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Migrating a legacy codebase to TypeScript strict mode

NEO Campus Editorial8 April 20266 min read
Migrating a legacy codebase to TypeScript strict mode

Strict mode is the difference between TypeScript that catches bugs and TypeScript that decorates JavaScript. Getting there in a legacy codebase requires patience.

Turn on flags one at a time

Start with noImplicitAny, then strictNullChecks, then the rest. Each flag is its own migration project.

Quarantine the worst files

Use ts-expect-error sparingly and with comments explaining the debt. Track them in a single dashboard.

Lint the loopholes

Forbid any, as casts, and non-null assertions in new code via ESLint. The codebase will stop regressing while you fix the old debt.

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