Skip to content

Migration playbook: JavaScript → Strict TypeScript

JavaScript to strict TypeScript migration

Convert a JavaScript codebase to strict TypeScript file by file, so type errors are caught at compile time instead of in production.

Talk to an engineer
Custom software

Migration drivers

Why teams make this move

Driver 01

Runtime TypeErrors

Frequent 'Cannot read properties of undefined' crashes in production.

Driver 02

Fear of refactoring

Engineers avoiding changes to core business logic because nothing checks them at compile time.

Driver 03

Slow onboarding

New developers guessing at implicit data shapes passed through nested functions.

Execution sequence

How the migration runs

Each phase ends with a check you can verify — data parity, error rates, latency — and the rollback path is agreed before any traffic moves.

  1. 01Phase

    Compiler setup and codemods

    Setting up tsconfig with allowJs and running automated migration codemods.

  2. 02Phase

    Domain and API types

    Defining canonical types for database models, API inputs and business entities.

  3. 03Phase

    Module-by-module conversion

    Converting files from .js and .jsx to .ts and .tsx, from leaf utilities up to core controllers.

  4. 04Phase

    Strict checking in CI

    Enabling strict and noImplicitAny, and adding a TypeScript check to CI.

Risk prevention

Pitfalls that derail this migration

Risk 01

Overusing 'any'

Sprinkling 'any' to silence the compiler, which gives a false sense of safety.

Risk 02

Inaccurate third-party types

Outdated @types packages that don't match what the library actually returns.

Risk 03

Stalling feature work

Attempting a big-bang conversion instead of agreeing file-by-file standards.

Before and after

What we measure

We take a baseline before any change and report the same numbers after cutover, from your own tools. They are the evidence of whether the migration worked — not figures promised in advance.

Typed files
Share of files converted, tracked per sprint
Type errors
tsc errors under strict mode, tracked down to zero
Runtime errors
TypeErrors per week in your error tracker

Questions

Frequently asked migration questions

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.

Rehearse the cutover before the real one

Tell us about your data volume, traffic and timeline. An engineer will reply within one business day to set up a call about the migration plan and its rollback path.

Request a strategy call