---
outline: deep
---

# Quickstart

Cap is a modern, lightweight, open-source CAPTCHA alternative using SHA-256 proof-of-work.

Unlike traditional CAPTCHAs, Cap:

- Is fast and unobtrusive
- Uses no tracking or cookies
- Uses proof-of-work instead of intrusive puzzles
- Is fully accessible and self-hostable

Here, try it yourself:

<Demo />

## Components

Cap consists mainly of the **widget** (can be used invisibly) and **server** (you can use the Standalone server instead). Alternatively, M2M is also supported and there's also a checkpoint middleware similar to Cloudflare.

This guide details how to use the usual setup. You can find guides on using the [Standalone server](./standalone/index.md), [M2M solver](./solver.md), and [checkpoint middleware](./middleware/index.md) in their respective sections.

We highly recommend checking out the [Standalone mode](./standalone/index.md) as it's complete, fast, simple to set up, and works with any language that can make HTTP requests. It also includes a dashboard, API key support, and more.

## Client-side

Start by adding importing the Cap widget library from a CDN:

::: code-group

```html [jsdelivr]
<script src="https://cdn.jsdelivr.net/npm/@cap.js/widget"></script>
```

```html [unpkg]
<script src="https://unpkg.com/@cap.js/widget"></script>
```

:::

::: warning  
You should optimally pin a specific version to avoid breaking changes instead of using @latest  
:::

Next, add the `<cap-widget>` component to your HTML.

```html
<cap-widget id="cap" data-cap-api-endpoint="<your cap endpoint>"></cap-widget>
```

You'll need to start a server with the Cap API running at the same URL as specified in the `data-cap-api-endpoint` attribute. In the server-side example we provided, it's set to `/api`, but you can change this by replacing every `app.post('/api/...', ...)` to `app.post('/<endpoint>/...', ...)`. We'll tell you how to set up the server in the next section.

Then, in your JavaScript, listen for the `solve` event to capture the token when generated:

```js{3}
const widget = document.querySelector("#cap");

widget.addEventListener("solve", function (e) {
  const token = e.detail.token;

  // Handle the token as needed
});
```

Alternatively, you can use `onsolve=""` directly within the widget or wrap the widget in a `<form></form>` (where Cap will automatically submit the token alongside other form data. for this, it'll create a hidden field with name set to its `data-cap-hidden-field-name` attribute or `cap-token`).

## Server-side

Cap is fully self-hosted, so you'll need to start a server exposing an API for Cap's methods running at the same URL as specified in the `data-cap-api-endpoint` attribute. This is easy since we've already pre-made a library to help you generate and validate challenges for you.

At least Node 14 is required. Most modern Bun or Deno versions should work too.

Start by installing it:

::: code-group

```bash [bun]
bun add @cap.js/server
```

```bash [npm]
npm i @cap.js/server
```

```bash [pnpm]
pnpm i @cap.js/server
```

:::

::: tip  
Don't use JavaScript on your backend? Try the [Standalone mode](./standalone/index.md).  
:::

Now, you'll need to change your server code to add the routes that Cap needs to work. Here's an example:

::: code-group

```js [elysia]
import { Elysia } from "elysia";
import Cap from "@cap.js/server";

const cap = new Cap({
  tokens_store_path: ".data/tokensList.json",
});

new Elysia()
  .post("/api/challenge", () => {
    return cap.createChallenge();
  })

  .post("/api/redeem", async ({ body, set }) => {
    const { token, solutions } = body;

    if (!token || !solutions) {
      set.status = 400;
      return { success: false };
    }

    return await cap.redeemChallenge({ token, solutions });
  })
  .listen(3000);

console.log(`🦊 Elysia is running at http://localhost:3000`);
```

```js [fastify]
import Fastify from "fastify";
import Cap from "@cap.js/server";

const fastify = Fastify();
const cap = new Cap({
  tokens_store_path: ".data/tokensList.json",
});

fastify.post("/api/challenge", (req, res) => {
  res.send(cap.createChallenge());
});

fastify.post("/api/redeem", async (req, res) => {
  const { token, solutions } = req.body;
  if (!token || !solutions) {
    return res.code(400).send({ success: false });
  }

  res.send(await cap.redeemChallenge({ token, solutions }));
});

fastify.listen({ port: 3000, host: "0.0.0.0" }).then(() => {
  console.log("Server is running on http://localhost:3000");
});
```

```js [bun.serve]
import Cap from "@cap.js/server";

const cap = new Cap({
  tokens_store_path: ".data/tokensList.json",
});

Bun.serve({
  port: 3000,
  routes: {
    "/api/challenge": {
      POST: () => {
        return Response.json(cap.createChallenge());
      },
    },
    "/api/redeem": {
      POST: async (req) => {
        const body = await req.json();
        const { token, solutions } = body;

        if (!token || !solutions) {
          return Response.json({ success: false }, { status: 400 });
        }

        return Response.json(await cap.redeemChallenge({ token, solutions }));
      },
    },
  },
});

console.log(`Server running at http://localhost:3000`);
```

```js [hono]
import { Hono } from "hono";
import Cap from "@cap.js/server";

const app = new Hono();
const cap = new Cap({
  tokens_store_path: ".data/tokensList.json",
});

app.post("/api/challenge", (c) => {
  return c.json(cap.createChallenge());
});

app.post("/api/redeem", async (c) => {
  const { token, solutions } = await c.req.json();
  if (!token || !solutions) {
    return c.json({ success: false }, 400);
  }

  return c.json(await cap.redeemChallenge({ token, solutions }));
});

export default {
  port: 3000,
  fetch: app.fetch,
};
```

```js [express]
import express from "express";
import Cap from "@cap.js/server";

const app = express();
app.use(express.json());

const cap = new Cap({
  tokens_store_path: ".data/tokensList.json",
});

app.post("/api/challenge", (req, res) => {
  res.json(cap.createChallenge());
});

app.post("/api/redeem", async (req, res) => {
  const { token, solutions } = req.body;
  if (!token || !solutions) {
    return res.status(400).json({ success: false });
  }
  res.json(await cap.redeemChallenge({ token, solutions }));
});

app.listen(3000, () => {
  console.log("Listening on port 3000");
});
```

:::

Note that these are the simplest examples possible. You should adapt them to your needs. Usually, you'll want to:

- Use an actual database such as SQLite or Redis for storing tokens and challenges. This is done for you in the [Standalone server](./standalone/index.md)

- Add some kind of actual ratelimiting. This is also done for you in the [Standalone server](./standalone/index.md)

### Token validation

Once the token is generated and captured, you can use it later to validate the user's identity. You can do this by calling `await cap.validateToken` in your server-side code:

```js
await cap.validateToken("..."); // returns { success: Boolean }
```

Note that the token will immediately be deleted after this. To prevent this, use `await cap.validateToken("...", { keepToken: true })`.