# Type Checking

Catch type mismatches at analysis time, not at runtime. Especially valuable in large codebases and across refactors.

## Tools by Language

| Language | Tool | Notes |
|---|---|---|
| Python | `mypy`, `pyright` | not built-in; add type hints manually |
| JavaScript/TS | `tsc`, `flow` | TypeScript is the dominant choice |
| Go | built-in | the compiler enforces types |
| C++ | built-in + `clang-tidy` | compiler + static analysis |

## Python — `mypy`

```bash
mypy src/
```

Configure in `pyproject.toml`:

```toml
[tool.mypy]
python_version = "3.12"
strict = true
ignore_missing_imports = true
```

Start with `strict = false` and tighten incrementally. Add `# type: ignore` sparingly — only where third-party stubs are missing.

## TypeScript — `tsc`

```bash
tsc --noEmit        # type-check without emitting JS
```

`tsconfig.json`:

```json
{
  "compilerOptions": {
    "strict": true,
    "noEmit": true,
    "target": "ES2022"
  }
}
```

## Tips

- Type checking and linting are separate concerns — run both
- In Python, annotate function signatures first; body annotations add less value
- `pyright` (Pylance) gives faster feedback in VS Code; `mypy` is CI-standard