William Hein
Verified Expert in Engineering
JavaScript Developer
Aarhus, Denmark
Toptal member since June 3, 2026
William builds software for the hard cases, including real-time trading infrastructure, multi-agent AI automation, and B2B sales-intelligence dashboards. He has spent more than five years shipping production code in TypeScript, Go, Python, and several other languages, building autonomous systems that run against unreliable real-world data. William takes a project from the first commit through production and replies fast.
Portfolio
Experience
- JavaScript - 8 years
- Python - 7 years
- Git - 7 years
- TypeScript - 6 years
- SQL - 6 years
- Node.js - 6 years
- PHP - 5 years
- Go - 4 years
Preferred Environment
MacOS, Visual Studio Code (VS Code), Git, Docker, Podman, PostgreSQL, Cloudflare, Amazon Web Services (AWS), TypeScript, Python, Code Auditing, Code Refactoring, Performance Optimization, Artificial Intelligence (AI), Documentation, GitHub, Web Development, Databases, Code Review, Back-end Architecture, Back-end Development, REST APIs, Communication, Multi-tenant Architecture, Debugging, Database Architecture, Full-stack, Algorithms, Software as a Service (SaaS), Websites, Web, Web App Development, Front-end Development, NPM, Web Security, Web Applications, Software Development, Software Design, Development, Database Management
The most amazing...
...solution I've built is sentient-evolve AI agents that evolve a trading strategy, write the code, and fix their own compile errors unattended.
Work Experience
Founder and Lead Developer
Self-employed
- Built the Murph sales intelligence dashboard leveraging Next.js, TypeScript, and PostgreSQL from the initial commit, enhancing LinkedIn outreach SaaS through ICP-driven commenting.
- Architected a multi-agent system that autonomously evolves trading algorithms.
- Engineered a feedback loop where specialized AI agents analyze performance, generate code improvements, and self-correct compilation errors in isolated Docker containers.
- Built iq-data-server in Go to ingest real-time WebSocket streams from 7+ major exchanges, including Binance, Bybit, and Coinbase.
- Hardened the high-frequency ingest path by eliminating race conditions and head-of-line blocking, bounding unbounded tracker growth, and removing unsafe upsert patterns.
- Enhanced a production Flutter iOS and Android app, built a companion MusicFix web utilizing Next.js with 300+ files from scratch, and developed a Firebase back end with strict Firestore security rules.
- Engineered a distributed scraping platform with a producer-consumer architecture and worker threads handling high-concurrency ingestion from Ticketmaster and other vendors.
- Developed rolly-extension, a Chrome extension for automated trading, implementing human-emulation algorithms to bypass modern bot detection systems.
- Implemented human-emulation algorithms such as randomized event dispatching and heuristic DOM recovery to bypass modern bot detection systems.
- Led development of a scalable, real-time data processing service in Rust, handling millions of events daily with ultra-low latency. Improved system throughput by 40% and reduced server costs.
Senior Applied AI Engineer (Search & Retrieval)
eCommerce Comparison Platform
- Evaluated the platform’s AI-powered search and matching capabilities.
- Identified root causes of search accuracy and product matching issues.
- Improved search relevance across the platform. Assess and enhance semantic search and embedding-based retrieval workflows.
- Improved product matching and entity resolution quality. Proposed scalable technical solutions to improve search quality and user experience.
- Helped define the future technical direction of the platform’s search and matching systems.
CTO & Lead Engineer
CryptoIQ
- Built the autonomous strategy-evolution engine, sentient-evolve: a TypeScript orchestrator that runs three specialized AI agents.
- Ensured each candidate strategy runs in an isolated, non-root Docker container with resource limits, every attempt recorded in SQLite, committed to a timestamped git branch on success, and rolled back on failure, with evolutions running in parallel.
- Built a Go-based market data platform that ingests real-time WebSocket feeds from Binance, Bybit, Coinbase, Kraken, and OKX, normalizes exchange data into a unified model, and stores high-frequency trading data in PostgreSQL.
- Supervised processes with automatic backoff-restart and a backfill path that closes gaps in the historical record after any disconnect. I built this data server and still operate and maintain it in production today.
- Built a Chrome extension that automates trading using human-like interactions, randomized events, and DOM recovery techniques. Designed with strong containment, audit logging, and rollback mechanisms for reliable execution.
- Reviewed Rust code regularly for performance bottlenecks, memory leaks, and idiomatic best practices.
- Onboarded quickly onto complex Rust codebases, delivering new features within tight deadlines.
Developer
XSpectar
- Built xspectar-nft-daemon, a local WebSocket server for subscribing to the XRP Ledger stream, enabling real-time updates for NFT transactions.
- Implemented a verification system for user token holdings, ensuring secure and verified participation.
- Processed live transactions to verify NFT ownership and instantly update user roles across Discord and internal databases, enhancing user experience.
Technical Support
True Energy A/S
- Provided technical support for app-related issues, acting as a bridge between users and the engineering team to resolve bugs in the EV charging platform.
- Resolved technical issues for clients, ensuring high levels of customer satisfaction.
- Implemented process improvements that enhanced response times for technical inquiries.
Experience
MusicFix — Cross-platform App & Web Platform
https://musicfix.dkSeparately, I built the companion web platform, musicfix-web, from scratch in Next.js and TypeScript, with Tailwind, Radix UI, TanStack Query, geo-based search, internationalization, and the same Stripe and Firebase integration. The result is a single app and a single web surface sharing a single back end.
Orpheus Festudvalget — Ticketing & Entry Platform
https://orpheus.dkThe system issues tickets as generated PDFs (Puppeteer) and delivers them by email, then validates entry through QR codes scanned in a dedicated admin panel that tracks attendance in real time.
To keep design iteration fast, I wrote a custom "Webflow Compiler" build pipeline that takes the static Webflow export and rewires it into pages driven by the live API, so visual updates ship without touching back-end code. I later modernized the whole stack into a containerized Docker/Podman deployment and hardened the checkout flow with idempotency keys and session fixes.
ChargeControl — Native macOS Battery Tool for Apple Silicon
https://github.com/will2022/ChargeControlThe SwiftUI app and menu-bar UI communicate with the daemon over an XPC protocol, and a cc CLI exposes the same controls for automation. The daemon reads real-time telemetry (wattage, voltage, amperage, cycle count, and thermal sensors) and persists it to a SQLite history database with daily aggregation, charted in-app with Swift Charts.
Safety features include heat protection that pauses charging above a temperature threshold, smart hysteresis to avoid micro-cycling, and MagSafe LED state sync. I shipped v1.0.0 and distributed it through a dedicated Homebrew tap.
TypeScript Bridge Between Discord and Minecraft
https://github.com/will2022/OverflowBridgeEducation
Higher Technical Examination in Communication/IT A & Programming B
Tekniske Gymnasium - Vejle, Denmark
Skills
Libraries/APIs
Node.js, React, jQuery, Stripe, REST APIs, React Router, React Query, Vue, NetSuite API, SQLAlchemy, Pydantic, Coinbase API, Twilio API, Stripe API, Claude API, Google API, Stripe Connect, Auth, API Development, Google Drive API, OpenAI API, Shopify API, Highcharts, Pandas, NumPy, GraphQL API, Stripe Connect API, React Navigation, Vue 3, Social Media APIs, Google APIs, LinkedIn API, SOAP APIs, Google Calendar API, WhatsApp API, Google Maps SDK, WooCommerce API, Google Maps, Shopify Storefront API, HubSpot API, Azure API Management, Google Places API, Liquid, Mapbox API, Intercom API, Entity Framework, Playwright, Puppeteer, Three.js, Socket.IO, WordPress API, OpenAPI, Chart.js, WebGL, Google Maps API, Mapbox GL, WebRTC, Inertia, ServiceNow REST API, Babylon.js, Facebook API, SolidJS, FFmpeg, QuickBooks API, Protobuf, Binance API
Tools
Git, Claude, Claude Code, GitHub, Zapier, Auth0, Terraform, Figma, Apache, Trello, Microsoft Graph, Microsoft Web Services, Amazon Simple Notification Service (SNS), Expo, LearnDash, ChatGPT, Prisma, Jira, Firebase Authentication, Firebase Cloud Messaging (FCM), Webpack, Docker Compose, BigQuery, Gradle, Sentry, Wix, NGINX, Shopify Plus, Celery, Tableau, VPN, Azure OpenAI Service, Codex, NPM, GIS, Google Workspace, Atlassian, Slack, Seos, SAP Fieldglass, Whisper, Odoo, Microsoft Power BI, Spreadsheets, Algorithm Design, Pytest, Google Sheets, Microsoft Excel, Amazon CloudWatch, GitLab CI/CD, Azure Application Insights, Microsoft Power Apps, Microsoft Power Pages, SAP Remote Function Call (RFC), Adobe Experience Design (XD), Prettier, Visual Studio, Amazon EKS, RabbitMQ, AWS Key Management Service (KMS), AWS IAM, Amazon Elastic Container Registry (ECR), Amazon ElastiCache, Flyway, n8n, Windsurf, Google Analytics, Astro, ComfyUI, SuiteTalk, Android Auto, ServiceNow App Engine Studio, Apple Push Notifications, Amazon Elastic Container Service (ECS), Vitest, Shadcn, Shadcn UI, Acumatica, Hotwire, Bitbucket, Fastlane, Zoom, Manhattan OMS, Adobe CC, Adobe, Helm, TestFlight, SAP Crystal Reports
Languages
TypeScript, Python, Go, PHP, C#, SQL, JavaScript, Swift, HTML5, CSS, HTML, Java, Shopify Liquid, Rust, CSS3, Zoho Deluge, GraphQL, C#.NET, Python 3, SCSS, Elixir, VB.NET, Visual Basic .NET (VB.NET), Excel VBA, Visual Basic for Applications (VBA), C++, Dart, Bash, SuiteScript 1, SuiteScript 2, Snowflake, Kotlin, Gremlin, Ruby, XML
Frameworks
Django, Flask, Laravel, CodeIgniter, Symfony, Next.js, Express.js, Flutter, React Native, OAuth 2, Redux, LangGraph, Angular, ASP.NET, Svelte, Bootstrap, Tailwind CSS, JSON Web Tokens (JWT), NestJS, SharePoint Framework (SPFx), Material UI, Laravel 11, Nuxt.js, Nuxt 3, Quasar, Vanilla JS, Shopify Hydrogen, AWS HA, .NET, .NET 8, ASP.NET Core, .NET Core, Ruby on Rails (RoR), Sitecore JSS, Jest, Ruby on Rails 7, Selenium, VB.NET WinForms, Core ML, Unity, Spring AI, Spring, Spring Boot, gRPC, Swagger, Ionic, Scrapy, AdonisJS, SwiftUI, Agentic Frameworks, Windows PowerShell, Streamlit, LlamaIndex, Fastify, Ghidra, Electron
Paradigms
Code Refactoring, Automation, Visual Design, UI Design, Model Context Protocol (MCP), Database Design, DevOps, Event-driven Architecture, Web App Design, Back-end Architecture, Serverless Architecture, Microservices, Real-time Systems, Mobile Development, UX Design, Web Performance Optimization (WPO), Web UI Design, Web UX Design, Mobile Design, Mobile UI Design, Testing, Refactoring, Search Engine Optimization (SEO), Marketplace Platforms, Automated Testing, Continuous Delivery (CD), Continuous Integration (CI), Agile, Software Testing, Object-oriented Design (OOD), Microservices Architecture, Role-based Access Control (RBAC), Microfrontends, Fast Healthcare Interoperability Resources (FHIR), ETL, Test-driven Development (TDD), HIPAA Compliance, E2E Testing, Event-driven Design (EDD), Kanban
Platforms
MacOS, Visual Studio Code (VS Code), Amazon Web Services (AWS), Windows, Linux, Firebase, iOS, Kubernetes, WordPress, Vercel, Azure, Shopify, AWS Lambda, Azure Functions, SharePoint, AWS IoT, Webflow, HubSpot, Velo by Wix, Mapbox, WooCommerce, Mobile, Twilio, Amazon EC2, Microsoft Power Automate, Web, Heroku, Sitecore Experience Manager Cloud (XM Cloud), Cloud Native, Clerk, LangSmith, Kajabi, Microsoft Power Platform, Drupal, Drupal 10, Drupal 9, Docker, Netlify, Android, Google Cloud Platform (GCP), Databricks, Apache Kafka, Oracle, ARCore, Bybit, Windows Server, Android TV, PostHog, Cin7, OpenTok, Google Ads, Algolia, Craft CMS, ServiceNow Software Asset Management (SAM), TaxJar, Zoho CRM, Shippo, Replit, RevenueCat, Telnyx, Mixpanel, Klaviyo
Storage
PostgreSQL, NoSQL, MySQL, MariaDB, MongoDB, Amazon S3 (AWS S3), Database Migration, JSON, Amazon DynamoDB, Databases, Elasticsearch, Microsoft SQL Server, Database Architecture, Data Validation, Data Pipelines, SQLite, WP Engine, Azure SQL, Neo4j, Azure Cosmos DB, Cloud Firestore, Data Integration, Database Modeling, Database Management, Redis, Amazon Aurora, Google Cloud, PostGIS, IndexedDB, Cloud Deployment, Azure Cloud Services, Azure Active Directory, Object Storage, Cassandra, ClickHouse, Microsoft Entra ID
Industry Expertise
Project Management, Automotive, Web Design, Healthcare, Social Media, Telecommunications, Marketing
Other
FastAPI, Scraping, Full-stack, Trading, Information Technology, Front-end, Full-stack Development, Google, Programming, Software Architecture, API Integration, Back-end, Code Auditing, Payment APIs, Performance Optimization, Cursor AI, Data Engineering, Prompt Engineering, Artificial Intelligence (AI), Supabase, Architecture, Design Systems, Front-end Architecture, AI Agents, AI Automation, Large Language Models (LLMs), Documentation, APIs, Webhooks, OpenAI, Third-party API orchestration with webhooks/polling, Computer Vision, NetSuite, NetSuite CRM, Agentic AI, Multi-agent Systems, User Interface (UI), User Experience (UX), Workflows, Application Modernization, Document Processing, PDF, LangChain, Figma to Shopify, Gemini, High-performance Design, Distribution, Real-time Data, Query Optimization, Containerization, CI/CD Pipelines, Fintech, AI Development, Product Management, Dashboards, Cloud Architecture, Workflow Automation, Authentication, Authorization, Live Chat, System Architecture, API Design, Command-line Interface (CLI), Startups, Azure Function App, App Design, eCommerce, Agentic Coding, Zoho, Microsoft Graph API, Web Scraping, Data Mining, Data Analysis, Amazon API Gateway, Identity & Access Management (IAM), Geolocation, Mobile Architecture, AI-generated Code, Performance, Progressive Web Applications (PWAs), Web Crawlers, Infrastructure, Fractional CTO, Web Development, Mobile Apps, Financials, Payments, Invoicing, Code Review, Mobile Back-end, Mobile App Development, AWS Cloud Architecture, Back-end Development, Payment Processing, SaaS Architecture, IT Audits, Minimum Viable Product (MVP), MVP Design, Agentic Workflow Design, Communication, Tailwind UI, Troubleshooting, Database Optimization, Multi-tenant Architecture, OutSystems, Education Technology (Edtech), Learning Management Systems (LMS), Debugging, CTO, Automotive Systems, Network Engineering, Censorship Measurement, Retrieval-augmented Generation (RAG), ReAct Agents, Third-party APIs, Third-party Integration, Workflow, Recommendation Systems, SaaS, AI Architecture, DocuSign, Analytics, Metabase, Vite, Solution Architecture, API Documentation, System Integration, Data Visualization, RESTFul APIs, Charts, Database Schema Design, GraphDB, Data Modeling, Data Migration, Consulting, Booking, Integrations, Website Integration, Deployment, Marketplaces, Firebase Cloud Functions, Firebase Hosting, Vibe Coding, Delivery, Data Dictionary Development, Cloud, WordPress Themes, WordPress Design, LinkedIn, Database Normalization, Social Networks, CMS, Vector Databases, Web Content Accessibility Guidelines (WCAG), WCAG 2, VPS, Headless Commerce, Inventory Management, Google Calendar, Scheduling, Video APIs, UI Animation, Symphony, Content Delivery Networks (CDN), Supabase Auth, Headless WordPress, UI Development, Network Protocols, Distributed Systems, Migration, Operating Systems, TCP/IP, HTTP, Algorithms, ServiceNow, Generative Artificial Intelligence (GenAI), Natural Language Processing (NLP), Technical Consulting, Economics, Anthropic, Real Estate, Data Scraping, LinkedIn Sales Navigator, Custom Shopify Apps, Shopify Theme, Software as a Service (SaaS), Healthcare Software, Privacy, Enterprise Resource Planning (ERP), QA Automation, SOAP, Shipping, HubSpot CRM, AI Chatbots, MDM, Mobile Device Management (MDM), Website Design, Website Redesign, Websites, Chrome Extensions, Azure AI Document Intelligence, Hands-on Implementation, High Code Quality, Leadership, Scaling, Technical SEO, IT Consulting, Reviews, Bun, AI-assisted Development, Software Engineering, API Management, Terraform Cloud, Amazon Bedrock AgentCore, Data Processing, Workflow Automation & System Integration, Product Ownership, Cloud Engineering, Cloud Platforms, Web App Development, Figma to React, Front-end Development, Technical Architecture, Concurrency, Multi-tenant SaaS, Operations Management Systems (OMS), Warehouse Management System (WMS), Third-party Logistics, Server-side Rendering (SSR), SAP, Financial APIs, Cross-platform App Development, GitHub Actions, Web Security, Web Applications, Product Development, Software Development, Scalable Architecture, Jira Administration, System Administration, Data, Pipelines, Problem Solving, Digital Design, Single Sign-on (SSO), Sitecore, Software Design, Billing, Convex, RESTful Web Services, Vendor Management, Real-time Tracking, Multiplayer, API Connectors, Middleware, Engineering Management, App Store Connect, Google Play Store, Optimization, AI Voice Agents, ElevenLabs Solutions, Speech-to-Text (STT), Medical Software, Email Automation, App Store, Dashboard Development, Lean, Product Design, Web App UX, RTL, Order Management, Ruby on Rails 8, HubSpot Sales Hub, Reporting, Product Mindset, AI Programming, AI Design, Mobile-first Design, Videos, Quantitative Finance, Financial Modeling, Quantitative Analysis, Quantitative Modeling, Executive Leadership, IT Management, Finance APIs, Investment Banking, Securities, Finance, Statistical Modeling, AI Governance, AI Testing, OOP Designs, Software Documentation, Package Design, HubSpot Service Hub, Landing Pages, Google Analytics 4 (GA4), Web Marketing, Affiliate Marketing, Automations, Data Extraction, Scripting, Supply Chain, Supply Chain Operations, Apple Pay, Google Pay, Early-stage Startups, Accounting Systems, Azure Data Lake, Data Build Tool (dbt), Data Quality, Data Warehousing, Financial Data, Financial Market Data, Intercom, Shopify Customizations, Shopify Design, Meta API, Payment Gateways, CSV, Excel Expert, Excel 365, Azure Blog Storage, Cloud Security, Microsoft Azure, role based access control, AI Engineering, Data Analytics, Log Analysis, Microsoft Dataverse, Development, Content Management Systems (CMS), Digital Marketing, Google SEO, Push Notifications, Application Security, Bots, In-app Purchase (IAP), In-app Purchases, Subscription Management, Subscriptions, User Authentication, Financial Markets, Real-time Communication (RTC), Cross-browser Compatibility, Technical Writing, Web Standards, Community, Developer Relations, BAPI, Business Application Programming Interface, SAP Open Data Protocol (OData), Data Encryption, Data Governance, Browser Automation, Legacy Code, Subscription Processing, Portals, Web Administration, Training, Data Security, Spotify, Responsive UI, Graphic Design, Excel Macros, Bookkeeping, Generative AI, Rich Communication Services (RCS), Security Architecture, Bug Fixes, iPhone, Large Data Sets, RSS Feeds, Production Engineering, Control Systems, DNP3, Modbus, Modbus Protocol, NATS, SCADA, Podman, Cloudflare, ESLint, Non-fungible Tokens (NFT), Crypto, WebSockets, GRAPH, Discord, Security, 3D Maps, Amazon RDS, Technical Leadership, WordPress Plugins, Data Science, Geospatial Data, Machine Learning, Machine Learning Operations (MLOps), AI Integration, OAuth, Video Processing, Intuit QuickBooks, User Flows, AI Tools, Integration, HL7, SQL Server, eCommerce UX, Autonomous AI, ACH Payments, Video Calling, Serverless, Squarespace, Cloud Infrastructure, IIS, AI Agent Orchestration, QuickBooks Online, Observability, Hooks, PWA, Maps, Domain Migration, Object Detection, Optical Character Recognition (OCR), Deep Learning, CRM, Emailers, Outlook, RedPanda, Data Infrastructure, GCP DevOps, Probability Theory, Amazon Neptune, Hotels & Lodging, Reservation Flow, Google Tag Manager, Human-in-the-loop (HITL), Meta Ads, TikTok Ads, OData, Base44, Fashion, Luxury, Luxury Goods, Advertising Technology (Adtech), CarPlay, ServiceNow CSM, ServiceNow Integration Hub, ServiceNow Performance Analytics, Solution Design, CAG, AI Avatars, Avatars, Chatbots, Large Language Model Operations (LLMOps), RAG Systems, 3D Rendering, Business Process Automation, Facebook, Leads, Lead Generation, HIPAA, TaxJar API, LLM Integration, Web Push Notifications, Agentic AI Systems, Image Recognition, SDET, Lovable, App Store Acceptance Process, QR Codes, HIPAA Electronic Data Interchange (EDI), SOC 2, Healthtech, Railway, I am buying a company and want an expert to review the code to make sure that it does what the seller says it does and as a second step to make it more efficient so that I can copywrite it, AWS Elemental MediaConvert, AWS DevOps, Video Encoding, Life Science, AI-based Content SEO, AI-enabled Search, Answer Engine Optimization (AEO), Advisory, Robotic Process Automation (RPA), Agentic RAG Systems, RAG Architecture, Scalability, Pinecone, Conversational AI, Knowledge Graphs, Video Streaming, State Machines, Event-driven Systems, Financial Services, Turborepo, Monorepos, Mortgages, Accessibility, Malware Analysis, Apache Cassandra, Big Data, Orchestration, SEO Tools, Taxation, Monday.com, HubSpot Marketing Hub, Data Aggregation, GeoJSON, 2D Image Processing, Image Processing, Telephony, Manufacturing Resource Planning (MRP), Apify SDK, FaunaDB, claw code, Process Mining, Polars, Microsoft Planner, Traffic Analysis, Web Analytics, Website Traffic, Network Management, White Labeling, Attention to Detail, Speech Recognition, Text-to-Speech (TTS), Large Language Models / LLM, CPaaS, Messaging, Planning, Manual QA, IoT, Chargebee
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