/typescript-v6

TypeScript 6 Migration Reference

Use this reference when a project is moving from TypeScript 5.x-era assumptions into TypeScript 6 behavior.

High-Impact Migration Changes

1. Re-check ambient globals

Projects that rely on Node.js, test, Worker, or Bun globals should make types explicit instead of relying on broad ambient discovery.

{
  "compilerOptions": {
    "types": ["node", "jest"]
  }
}

2. Make rootDir explicit

{
  "compilerOptions": {
    "rootDir": "./src",
    "outDir": "./dist"
  },
  "include": ["src/**/*"]
}

This avoids surprising emit-path shifts such as dist/src/....

3. Replace deprecated config early

Look for old settings like:

• moduleResolution: "node" / "node10"
• baseUrl
• downlevelIteration
• target: "es5"

Move to bundler or nodenext, direct paths, and more modern targets first.

4. Use the compiler before editing code

npx tsc --noEmit
npx tsc --showConfig
npx tsc --explainFiles
npx tsc --traceResolution

These commands usually explain the upgrade faster than rewriting source files blindly.

5. Keep ignoreDeprecations temporary

{
  "compilerOptions": {
    "ignoreDeprecations": "6.0"
  }
}

Use it to create breathing room, then remove it once the real replacements land.

Migration Checklist

• rootDir is explicit if sources live below the tsconfig.json
• types is explicit where ambient globals matter
• moduleResolution is bundler or nodenext
• deprecated options are removed or scheduled for removal
• tsc --showConfig and tsc --explainFiles were used if behavior is still surprising
• runtime support for Temporal, RegExp.escape, and Map.getOrInsert* was checked separately

Quick Reference