---
title: "Snippets"
manual: "Headless"
version: "main"
permalink: "https://docs.typo3.org/permalink/friendsoftypo3/headless:developer-snippets@main"
source: "Developer/Snippets.rst"
rendered: "2026-10-14T09:29:03+00:00"
---

# Snippets {#developer-snippets}

Drop-in recipes for the most common questions.

## Customise file output (URL signing, focus point, etc.) {#customise-file-output-url-signing-focus-point-etc}

Listen to `EnrichFileDataEvent`. See [Events you can listen to](https://docs.typo3.org/permalink/friendsoftypo3/headless:developer-events@main).

## Override the JSON form output {#override-the-json-form-output}

Implement
`FriendsOfTYPO3HeadlessFormDecoratorDefinitionDecoratorInterface`
or extend `AbstractFormDefinitionDecorator`, then point the form's
`renderingOptions.formDecorator` at it. See [EXT:form](https://docs.typo3.org/permalink/friendsoftypo3/headless:integrations-form@main).

## Add a JSON field to every page response {#add-a-json-field-to-every-page-response}

Extend the `page` object in TypoScript (loaded after the set):

```typoscript
page.10.fields {
  myField = TEXT
  myField.value = static value
  myDynamic = TEXT
  myDynamic.data = TSFE:id
}
```

## Read the current site's frontend URL {#read-the-current-site-s-frontend-url}

Inject `FriendsOfTYPO3HeadlessUtilityHeadlessFrontendUrlInterface`
and call `withRequest($request)->getFrontendUrl()`. Returns the
configured `frontendBase` for the active site and language.

## Detect headless mode in your own code {#detect-headless-mode-in-your-own-code}

Inject `FriendsOfTYPO3HeadlessUtilityHeadlessModeInterface` and
call `isEnabledFor($request)`. FULL mode always returns `true`; MIXED
mode only when the request's first `Accept` header value is exactly
`application/json` (the strict match is the API contract).

## Encode JSON safely from your own code {#encode-json-safely-from-your-own-code}

Inject `FriendsOfTYPO3HeadlessJsonJsonEncoderInterface` instead of
calling `json_encode` directly. It applies HTML-attribute-safe hex
flags (`JSON_HEX_APOS | JSON_HEX_AMP`) and the `headless.prettyPrint`
feature flag for free. Note that it does **not** throw on encoding
failures — a `JsonException` is caught, logged as critical, and the
string `"[]"` is returned instead.

## Decode nested JSON inside an array {#decode-nested-json-inside-an-array}

Inject `FriendsOfTYPO3HeadlessJsonJsonDecoderInterface` and call
`decode($array)`. Any string value that looks like JSON gets decoded
(nested structures come back as `stdClass` objects, the outer array
stays an array). Useful when stitching multiple `JSON` cObjects
together.

## Process a file with the same shape as the default response {#process-a-file-with-the-same-shape-as-the-default-response}

Inject `FriendsOfTYPO3HeadlessUtilityFileUtilityInterface` and
call `process($fileReference, ProcessingConfiguration::fromOptions($opts))`.
The output matches what content elements receive — your custom
endpoints stay shape-compatible with the rest of the JSON API.