Due to security/privacy concerns, JSON REST API (WP API) only allows post meta data (custom field value) for authenticated users.
This plugin is an easy way to make post meta available when unauthenticated.
After activate the plugin, go to WP API Customizer option page.
Click the plus icon to add rows.
Set JSON attributes and custom field names in your posts that you want to use when unauthenticated.
The JSON REST API will provide the data as “post_meta” like the following:
"post_meta":{"your_attribute_a":"Custom Field A value","your_attribute_b":"Custom Field B value"}