By Fresh authors

At their core, routes describe how a request for a given path should be handled, and what the response should be. To do this, routes have two main parts: the handler, and the component. A route can have either one, or both, but never neither.

The handler is a function that is called for every request to the route. It needs to return a response that is then sent to the client. The response could be anything: a plain text string, a JSON object, an HTML page, a WebSocket connection, a streaming file, or pretty much anything else. The handler is passed a render function that it can call to invoke rendering a component.

The component is the template for a page. It is a JSX element that is rendered on the server. The page component gets passed props that can be used by it to determine exactly what should be rendered. By default components receives props consisting of: the request URL, the matches from the URL pattern match, and any data passed to the handler's render function.

Let's look at a basic route that returns a plain text string:

// routes/plain.tsx

import { HandlerContext, Handlers } from "$fresh/server.ts";

export const handler: Handlers = {
  GET(_req: Request, _ctx: HandlerContext) {
    return new Response("Hello World");
  },
};

To define a handler, one needs to export a handler function or object from the route module. If the handler is an object, each key in the object is the name of the HTTP method that the handler should be called for. For example the GET handler above is called for GET requests. If the handler is a function, it is called for all requests regardless of the method. If an HTTP method does not have a corresponding handler, a 405 HTTP error is returned.

Now, let's render some HTML using the route component:

// routes/html.tsx

/** @jsx h */
import { h } from "preact";
import { PageProps } from "$fresh/server.ts";

export default function Page(props: PageProps) {
  return <div>You are on the page '{props.url.href}'.</div>;
}

The page component needs to be the default export of the route module. It is passed props that can be used to render the page.

As you can see in the second example, if no handler is explicitly defined a default handler is used that just renders out the page component if present. You can also override the default handler though to modify how exactly rendering should work.

In the below example, a custom handler is used to add a custom header to the response after rendering the page component.

// routes/html.tsx

/** @jsx h */
import { h } from "preact";
import { HandlerContext, Handlers, PageProps } from "$fresh/server.ts";

export const handler: Handlers = {
  async GET(_req: Request, ctx: HandlerContext) {
    const resp = await ctx.render();
    resp.headers.set("X-Custom-Header", "Hello World");
    return resp;
  },
};

export default function Page(props: PageProps) {
  return <div>You are on the page '{props.url.href}'.</div>;
}