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:

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

Array:

// OpenAPI: type: array, items: {type: string}
type Names = string[];

Enum:

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

oneOf (Union):

// OpenAPI: oneOf: [{$ref: Cat}, {$ref: Dog}]
type Pet = Cat | Dog;

allOf (Intersection/Extends):

// OpenAPI: allOf: [{$ref: Base}, {type: object, properties: ...}]
interface Extended extends Base {
  extraField: string;
}

Code Generation

File Header

/**
 * 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:

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:

// 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:

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)

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)

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

Complete Example

Input (OpenAPI):

{
  "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):

/**
 * 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 |