How to Disable the WordPress JSON REST API without plugin

The WordPress JSON REST API can be useful for developers building apps using WordPress. Other site owners may find it unusable – and it could also potentially expose your website to DDoS attacks that can be resource intensive, hence slowing down the site.

The following code snippet will render the WordPress REST API disabled by returning an instance of WP_Error when it hits the authentication process,

Instructions
Add the this snippet to your functions.php file.

*Make sure to test that your site works as intended after disabling the WordPress JSON REST API.

Related links:
https://make.wordpress.org/core/features-as-plugins/
https://stackoverflow.com/questions/31518090/jetpack-json-rest-api-and-wp-api