TypeScript 6.0 breaking changes are not theoretical. With strict mode now default, ES5 dropped, and ESNext module resolution required, this is the most disruptive TypeScript release since 2.x. Migration is not optional: if you rely on downstream dependencies, your next npm install could break CI unless you’re ready. The break rate is not subtle — 70% of codebases that dodged strict mode now face instant red builds. Here’s what’s changed, why, and how to actually migrate without a week of pipeline outages.
What are the key breaking changes in TypeScript 6.0?
TypeScript 6.0 introduces three headline breaking changes:
Strict mode defaults to true
Previously, TypeScript shipped with strict: false unless you opted in. This masked a huge class of unsound code, but “worked” for legacy projects. That’s over. In 6.0, strict: true is enforced unless you explicitly override it.
