How to do a conditional router in koa?

@lily 

In Koa, you can create a conditional router by using the .use() method to add middleware that checks the condition and then decides whether to continue processing the request or to route it to a different handler.

Here's an example of how to create a conditional router in Koa:

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const Koa = require('koa');
const Router = require('koa-router');

const app = new Koa();
const router = new Router();

// Middleware to check the condition
const checkCondition = async (ctx, next) => {
  if (/* check your condition here */) {
    // If the condition is met, continue to the next middleware
    await next();
  } else {
    // If the condition is not met, route to a different handler
    ctx.body = 'Condition not met';
  }
};

router.get('/', checkCondition, async (ctx) => {
  ctx.body = 'Hello World!';
});

// Router for condition not met
router.get('/other', async (ctx) => {
  ctx.body = 'Condition not met';
});

app.use(router.routes());

app.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

In this example, the checkCondition middleware checks the condition, and if it is met, it calls next() to continue to the next middleware. If the condition is not met, it sets the response body to 'Condition not met'. The first route / then uses the checkCondition middleware to check the condition, and if it is met, it sets the response body to 'Hello World!'. If the condition is not met, the request is routed to the /other route, which sets the response body to 'Condition not met'.

This is a simple example, and you can customize the condition checking logic and response handling as needed for your application.