What is the purpose of Koa.js' 'app.use()' method?

Koa.js - Interview Questions

The app.use() method in Koa.js is used to register middleware functions that will be executed in the order they are defined when an HTTP request is received. Middleware functions in Koa.js are central to handling various aspects of the request-response lifecycle, such as processing incoming requests, modifying the response, and controlling the flow of the application.

Here's an overview of the purpose and usage of the app.use() method in Koa.js :

1. Registering Middleware Functions : The primary purpose of app.use() is to register middleware functions with the Koa application. Middleware functions are functions that have access to the Koa context object (ctx) and the next function, which allows them to pass control to the next middleware in the stack.

const Koa = require('koa');
const app = new Koa();

// Registering a middleware function
app.use(async (ctx, next) => {
  // Middleware logic
  await next(); // Pass control to the next middleware
});?

2. Order of Execution : Middleware functions are executed in the order in which they are registered using app.use(). The order of execution is often referred to as the "downstream" flow. Each middleware function can perform specific tasks and decide whether to continue to the next middleware by calling await next().

app.use(async (ctx, next) => {
  // Middleware 1 logic
  await next(); // Continue to the next middleware
});

app.use(async (ctx, next) => {
  // Middleware 2 logic
  await next(); // Continue to the next middleware
});?

3. Handling Asynchronous Operations : Middleware functions can handle asynchronous operations using mechanisms like async/await or generators (in older versions of Koa.js). This allows developers to write asynchronous code in a more readable and sequential manner.

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

4. Modifying the Context Object : Middleware functions have access to the Koa context object (ctx), which encapsulates the request, response, and other information. Middleware can read from and modify the ctx object to influence the behavior of subsequent middleware or control the response.

app.use(async (ctx, next) => {
  // Accessing and modifying the context object
  console.log(`Received request: ${ctx.method} ${ctx.url}`);
  ctx.body = 'Hello, Koa!';
  await next(); // Continue to the next middleware
});?

5. Error Handling : Middleware functions can handle errors by using try/catch blocks or by throwing an error using ctx.throw(). This allows for structured error handling within the middleware stack.

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

6. Composing Middleware : The app.use() method allows developers to compose a stack of middleware functions, each responsible for a specific aspect of the application. This promotes modularity and maintainability in the codebase.

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