Include ACF fields in WP-API response

functions.php

<?php
/**
 * Include ACF fields in REST API response
 *
 * Post, Page or Custom Posttype
 */
function my_theme_acf_to_rest_api( $response, $post, $request ) {
	if ( ! function_exists( 'get_fields' ) ) {
		return $response;
	}

	if ( isset( $post ) ) {
		$response->data['acf'] = get_fields( $post->ID );
	}

	return $response;
}
add_filter( 'rest_prepare_post', 'my_theme_acf_to_rest_api', 10, 3 );
add_filter( 'rest_prepare_page', 'my_theme_acf_to_rest_api', 10, 3 );
//add_filter( 'rest_prepare_{my_custom_posttype}', 'my_theme_acf_to_rest_api', 10, 3 ); // Custom posttype


/**
 * Include ACF fields in REST API response
 *
 * Userdata
 */
function my_theme_acf_to_user_rest_api( $response, $user, $request ) {
	if ( ! function_exists( 'get_fields' ) ) {
		return $response;
	}

	if ( isset( $user ) ) {
		$response->data['acf'] = get_fields( 'user_' . $user->ID );
	}

	return $response;
}
add_filter( 'rest_prepare_user', 'my_theme_acf_to_user_rest_api', 10, 3 );

This code snippet is no longer required. WP-API Support has been included in the latest Advanced Custom Fields v5.11 release 🎉
https://www.advancedcustomfields.com/blog/acf-5-11-release-rest-api/
https://www.advancedcustomfields.com/resources/rest-api/