> For the complete documentation index, see [llms.txt](https://docs.authgear.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.authgear.com/customization/events-hooks/denohooks.md).

# JavaScript / TypeScript Hooks

JavaScript / TypeScript Hooks are written as a [ES2015 module](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules). The module is executed by [Deno](https://deno.land/).

The module **MUST** have a [default export](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/export#description) of a function taking 1 argument. The argument is the [event](/customization/events-hooks.md#event-shape). The function can either be synchronous or asynchronous.

If the Hook is registered for a blocking event, the function **MUST** return a value according to the [specification](/customization/events-hooks.md#blocking-events).

The Hooks **DO NOT** have access to files or the environment. They only have access to the external network.

The stdout and the stderr of the Hooks are both ignored. Your hooks **MUST NOT** assume anything about the arguments and the stdin of the module.

## Configure JavaScript / TypeScript Hooks in your Authgear project

* In the portal, go to **Advanced** > **Hooks**.
* Add your JavaScript / TypeScript Hooks handler in **Blocking Events** and **Non-Blocking Events**, depending on which event you want to listen to.
* Click **Save**.

## Examples

Here is an example of a Hook for a blocking event.

```typescript
import { HookEvent, HookResponse } from "https://deno.land/x/authgear_deno_hook@v1.6.1/mod.ts";
export default async function(e: HookEvent): Promise<HookResponse> {
  // This hook simply allows the operation, which is identical to no-op.
  return { is_allowed: true };
}
```

An example to mutate a JWT token

```typescript
import {HookResponse, EventOIDCJWTPreCreate } from "https://deno.land/x/authgear_deno_hook@v1.0.0/mod.ts";

export default async function(event: EventOIDCJWTPreCreate): Promise<HookResponse> {
  return { 
    is_allowed: true,
    mutations: {
      jwt:{
        payload:{
          ...event.payload.jwt.payload, //the original payload in the jwt
          "https://myapp.com": {
            "custom_field": "custom_value"
          }
        }
      }
    }
  };
}
```

Here is an example of a Hook for a non-blocking event.

```typescript
import { HookEvent } from "https://deno.land/x/authgear_deno_hook@v0.3.0/mod.ts";
export default async function(e: HookEvent): Promise<void> {
  // This hook does nothing, which is identical to no-op.
}
```

## TypeScript Definition

<https://deno.land/x/authgear_deno_hook> is a TypeScript definition that aids you in writing a Hook. You can see the full definition at <https://deno.land/x/authgear_deno_hook/mod.ts>

If you are a Visual Studio Code user, you can [set up your editor](https://deno.land/manual@v1.27.2/references/vscode_deno) to take full advantage of the definition.

Alternatively, you can edit your hook and use the Deno CLI to typecheck.

```bash
$ deno check YOUR_HOOK.ts
```


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://docs.authgear.com/customization/events-hooks/denohooks.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.