Upgrading to TypeScript 7: what actually breaks

TypeScript 7 is 8-12x faster, which makes the upgrade tempting to do on a Friday afternoon. Don't. The speed is real, but 7.0 is the release where the team cashed in every deprecation they've been sitting on and shipped without a programmatic API. Whether you can move depends less on your code than on your toolchain.

Here's the honest checklist, in the order the breakage actually hits you.

1. Options that are now hard errors

TypeScript 6 made these warnings. TypeScript 7 makes them errors, so tsc refuses to start rather than silently doing something else. Removed outright:

If you're on a modern setup, most of this is a no-op. If you're carrying a tsconfig.json that has been copy-pasted since 2019, this is where your afternoon goes.

2. Defaults that changed under you

More dangerous than the removals, because these don't error, they just change what your build means.

• strict: true is the default. If you were implicitly relying on strict being off, you now have every error strict catches, all at once. Set "strict": false explicitly if you're not ready — but note you're now opting out, in writing, which is the point.

• module: esnext is the default.

• noUncheckedSideEffectImports: true — a bare import "./thing" that resolves to nothing is now an error. This catches real bugs and a few false ones (side-effect imports of CSS or assets need the right ambient declarations).

• stableTypeOrdering: true, and it cannot be disabled. The checker is parallel now, so deterministic type ordering has to be enforced rather than assumed.

• rootDir now defaults to ./. If your tsconfig.json lives above your sources, your output layout just changed. Say what you mean:

  {
    "compilerOptions": {
      "rootDir": "./src"
    }
  }

  copy

• types defaults to [] instead of auto-discovering everything in node_modules/@types. This is a genuinely good change (auto-discovery was a common source of "why is describe a global in my production build") and a genuinely annoying one, because you must now list what you use:

  {
    "compilerOptions": {
      "types": ["node", "vitest/globals"]
    }
  }

  copy

  Missing globals after upgrading — process, describe, expect — is almost always this.

3. Checked JavaScript got stricter

If you run checkJs over a JS codebase with JSDoc types, several patterns are gone: values can no longer stand in for types (use typeof x), @enum is no longer recognized (use @typedef), the postfix ! non-null operator is out, and Closure-style function syntax isn't supported. Small surface, but if you're a big JSDoc shop it's the whole migration.

4. The one that actually decides your timeline: no API

TypeScript 7.0 ships with no programmatic API. Anything that imports typescript and walks the AST or asks the checker questions does not work on 7 yet. That includes:

• typescript-eslint (so: most people's lint setup)
• Vue, Svelte, Astro, MDX — anything that embeds or templates TypeScript
• Angular's template type-checking
• ts-morph, most codemods, most custom transformers

The supported path is to run both compilers side by side: TypeScript 7 for the fast CLI builds, TypeScript 6 via @typescript/typescript6 (an npm alias) for the tools that still need the API. A real API is expected in 7.1.

So the practical question is not "does my code compile under 7" but "which of my tools reach into the compiler". Most teams will find the answer is at least one.

What this means for a site like this one

This site is Next.js App Router plus MDX (that's how these threads are written). MDX needs the TypeScript API to type-check inside .mdx, so per the release notes, MDX projects stay on TypeScript 6 for now. The realistic setup is split:

• CLI type-check and CI: TypeScript 7. npm run typecheck is the single slowest thing in our pipeline, and it's the thing every push waits on.
• Editor and lint: TypeScript 6 until 7.1 lands the API, because typescript-eslint and the MDX toolchain need it.

That's not a clean win, and it's worth being clear-eyed about it rather than performing an upgrade for the changelog entry. A codebase this size type-checks in a couple of seconds either way. The 8-12x is transformative for a 130-second build and a rounding error for a 2-second one, so we'll take the CI half now and move the editor when the API is real.

A migration order that works

1. Upgrade to the latest TypeScript 6 first, and fix every deprecation warning. This is the actual work, and you can do it incrementally while your tooling still functions.
2. Modernize tsconfig.json: moduleResolution to bundler or nodenext, drop baseUrl, drop esModuleInterop: false, make rootDir and types explicit.
3. Turn strict on, or explicitly turn it off, so nothing changes silently under you.
4. Inventory what touches the compiler API: lint, test transforms, codemods, framework plugins. That list is your gate.
5. Run TypeScript 7 for CLI builds and CI, keep TypeScript 6 wherever the API is required.
6. Revisit when 7.1 ships an API and the ecosystem catches up.

Steps 1-3 are worth doing even if you never adopt 7. That's usually the sign of a well-designed breaking release: the migration path improves your codebase whether or not you arrive at the destination.