SungHyun Kim

• Joined a company that began as a toy project and helped grow it to 10K DAU and 300M KRW in monthly revenue, through to its acquisition.

Product Features and Operations

• Proposed and built "Create user settings quickly with AI": pinpointed the onboarding barrier by combining usage patterns, community feedback, and hands-on use of the service.
• Fixed chat data synchronization issues: consolidated scattered chat fetching, streaming, and cache invalidation logic, and redesigned it so server data is the single source of truth.
• Established update announcement and customer support processes and moved them in-service, replacing operations that had depended on external social media.

Product Performance

• Cut network traffic by more than 50% by removing duplicate API calls and reworking how data refreshes when streaming completes.
• Raised the main page Lighthouse performance score from 35 to 70: replaced JS-measured container widths with a CSS-based layout, introduced loading skeletons, and applied font lazy loading.
• Reduced messages re-rendered per streamed token from at least 20 to 1 by separating references so only the updating message re-renders.
• Reduced network requests when jumping to an in-chat search result from up to 50 to 1 by replacing sequential page fetches with server-side bidirectional pagination.

Development Environment

• Unified a legacy stack mixing MobX, zustand, SWR, and TanStack Query onto zustand and TanStack Query, and removed dead code for a net reduction of about 40,000 lines of production code.
• Defined and documented policies for server/client data boundaries, query key management, and streaming, establishing them as team conventions.
• Resolved security issues with user-authored JSX components by proposing AI pre-review and iframe isolation, then handled the resulting media query limitations with CSS container queries.

• Built i18n support and designed and implemented the full policy branching of the creator revenue settlement funnel per country, together with the product owner.
• Modularized the complex pricing policy of the comic store check-in/check-out system into a structure resilient to change.
• Worked with store staff to add features they actually needed on site: check-out recovery, memos, macros for handling inquiries, and automated inventory updates.

• Built and shared tools that automate repetitive team work such as insurance terminology review, cutting manual work time by more than 50%.
• Built components such as a TimePicker designed for keyboard interaction, accessibility (a11y), and type safety, to meet design requirements.

• Built the blog from scratch with Next.js, generating a table of contents and automating image path rewriting through custom remark plugins. Articles
• Built an AI-based automatic translation system to support English, improving global accessibility. Article
• Traced a deployment build failure to bundle size and cut the bundle by 70% by changing the design and writing third-party code. Article

• Rewrote the API server handling lecture attendance and assignment submissions from Go to Nest.js and Prisma.
• Built a Discord bot for online lecture attendance using discord.js and deployed it alongside the server.

• Wrote a Prisma syntax highlighting plugin, now listed as an official third-party grammar. highlight.js PR #4252
• Migrated the ESLint configuration of the Yorkie JS SDK monorepo to flat config. yorkie-js-sdk PR #1045
• Replaced lodash with es-toolkit, cutting the Yorkie JS SDK example bundle size by 98%. yorkie-js-sdk PR #1101
• Corrected a historical inaccuracy about `@@unscopables` in MDN documentation. mdn/content PR #34646
• Corrected a historical inaccuracy about `NaN` in MDN documentation. mdn/content PR #35496
• Fixed an incorrect comment in the JavaScriptCore engine source. WebKit PR #25696
• Translated and published a 120-page paper on the history of JavaScript. Published Site

• Geultto frontend meetup: "Starting Networking My Way". Slides
• BBConf: a brief history of computers, networks, and the web, clarifying common misconceptions. Slides
• How to sustain blogging for the long term and write better articles. Slides
• Taught a winter break algorithm course to about 100 university students in the Sinchon area. Lecture Materials

• Article on special comment formats in JavaScript, selected by Naver FE News (Feb 2024). Curation
• Deep dive tracing the history of closures from Turing machines to JavaScript, selected in Geultto 10th cohort. Article
• Explained type system variance through TypeScript, selected in Geultto 9th cohort. Article