How to allow access-control-allow-origin with koa?

@cortez.connelly 

To allow access-control-allow-origin with Koa, you can use the koa-cors middleware. Here's how you can do it:

1. Install the koa-cors package:

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npm install koa-cors

1. Import koa-cors in your Koa application:

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const Koa = require('koa');
const cors = require('koa-cors');

const app = new Koa();

// Use the cors middleware
app.use(cors());

1. Customize the CORS settings if needed:

You can pass options to the cors middleware to customize the CORS settings. For example, to allow requests from a specific origin, you can set the origin option:

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app.use(cors({
  origin: 'http://example.com',
}));

1. Handle preflight requests:

If your clients are making requests that require preflight requests (OPTIONS requests), you can use the koa-router middleware to handle these requests:

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

router.options('/*', (ctx) => {
  // Handle preflight requests
  ctx.status = 204;
});

app.use(router.routes());
app.use(router.allowedMethods());

By following these steps, you can easily allow access-control-allow-origin with Koa and customize the CORS settings as needed.