Detailed explanation of Koa Context object ctx core properties and usage methods

Koa's Context object (ctx) is a core concept that encapsulates Node.js's request and response objects into a single object, providing a more concise API through proxy pattern.

Core properties and methods of ctx object:

Request-related properties:

• ctx.request: Koa's Request object
• ctx.req: Node.js native request object
• ctx.request.url: Request URL
• ctx.request.method: Request method
• ctx.request.header: Request header object
• ctx.request.query: Parsed query string
• ctx.request.querystring: Raw query string
• ctx.request.path: Request path
• ctx.request.body: Request body (requires body-parser middleware)

Response-related properties:

• ctx.response: Koa's Response object
• ctx.res: Node.js native response object
• ctx.response.body: Response body
• ctx.response.status: Response status code
• ctx.response.header: Response header object
• ctx.response.type: Response Content-Type
• ctx.response.length: Response Content-Length

Convenient access (directly through ctx):

• ctx.url: Equivalent to ctx.request.url
• ctx.method: Equivalent to ctx.request.method
• ctx.header: Equivalent to ctx.request.header
• ctx.query: Equivalent to ctx.request.query
• ctx.body: Equivalent to ctx.response.body
• ctx.status: Equivalent to ctx.response.status
• ctx.type: Equivalent to ctx.response.type

Other important properties:

• ctx.state: Recommended namespace for passing information between middleware
• ctx.app: Application instance reference
• ctx.cookies: Cookie manipulation utility
• ctx.throw(): Throw HTTP errors
• ctx.assert(): Assertion check

Code example:

javascript
app.use(async (ctx) => {
  // Get request info
  const url = ctx.url;
  const method = ctx.method;
  const query = ctx.query;
  
  // Set response
  ctx.status = 200;
  ctx.type = 'application/json';
  ctx.body = { message: 'Hello Koa' };
  
  // Use state to pass data
  ctx.state.user = { id: 1, name: 'John' };
  
  // Throw error
  if (!ctx.query.token) {
    ctx.throw(401, 'Token required');
  }
});

Design advantages of ctx object:

1. Conciseness: Reduces code redundancy, improves development efficiency
2. Consistency: Unified API interface, reduces learning cost
3. Flexibility: Supports both convenient access and direct native object manipulation
4. Extensibility: Can extend ctx object functionality through middleware