register_rest_field()

function register_rest_field( $object_type, $attribute, $args = array() ) {
	$defaults = array(
		'get_callback'    => null,
		'update_callback' => null,
		'schema'          => null,
	);

	$args = gc_parse_args( $args, $defaults );

	global $gc_rest_additional_fields;

	$object_types = (array) $object_type;

	foreach ( $object_types as $object_type ) {
		$gc_rest_additional_fields[ $object_type ][ $attribute ] = $args;
	}
}

// Make sure to use PHP >= 5.4
add_action(
	'rest_api_init',
	function () {
		// Field name to register.
		$field = 'my_field';
		register_rest_field(
			'post',
			$field,
			array(
				'get_callback'    => function ( $object ) use ( $field ) {
					// Get field as single value from post meta.
					return get_post_meta( $object['id'], $field, true );
				},
				'update_callback' => function ( $value, $object ) use ( $field ) {
					// Update the field/meta value.
					update_post_meta( $object->ID, $field, $value );
				},
				'schema'          => array(
					'type'        => 'string',
					'arg_options' => array(
						'sanitize_callback' => function ( $value ) {
							// Make the value safe for storage.
							return sanitize_text_field( $value );
						},
						'validate_callback' => function ( $value ) {
							// Valid if it contains exactly 10 English letters.
							return (bool) preg_match( '/A[a-z]{10}Z/', $value );
						},
					),
				),
			)
		);
	}
);

add_action( 'rest_api_init', 'create_api_posts_meta_field' );

function create_api_posts_meta_field() {

	// register_rest_field ( 'name-of-post-type', 'name-of-field-to-return', array-of-callbacks-and-schema() )
	register_rest_field( 'post', 'post-meta-fields', array(
	       'get_callback'    => 'get_post_meta_for_api',
	       'schema'          => null,
	    )
	);
}

function get_post_meta_for_api( $object ) {
	//get the id of the post object array
	$post_id = $object['id'];

	//return the post meta
 	return get_post_meta( $post_id );
}

$meta = [
    'key' => 'my_post_type_name',
    'description' => 'This is the name of my post type',
    'type' => 'string'
];

register_rest_field('my_post_type', $meta['key'], [
    'get_callback' => function ($params) use ($meta) {
        return get_post_meta($params['id'], $meta['key'], true);
    },
    'update_callback' => function ($value, $object, $fieldName){
        return update_post_meta($object->ID, $fieldName, $value);
    }
]);

function add_last_date_data() {
  //Last date
  register_rest_field( 'post',
    'last_date',
    array(
        'get_callback'  => 'rest_get_last_date',
        'update_callback'   => null,
        'schema'            => null,
     )
  );
}
function rest_get_last_date( $object ) {
    //get the Post Id
    $post_id = $object['id'];
	global $gcdb;
	$sql = "SELECT * FROM gc_it_job_details WHERE post_id = '$post_id'"; //gc_it_job_details is job table
	$results = $gcdb->get_row($sql);
		if(count($results) > 0) {
			//We have this post_id row in the table
			return $results->last_date;	//last_date is the column name, change to your's
    
		} else return ""; //return nothing 
		
    
}
add_action( 'rest_api_init', 'add_last_date_data' );

// assume there custom post_type named events 
add_action( 'rest_api_init', function() {
    register_rest_field( 'events', 'event_date', array(

        'get_callback' => function( $comment_arr ) {
            $comment_obj = get_field('event_date', $comment_arr['id'] );
            return $comment_obj;
        },

        'update_callback' => function( $karma, $comment_obj ) {
            update_field('event_date', $karma, $comment_arr['id'] );
            return true;
        },

        'schema' => array(
            'event_date' => __( 'event_date' ),
            'type'        => 'text'
        ),

    ));

});