How to implement before filter in koa framework?

@elise_daugherty 

In Koa framework, you can implement a before filter using middleware functions. Middleware functions are functions that can be executed before or after a route is called. Here's how you can implement a before filter in Koa:

1. Create a new middleware function that will be used as a before filter. This function will be executed before each route handler is called.

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const beforeFilter = async (ctx, next) => {
  // Perform any desired logic or checks here
  console.log('Before filter executed');

  // Call next() to continue executing the next middleware or route handler
  await next();
};

1. In your server file, use the app.use() method to register your before filter middleware. Make sure to add it before any route handlers that need the filter to be applied.

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

// Register the before filter middleware
app.use(beforeFilter);

// Add your route handlers here...
app.use(async (ctx) => {
  ctx.body = 'Hello Koa!';
});

app.listen(3000);

Now, whenever a request is made to your Koa server, the beforeFilter middleware will be executed before the route handler is called. You can perform any desired logic or checks in the before filter, such as authentication, logging, or modifying the request/response objects.

Remember to call next() in the before filter to ensure that the next middleware or route handler is executed.