Order of Execution :  Middleware functions are executed in the order in which they are defined using the app.use() method. This order is referred to as the "downstream" flow.


Context Object (ctx) : Each middleware function receives the context object (ctx) as a parameter. The context object encapsulates the request, response, and other related information. Middleware functions can read from and modify the ctx object.

Next Function : Middleware functions can optionally call the next function to pass control to the next middleware in the stack. If the next function is not called, the downstream flow is halted, and subsequent middleware functions are not executed.

app.use(async (ctx, next) => {
  // Middleware logic before calling the next middleware
  await next(); // Call the next middleware
  // Middleware logic after the next middleware has completed
});?

Async/Await : Middleware functions can leverage async/await for handling asynchronous operations. This makes it easy to write asynchronous code in a synchronous style, enhancing readability.

app.use(async (ctx, next) => {
  // Asynchronous logic
  await someAsyncFunction();
  await next(); // Continue to the next middleware
});?

Error Handling : Middleware functions can handle errors by either using try/catch blocks or by throwing an error using ctx.throw(). If an error occurs, the downstream flow is interrupted, and the control is transferred to the closest error-handling middleware or the global error event.

app.use(async (ctx, next) => {
  try {
    // Some logic
    await next();
  } catch (error) {
    // Handle the error
    ctx.status = 500;
    ctx.body = 'Internal Server Error';
  }
});?

Composition : Middleware functions can be composed to perform specific tasks. Developers often create reusable middleware functions and compose them to build complex applications. This promotes a modular and maintainable codebase.

const loggingMiddleware = async (ctx, next) => {
  console.log(`Received request: ${ctx.method} ${ctx.url}`);
  await next();
};

const authenticationMiddleware = async (ctx, next) => {
  // Authentication logic
  await next();
};

app.use(loggingMiddleware);
app.use(authenticationMiddleware);?