By Fresh authors

Forms are a common mechanism for letting users interact with applications. In the last few years it has become more and more common for web applications to move form submission entirely to the client. This can have useful properties for interactivity, but it is much worse for resiliency and user experience as a whole. Browsers have great built in systems for form submission, revolving around the HTML <form> element.

Fresh builds the core of its form submission infrastructure around the native <form> element. This page explains how to use <form> in Fresh, and the next chapter explains how to progressively enhance your forms with client side JavaScript to make them more interactive.

The way forms work in the browser, is that they perform an HTML navigation action when the user submits the form. In most cases this means that when the form is submitted, a GET or POST request is sent to the server with the form data, which then responds with a new page to render.

Fresh can handle both GET and POST requests through the custom handlers feature of routes. The handlers can perform any necessary processing on the form data, and then pass data to the ctx.render() call to render a new page.

Here is an example implementing a search form that filters an array of names server side:

// routes/search.tsx

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

const NAMES = ["Alice", "Bob", "Charlie", "Dave", "Eve", "Frank"];

interface Data {
  results: string[];
  query: string;
}

export const handler: Handlers<Data> = {
  GET(req, ctx) {
    const url = new URL(req.url);
    const query = url.searchParams.get("q") || "";
    const results = NAMES.filter((name) => name.includes(query));
    return ctx.render({ results, query });
  },
};

export default function Page({ data }: PageProps<Data>) {
  const { results, query } = data;
  return (
    <div>
      <form>
        <input type="text" name="q" value={query} />
        <button type="submit">Search</button>
      </form>
      <ul>
        {results.map((name) => <li key={name}>{name}</li>)}
      </ul>
    </div>
  );
}

When the user submits the form, the browser will navigate to /search with the query set as the q query parameter in the URL. The GET handler will then filter the names array based on the query, and pass it to the page component for rendering.