Add a new field in existing REST API's response.

Add-field-in-rest-api.php

// Add a new field in REST API's on an existing WordPress object type.
// ref - https://developer.wordpress.org/reference/functions/register_rest_field/

add_action( 'rest_api_init', 'create_api_posts_meta_field' );
function create_api_posts_meta_field() { 
    register_rest_field( 
		  'POST_TYPE', 									                          // Post type {string/array}
		  'post-meta', 										                      // Field name {string}
		  array(												                  // Callback {array}
           'get_callback'    => 'get_post_meta_for_api',
           'schema'          => null,
        )
    );
}
 
function get_post_meta_for_api( $object ) {
	$post_id = $object['id'];                                   				// Post's Id
	$post_meta_fields = get_post_meta( $post_id );              				// Fetch post's meta data
	
	foreach( $post_meta_fields as $key => $val):
		if($key[0] == '_') { 								                    // if meta key start with '_' found
			unset($post_meta_fields[$key]);					                	// remove that key from returning array
		}
	endforeach;
  
  return $post_meta_fields;
}