6), TypeScript, API Integration, JSX, Test-driven Development (TDD), HTML5, DOM, REST, Git, Web Development, Cypress, Single-page Applications (SPAs), Front-end Design, Front-end Development, Responsive Web Design (RWD)

React Developer

2021 - 2022
Quantox Technology
  • Implemented major solutions and critical bug fixes related to cross-browser and cross-device optimization issues, drastically improving the client's customer retention rate and keeping more than 60,000 active users on their platform.
  • Developed the project architecture from the ground up.
  • Maintained and improved legacy code by constantly proposing new solutions and implementing new features according to industry standards and best practices.
  • Helped team members with the challenges faced during the project development process and problem-solving.
  • Developed and maintained internal projects and applications and actively contributed to the company's non-commercial endeavors like blockchain hackathons, research and development activities, and workshops.
Technologies: Next.js, Node.js, React, Redux, SCSS, Sass, Axios, React Hook Form, Context API, Material UI, Jest, Enzyme, BrowserStack, DotCMS, TypeScript, Pixel Perfect, HTML, Styled-components, GitHub, MacOS, Visual Studio Code (VS Code), JavaScript, CSS, Lerna, Functional Programming, Front-end, APIs, REST APIs, React Redux, Application State Management, Reusable Libraries, Agile, User Experience (UX), User Interface (UI), Responsive Design, ECMAScript (ES6), API Integration, JSX, Test-driven Development (TDD), Tailwind CSS, HTML5, DOM, REST, Git, Web Development, WebSockets, Cypress, Single-page Applications (SPAs), Front-end Design, Front-end Development, Server-side Rendering (SSR), Responsive Web Design (RWD)

Lead React Developer (Freelance)

2021 - 2022
Starcier
  • Led a team of three developers in building an enterprise platform.
  • Helped and mentored team members with the challenges faced during the project development process and problem-solving.
  • Scored a significant client for the company by delivering a time-critical proof-of-concept web app within two months.
  • Created a global portal through which all of the client's professionals can gain access to their portfolio companies, a worldwide network of high-profile SMEs and C-level executives, relevant business insights, and research.
  • Implemented an authentication infrastructure using the Okta platform.
  • Developed a complete front-end architecture from the ground up.
Technologies: JavaScript, React, Redux, Material UI, Axios, Lodash, React Hook Form, Context API, Okta, Pixel Perfect, HTML, Styled-components, GitHub, MacOS, Visual Studio Code (VS Code), CSS, Functional Programming, Front-end, APIs, REST APIs, React Redux, Application State Management, Reusable Libraries, Agile, User Experience (UX), User Interface (UI), Responsive Design, ECMAScript (ES6), TypeScript, API Integration, JSX, Tailwind CSS, HTML5, DOM, REST, Git, Web Development, Single-page Applications (SPAs), Front-end Design, Front-end Development, Responsive Web Design (RWD)

React Developer

2020 - 2021
JAGGAER
  • Created customized components according to design specifications using Material-UI, styled-components, and styled tools libraries.
  • Contributed to the development of a core part of the platform—a form designer feature with which users can create custom forms according to their businesses requirements.
  • Managed custom application components library and codebase documentation using React Styleguidist.
  • Participated in developing and maintaining the overall codebase and project architecture mono repository using Lerna.
  • Refactored critical parts of the application into reusable modules and separate npm packages.
  • Improved the application's development and maintenance by constantly proposing reusable and performance-oriented solutions.
Technologies: React, Redux, JSON, Schemas, Axios, Context API, Lodash, Material UI, Lerna, Webpack, Styled-components, NPM, Pixel Perfect, Storybook, HTML, GitHub, Linux, Visual Studio Code (VS Code), JavaScript, CSS, TypeScript, Functional Programming, Front-end, APIs, REST APIs, React Redux, Application State Management, Reusable Libraries, Agile, User Experience (UX), User Interface (UI), Responsive Design, ECMAScript (ES6), API Integration, JSX, Test-driven Development (TDD), HTML5, DOM, REST, Git, Web Development, Single-page Applications (SPAs), Front-end Design, Front-end Development, Responsive Web Design (RWD)

React Developer (Freelance)

2019 - 2020
PatronGG
  • Implemented an authentication infrastructure using Auth0.
  • Created highly customized and reusable React components according to design specifications using the Material-UI library.
  • Translated complex business requirements into a well-organized front-end architecture.
  • Delivered cross-browser optimization for all application features.
  • Built features and functionalities with speed and performance in mind.
Technologies: JavaScript, React, Context API, Formik, Material UI, Normalizr, Lodash, React Router, Yup, Auth0, Jest, Enzyme, JSDoc, Pixel Perfect, HTML, GitHub, Visual Studio Code (VS Code), CSS, Functional Programming, Front-end, APIs, REST APIs, Application State Management, Reusable Libraries, Agile, User Experience (UX), User Interface (UI), Responsive Design, ECMAScript (ES6), API Integration, JSX, Test-driven Development (TDD), HTML5, DOM, REST, Git, Web Development, Single-page Applications (SPAs), Front-end Design, Front-end Development, Responsive Web Design (RWD)

Front-end Developer

2018 - 2020
Teodesk
  • Contributed to the design, architecture, and development of a project management application.
  • Resolved critical bugs concerning the UI and front-end architecture, which fixed important performance issues.
  • Refactored and organized the application's UI codebase using the BEM methodology with Less, making it more scalable and maintainable.
  • Implemented a task tracking Gantt chart, which improved the overview and monitoring of the overall project progress through a transparent timeline.
  • Ensured the high quality of the project by proposing and implementing enhancements.
  • Developed and maintained other internal projects and applications.
Technologies: JavaScript, React, Knockout (Knockout.js), Less, BEM, Pixel Perfect, HTML, GitHub, Visual Studio Code (VS Code), CSS, Functional Programming, Front-end, APIs, REST APIs, Application State Management, Reusable Libraries, Agile, User Experience (UX), User Interface (UI), Responsive Design, ECMAScript (ES6), API Integration, JSX, HTML5, DOM, REST, Git, Web Development, Single-page Applications (SPAs), Front-end Design, Front-end Development, Responsive Web Design (RWD)

Front-end Developer

2017 - 2018
Diamond Code
  • Developed an app that optimized business processes and boosted communication between over 50 clients' operators and employees.
  • Built new features across different application modules, drastically improving the overall UI/UX.
  • Implemented important bug fixes, which improved the application's customer retention rate.
  • Upgraded the codebase by refactoring legacy code and reorganizing the application architecture.
Technologies: JavaScript, React, Sass, Pixel Perfect, HTML, GitHub, Visual Studio Code (VS Code), CSS, Functional Programming, Front-end, APIs, REST APIs, Application State Management, Reusable Libraries, Agile, User Experience (UX), User Interface (UI), Responsive Design, ECMAScript (ES6), API Integration, JSX, HTML5, DOM, REST, Git, Web Development, Single-page Applications (SPAs), Front-end Design, Front-end Development, Responsive Web Design (RWD)

Experience

Tangram iptiQ

https://lifeprotect.zurich.ch/
A React-based application for digital insurance. As a front-end developer on this project, I implemented new features, resolved critical bug fixes, planned and rethought project architecture with the team, developed and maintained a reusable analytics tool, and integrated third-party solutions and services.

GA Compass

GA Compass is a globally available web application that lets investment professionals access portfolio companies, a global network of high-profile SMEs and C-level executives, relevant business insights, and research within business models, pricing, digital marketing.

Jaggaer One

https://www.jaggaer.com/solutions/
Jaggaer One is a SaaS application developed as a multi-project mono repository. It is an all-in-one procurement software that provides modular solutions from source to pay, like eProcurement, spend analytics, inventory management, supply chain collaboration, and more.

Skills

Libraries/APIs

React, React Redux, Context API, REST APIs, Node.js, Formik, Normalizr, Lodash, React Router, Yup, React Hook Form, Redux-Saga, MobX, Recharts, Zod, TanStack Table, React Query, HeroUI, Stripe, OpenAPI

Tools

JSX, GitHub, Lerna, Git, Auth0, Webpack, BrowserStack, NPM

Languages

JavaScript, CSS, TypeScript, ECMAScript (ES6), HTML5, HTML, Sass, Less, SCSS, GraphQL, C#.NET

Paradigms

Agile, Test-driven Development (TDD), REST, Responsive Web Design (RWD), Functional Programming, BEM, Unit Testing

Frameworks

Redux, Next.js, Jest, Tailwind CSS, Knockout (Knockout.js), Material UI, Cypress

Platforms

Visual Studio Code (VS Code), Windows, MacOS, Linux, DotCMS, Tealium

Storage

JSON

Other

Front-end, Application State Management, Reusable Libraries, Responsive Design, API Integration, DOM, Single-page Applications (SPAs), Front-end Design, Front-end Development, Styled-components, Storybook, Axios, Enzyme, Pixel Perfect, APIs, User Experience (UX), User Interface (UI), Web Development, WebSockets, Server-side Rendering (SSR), JSDoc, Okta, Tealium iQ Tag Management, Schemas, TradingView, Radix UI, Zustand, FontAwesome, Turborepo, Chakra UI, Vite, SWR

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