Will set up a Rest API endpoint that will give all our posts

Reference guide: wordpress link

It will be a 2 step process

1. Register Route

add_action( 'rest_api_init', function() {

	register_rest_route( 'myplugin/v1', '/author/(?P\d+)', array(
		'methods' => 'GET',
		'callback' => 'my_awesome_func',
	) );
	register_rest_route( 'myplugin/v1', '/allposts', array(
		'methods' => 'GET',
		'callback' => 'custom_get_all_posts',
	) );
});

2. Get the route to pull all the posts

function custom_get_all_posts( $data ) {
	$posts = get_posts( array(
	) );
	$args = array(
		'numberposts' => 10
	);
	if ( empty( $posts ) ) {
		return null;
	}
	return $posts;
}

Another example would be:

Having already an ID from the post, we can create a custom function which will get the data from that post.

function get_post_by_id( $data ) {
	$args = array(
		'p'	=> $data['id'],
		'post_type'	=> 'post'
	);
	$post = new WP_Query($args);
	return $post;
}