# 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
```


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