Migrated a live govtech SaaS while fire departments kept using it.

What changed

Moved one layer at a time: runtime, language, data fetching, forms, maps, auth, and tenant configuration.

Measured outcomes

Whole frontend migrated, weekly releases preserved

JS → TS

domain modules shipped on the new stack

7

release gaps during the migration

0

Highlights

• Moved the whole frontend off JavaScript on CRA onto TypeScript on Vite in roughly three months without blocking a release. Leaned on the allowJs flag to convert module by module, starting with forms and data hooks where types kept catching real bugs.
• Replaced hand-rolled fetch hooks with RTK Query. Tag-based cache invalidation removed a long tail of stale-data bugs and loading states that disagreed with each other.
• Made Zod schemas the shared contract for forms and API edges. The same schema drove validation, component state, and DTO typing through react-hook-form.
• Built autocomplete across server records, ArcGIS suggestions, and tenant-defined locations. Each source could fail without killing the typeahead.
• Shipped ArcGIS occupancy maps with drag-and-drop safety markers, persistent zoom and pan, and staged marker updates that stopped flicker during edits.
• Modeled 2FA as an explicit state machine: credentials submitted, awaiting code, hydrating session, recovery code, and refresh mid-flow all had named transitions.
• Moved tenant config from build-time .env to a server-driven settings endpoint for theme tokens, default map coordinates, and admin UI shape. New municipalities no longer required a redeploy.
• Added guided inspection text with backoff, deterministic fallback, and human review before save. Drafted language stayed editable and never blocked the inspection flow.
• Built the checklist-template admin: drag-and-drop sections and items, fee-type fields, and cascading deletes for the authoring surface the product depends on.
• Built the contractor portal as its own routing tree with role-scoped permit lists, activities, invoices, and document uploads.

Outcomes

• For 20 months I was the sole frontend engineer on a two-engineer team. Six migrations shipped without creating a deploy gap.
• Frontend stopped being the scary part of the roadmap. Feature work and migration work moved together instead of competing.
• New municipalities could be configured by settings instead of redeploying.
• The platform was live with US municipal fire departments and building-inspection offices before my engagement ended.

Stack

• React 18
• TypeScript
• Vite
• Redux Toolkit
• RTK Query
• react-hook-form
• Zod
• ArcGIS Maps SDK
• MUI
• MUI DataGrid Pro
• Mobiscroll
• Model API
• Biome
• Yarn 4 PnP