Show Post Content with AJAX

Create the JSON endpoint that accepts a WP_Post ID.

add_action( 'rest_api_init', function() {
    register_rest_route( 'api/v2', '/post/(?<id>\d+)', array (
        'methods'  => 'GET',
        'callback' => 'get_post_content',
    ) );
} );

Handle the endpoint to return a post’s content based on the ID passed.

function get_post_content( WP_REST_Request $request ) {

    $post = get_post( absint( $request->get_param( 'id' ) ) );

    if ( ! $post ) {
        return 'Invalid ID';
    }

    return apply_filters( 'the_content', $post->post_content );
}

Get the data with jQuery.

<script>
jQuery.get( "/wp-json/api/v2/post/504", function( data ) {

    jQuery( ".result" ).html( data );

});
</script>

---

(Optional) – Localize the REST URL .

wp_localize_script( 'wp-api', 'wpApiSettings', array( 'root' => esc_url_raw( rest_url() ) ) );

<script>
jQuery.get( wpApiSettings.root + "api/v2/post/504", function( data ) {
    jQuery( ".result" ).html( data );
});
</script>