Add custom post meta to WordPress REST API

custom-fields.php

<?php

function get_custom_post_meta_cb($object, $field_name, $request) {
    return get_post_meta( $object['id'], $field_name, true );
}

function get_custom_post_meta_cb_the_content($object, $field_name, $request) {
    return apply_filters( 'the_content', get_post_meta( $object['id'], $field_name, true ) );
}

function update_custom_post_meta_cb($value, $object, $field_name) {
    if ( ! $value || ! is_string( $value ) ) { return; }
    return update_term_meta( $object->ID, $field_name, $value );
}

function api_register_rest_field($post_type, $field_id, $isEditor = false) {
    if ( isset( $post_type ) && isset( $field_id ) ) {
        if ($isEditor) {
            $get_callback = 'get_custom_post_meta_cb_the_content';
        } else {
            $get_callback = 'get_custom_post_meta_cb';
        }

        register_rest_field($post_type, $field_id, [
            'get_callback' => $get_callback,
            'update_callback' => 'update_custom_post_meta_cb',
            'schema' => null
        ]);
    }
}

function register_rest_custom_fields() {
    api_register_rest_field( 'page', 'about_como_construimos', false );
    api_register_rest_field( 'page', 'about_analise_fundamentos', true );
    api_register_rest_field( 'page', 'about_estruturacao_fidc', true );
    api_register_rest_field( 'page', 'about_revisao_monitoramento', true );
    api_register_rest_field( 'page', 'about_valores_text', false );
    api_register_rest_field( 'page', 'about_valores_image', false );
    api_register_rest_field( 'downloads', 'download_file', false );
}
add_action('rest_api_init', 'register_rest_custom_fields');