01
'Cannot read properties of undefined' crashes
Implicit data shapes causing runtime crashes in production.
Runtime TypeErrors, undocumented data shapes and fear of refactoring
JavaScript to strict TypeScript code modernization
Move a JavaScript codebase to strict TypeScript module by module, so data-shape bugs are caught at compile time instead of in production.
Symptoms
Signs your platform has this problem
If several of these sound familiar, the plan below is where we would start.
02
Fear of refactoring core logic
No compile-time checks, so even small clean-ups feel risky.
03
Slow onboarding
New engineers spending days guessing function signatures and object shapes.
Remediation plan
How we fix it, step by step
Each phase ends with a measurement, so you can see what changed before the next one starts.
01
Compiler setup and codemods
Configuring tsconfig.json with allowJs and running automated AST codemods.
02
Domain contracts and entity types
Defining canonical types and Zod schemas for database models and API contracts.
03
Module-by-module conversion
Converting .js files to .ts from leaf utilities up to core route controllers.
04
Strict checks in CI
Enabling strict and noImplicitAny, and adding a TypeScript check to CI.
Technical checklist
Remediation checklist
What we check before a change goes to production:
- Configure tsconfig.json with strict compiler flags and path aliases
- Run codemods that infer types from existing JSDoc comments
- Define canonical TypeScript types for database and API schemas
- Add tsc --noEmit to CI so untyped pull requests fail
What we measure
We take a baseline first and report the same measurements after each change, from your own monitoring — evidence, not promised results.
- Strict coverage
- Files compiling under strict mode, tracked per sprint
- TypeErrors
- Runtime TypeErrors per week in your error tracker
- Release cadence
- Releases per month while the conversion runs
Related service
Custom software
Custom software for Indian SMEs and startups, built around your workflow with an agreed scope, review milestones, clear ownership and documented handover.
Explore Custom softwareQuestions
Questions about this remediation
We configure TypeScript with allowJs, so existing JavaScript files and new TypeScript files coexist in the same build while the conversion moves file by file.
It depends on codebase size and how much is already documented. We start with strict types at the boundaries — API contracts and database models — so the value arrives early, and estimate the rest after a file inventory.
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Want an engineer to look at this with you?
Send us the symptoms and any metrics you have. We'll reply within one business day, set up a call and agree what to measure before anything changes.