In Koa.js, errors can be handled using a combination of try/catch blocks and the ctx.throw() method. The ctx.throw() method is a convenient way to throw HTTP errors, and it will automatically set the appropriate HTTP status code and response body. Additionally, you can use try/catch blocks to catch and handle errors within your middleware functions.

Here's how you can handle errors in Koa.js :

Using try/catch : You can use try/catch blocks to catch errors within your middleware functions. If an error occurs, you can handle it and respond accordingly.

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

  } catch (error) {
    // Handle the error
    console.error('Error:', error);

    // Set an appropriate HTTP status code
    ctx.status = error.status || 500;

    // Set the response body
    ctx.body = {
      error: {
        message: error.message || 'Internal Server Error',
      },
    };
  }
});?

Using ctx.throw() : The ctx.throw() method can be used to throw an HTTP error. It takes two parameters: the HTTP status code and an optional error message.

app.use(async (ctx, next) => {
  // Your middleware logic

  // Throw a 404 Not Found error if a resource is not found
  if (!someResourceExists) {
    ctx.throw(404, 'Resource Not Found');
  }

  // Continue to the next middleware
  await next();
});?

The ctx.throw() method sets the appropriate HTTP status code and generates an error that can be caught by the closest try/catch block or a global error event.
Global Error Event : You can handle uncaught errors globally by listening to the 'error' event on the Koa application object. This is useful for handling errors that occur outside the context of a specific middleware function.

app.on('error', (err, ctx) => {
  console.error('Global Error:', err);

  // Set an appropriate HTTP status code
  ctx.status = err.status || 500;

  // Set the response body
  ctx.body = {
    error: {
      message: err.message || 'Internal Server Error',
    },
  };
});?

The global error event allows you to centralize error handling and respond consistently to errors that occur throughout your application.

By combining try/catch blocks, ctx.throw(), and the global error event, you can effectively handle errors in Koa.js applications. It's important to tailor error handling to the specific needs of your application and provide informative responses to clients.