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en/tutorials/ts/12ts-config

# TypeScript Config & Strict Mode

The `tsconfig.json` file controls how TypeScript compiles and type-checks your project.

## Basic Structure

```json
{
  "compilerOptions": {
    "target": "ES2020",
    "module": "ESNext",
    "strict": true,
    "outDir": "./dist",
    "rootDir": "./src"
  },
  "include": ["src/**/*"],
  "exclude": ["node_modules"]
}
```

**Remember:** Without `tsconfig.json`, TypeScript uses default options that may not suit your project.

## Strict Mode

`strict` enables all strict type-checking options:

- `strictNullChecks` — Prevents assigning null/undefined to non-nullable types
- `noImplicitAny` — Flags implicit `any` types
- `strictFunctionTypes` — Strict function parameter checking
- `strictPropertyInitialization` — Ensures class properties are initialized

**Always enable strict mode** — it catches the most common TypeScript bugs.

## Essential Options

```json
{
  "compilerOptions": {
    "target": "ES2020",
    "strict": true,
    "noEmit": true,
    "declaration": true,
    "sourceMap": true,
    "moduleResolution": "node",
    "esModuleInterop": true,
    "resolveJsonModule": true
  }
}
```

- `noEmit` — Type check only, no output (great for CI/CD)
- `declaration` — Generate `.d.ts` files for libraries
- `sourceMap` — Enable debugging

## Including and Excluding Files

```json
{
  "include": ["src/**/*"],
  "exclude": ["node_modules", "dist", "**/*.test.ts"],
  "files": ["src/main.ts"]
}
```

**Tip:** `include` uses glob patterns; `files` lists specific files.

## Project References

Split large projects into smaller, manageable units:

```json
{
  "extends": "./tsconfig.base.json",
  "compilerOptions": {
    "outDir": "./dist",
    "rootDir": "./src"
  },
  "references": [{ "path": "./tsconfig.lib.json" }]
}
```

**Remember:** Project references enable faster builds and better separation of concerns.

## Best Practices

- Always enable strict mode
- Use source maps in development for debugging
- Generate declarations for libraries with `declaration: true`
- Use project references for large codebases
- Enable `noUnusedLocals` and `noUnusedParameters` to clean up dead code

## Common Mistakes

- Not enabling strict mode misses many type errors
- Using a target that's too old forces unnecessary transpilation
- Not setting `rootDir` can cause confusing output paths
- Not excluding test files can cause type conflicts

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