openapi-to-typescript

# OpenAPI to TypeScript

Converts OpenAPI 3.0 specifications to TypeScript interfaces and type guards.

**Input:** OpenAPI file (JSON or YAML)
**Output:** TypeScript file with interfaces and type guards

## When to Use

- "generate types from openapi"
- "convert openapi to typescript"
- "create API interfaces"
- "generate types from spec"

## Workflow

1. Request the OpenAPI file path (if not provided)
2. Read and validate the file (must be OpenAPI 3.0.x)
3. Extract schemas from `components/schemas`
4. Extract endpoints from `paths` (request/response types)
5. Generate TypeScript (interfaces + type guards)
6. Ask where to save (default: `types/api.ts` in current directory)
7. Write the file

## OpenAPI Validation

Check before processing:

```
- Field "openapi" must exist and start with "3.0"
- Field "paths" must exist
- Field "components.schemas" must exist (if there are types)
```

If invalid, report the error and stop.

## Type Mapping

### Primitives

| OpenAPI     | TypeScript   |
|-------------|--------------|
| `string`    | `string`     |
| `number`    | `number`     |
| `integer`   | `number`     |
| `boolean`   | `boolean`    |
| `null`      | `null`       |

### Format Modifiers

| Format        | TypeScript              |
|---------------|-------------------------|
| `uuid`        | `string` (comment UUID) |
| `date`        | `string` (comment date) |
| `date-time`   | `string` (comment ISO)  |
| `email`       | `string` (comment email)|
| `uri`         | `string` (comment URI)  |

### Complex Types

**Object:**
```typescript
// OpenAPI: type: object, properties: {id, name}, required: [id]
interface Example {
  id: string;      // required: no ?
  name?: string;   // optional: with ?
}
```

**Array:**
```typescript
// OpenAPI: type: array, items: {type: string}
type Names = string[];
```

**Enum:**
```typescript
// OpenAPI: type: string, enum: [active, draft]
type Status = "active" | "draft";
```

**oneOf (Union):**
```typescript
// OpenAPI: oneOf: [{$ref: Cat}, {$ref: Dog}]
type Pet = Cat | Dog;
```

**allOf (Intersection/Extends):**
```typescript
// OpenAPI: allOf: [{$ref: Base}, {type: object, properties: ...}]
interface Extended extends Base {
  extraField: string;
}
```

## Code Generation

### File Header

```typescript
/**
 * Auto-generated from: {source_file}
 * Generated at: {timestamp}
 *
 * DO NOT EDIT MANUALLY - Regenerate from OpenAPI schema
 */
```

### Interfaces (from components/schemas)

For each schema in `components/schemas`:

```typescript
export interface Product {
  /** Product unique identifier */
  id: string;

  /** Product title */
  title: string;

  /** Product price */
  price: number;

  /** Created timestamp */
  created_at?: string;
}
```

- Use OpenAPI description as JSDoc
- Fields in `required[]` have no `?`
- Fields outside `required[]` have `?`

### Request/Response Types (from paths)

For each endpoint in `paths`:

```typescript
// GET /products - query params
export interface GetProductsRequest {
  page?: number;
  limit?: number;
}

// GET /products - response 200
export type GetProductsResponse = ProductList;

// POST /products - request body
export interface CreateProductRequest {
  title: string;
  price: number;
}

// POST /products - response 201
export type CreateProductResponse = Product;
```

Naming convention:
- `{Method}{Path}Request` for params/body
- `{Method}{Path}Response` for response

### Type Guards

For each main interface, generate a type guard:

```typescript
export function isProduct(value: unknown): value is Product {
  return (
    typeof value === 'object' &&
    value !== null &&
    'id' in value &&
    typeof (value as any).id === 'string' &&
    'title' in value &&
    typeof (value as any).title === 'string' &&
    'price' in value &&
    typeof (value as any).price === 'number'
  );
}
```

Type guard rules:
- Check `typeof value === 'object' && value !== null`
- For each required field: check `'field' in value`
- For primitive fields: check `typeof`
- For arrays: check `Array.isArray()`
- For enums: check `.includes()`

### Error Type (always include)

```typescript
export interface ApiError {
  status: number;
  error: string;
  detail?: string;
}

export function isApiError(value: unknown): value is ApiError {
  return (
    typeof value === 'object' &&
    value !== null &&
    'status' in value &&
    typeof (value as any).status === 'number' &&
    'error' in value &&
    typeof (value as any).error === 'string'
  );
}
```

## $ref Resolution

When encountering `{"$ref": "#/components/schemas/Product"}`:
1. Extract the schema name (`Product`)
2. Use the type directly (don't resolve inline)

```typescript
// OpenAPI: items: {$ref: "#/components/schemas/Product"}
// TypeScript:
items: Product[]  // reference, not inline
```

## Complete Example

**Input (OpenAPI):**
```json
{
  "openapi": "3.0.0",
  "components": {
    "schemas": {
      "User": {
        "type": "object",
        "properties": {
          "id": {"type": "string", "format": "uuid"},
          "email": {"type": "string", "format": "email"},
          "role": {"type": "string", "enum": ["admin", "user"]}
        },
        "required": ["id", "email", "role"]
      }
    }
  },
  "paths": {
    "/users/{id}": {
      "get": {
        "parameters": [{"name": "id", "in": "path", "required": true}],
        "responses": {
          "200": {
            "content": {
              "application/json": {
                "schema": {"$ref": "#/components/schemas/User"}
              }
            }
          }
        }
      }
    }
  }
}
```

**Output (TypeScript):**
```typescript
/**
 * Auto-generated from: api.openapi.json
 * Generated at: 2025-01-15T10:30:00Z
 *
 * DO NOT EDIT MANUALLY - Regenerate from OpenAPI schema
 */

// ============================================================================
// Types
// ============================================================================

export type UserRole = "admin" | "user";

export interface User {
  /** UUID */
  id: string;

  /** Email */
  email: string;

  role: UserRole;
}

// ============================================================================
// Request/Response Types
// ============================================================================

export interface GetUserByIdRequest {
  id: string;
}

export type GetUserByIdResponse = User;

// ============================================================================
// Type Guards
// ============================================================================

export function isUser(value: unknown): value is User {
  return (
    typeof value === 'object' &&
    value !== null &&
    'id' in value &&
    typeof (value as any).id === 'string' &&
    'email' in value &&
    typeof (value as any).email === 'string' &&
    'role' in value &&
    ['admin', 'user'].includes((value as any).role)
  );
}

// ============================================================================
// Error Types
// ============================================================================

export interface ApiError {
  status: number;
  error: string;
  detail?: string;
}

export function isApiError(value: unknown): value is ApiError {
  return (
    typeof value === 'object' &&
    value !== null &&
    'status' in value &&
    typeof (value as any).status === 'number' &&
    'error' in value &&
    typeof (value as any).error === 'string'
  );
}
```

## Common Errors

| Error | Action |
|-------|--------|
| OpenAPI version != 3.0.x | Report that only 3.0 is supported |
| $ref not found | List missing refs |
| Unknown type | Use `unknown` and warn |
| Circular reference | Use type alias with lazy reference |