Drupal8

This CodeLet set title and body field values using Ajax.
Place this CodeLet to your custom module file(.module) or Create a Custom module for Drupal 8 as per documentation https://www.drupal.org/docs/8/creating-custom-modules.

First of all add a field(field_link) to your content type. after that add an ajax to that field as per CodeLet.
Ajax example to replace multiple fields on ajaxCallback.
You set multiple fields value on ajax trigger in Drupal 8.

Use this CodeLet to remove "N/A" option from radio button list options.
Place this CodeLet to your custom module file(.module) or Create a Custom module for Drupal 8 as per documentation https://www.drupal.org/docs/8/creating-custom-modules

This CodeLet let you programmatically create a form page and render same form on a block.
Create custom module and follow CodeLet instruction to create a form and block programmatically.

This CodeLet Sends an email after a node of specific content type is created. The CodeLet uses Drupal8 API which sends and email upon node creation. This CodeLet implement hook_entity_insert & hook_mail.
Place this CodeLet on your MYMODULE.module file.

This CodeLet will display a block content in a region programmatically.
You have to implement hook_preprocess_HOOK() in THEMENAME.theme file of your theme directory.