How to fix src/index.ts is not under 'rootDir' in TypeScript

### How rootDir Affects Output Structure

The rootDir option controls the structure of your compiled output. Consider this project:

project/
├── src/
│   ├── index.ts
│   └── utils/
│       └── helper.ts
└── tsconfig.json

With rootDir: "./src":

{
  "compilerOptions": {
    "rootDir": "./src",
    "outDir": "./dist"
  }
}

Output:

dist/
├── index.js
└── utils/
    └── helper.js

Without rootDir (or with rootDir: "."):

{
  "compilerOptions": {
    "outDir": "./dist"
  }
}

Output:

dist/
└── src/
    ├── index.js
    └── utils/
        └── helper.js

The rootDir is stripped from the output path, so setting it correctly is important for deployment.

### Why TypeScript Enforces This

TypeScript's strict rootDir enforcement exists for good reason:

1. Predictable output: Every input file has an unambiguous output location
2. Prevent accidental compilation: Configuration files or scripts outside your source tree aren't accidentally compiled
3. Build reproducibility: The same input structure always produces the same output structure

If TypeScript allowed files outside rootDir, it would face two bad options:
- Write output files outside outDir (breaks isolation)
- Flatten the directory structure (loses organization)

### Working with Path Mapping

Path mapping in tsconfig.json doesn't bypass rootDir requirements:

{
  "compilerOptions": {
    "rootDir": "./src",
    "baseUrl": ".",
    "paths": {
      "@config/*": ["config/*"]    // config is outside src!
    }
  }
}

If config/ is outside src/, you'll still get the rootDir error even with path mapping. Solutions:

1. Move config into src: src/config/
2. Import config as a package, not source files
3. Use separate tsconfig for config with its own rootDir

### Declaration Files and rootDir

When using declaration: true, the same rootDir constraints apply to .d.ts output:

{
  "compilerOptions": {
    "rootDir": "./src",
    "outDir": "./dist",
    "declaration": true,
    "declarationDir": "./types"  // Must match rootDir structure
  }
}

Both JavaScript and declaration files respect the rootDir structure.

### Composite Projects and rootDir

In composite projects (used for project references), rootDir behaves differently:

{
  "compilerOptions": {
    "composite": true,
    // rootDir is inferred from the "include" pattern
    "outDir": "./dist"
  },
  "include": ["src/**/*"]
}

With composite: true, TypeScript is more strict about file organization because it builds incremental .tsbuildinfo files.

### Debugging rootDir Issues

See what TypeScript considers the rootDir:

# Show all files TypeScript is processing
npx tsc --listFiles

# Show detailed compiler options
npx tsc --showConfig

The --showConfig output will show the resolved rootDir value, which might be different from what you specified if TypeScript inferred it.

### Avoiding the Problem Entirely

Best practices to prevent this error:

1. Single source directory: Keep all source code in src/, all config in root
2. Explicit include: Use "include": ["src/**/*"] to be explicit
3. Let TypeScript infer: Don't set rootDir unless you need specific output structure
4. Separate configs: Use tsconfig.build.json for build, base tsconfig.json for IDE
5. Test isolation: Keep tests in src/__tests__/ or use separate tsconfig.test.json