You want to get exact class for particular plugin field.type, type in this in your devel for "entity_reference"

$field_type_manager = \Drupal::service('plugin.manager.field.field_type');
$class = $field_type_manager->getDefinition('entity_reference')['class'];

this will show you info

In drupal 7 when you moved modules from one directory to other, usually running drush rr (registry rebuild) would work, maybe you would need to clear caches. In drupal 8, this wont help. Paths are saved in APC and you will probably need to restart server.
For someone maybe 

Reading docs about drupal_add_js you might think you can add JS to header of html, but only thing you can choose is header or footer of your theme (as region), to add JS to header tag, you need to use 

In this post I will determine how many DB tables does each option you choose make.
 

Lets start with NODE, make a basic node, with just title and body, when we enter data for it, it will populare 6 tables.

1. node
2. node_revision
3. node__body
4. node_revision__body
5. node_field_data
6. node_field_revision

In lots of places you will see how to use drupal_http_request with drupal_http_build_query used on data, which is fine if you want to build regular query data (like the ones you have in URLs when using GET) but if you want to send JSON, you shouldn't use that, instead you need  json_encode function on data.

By default body field of a content type will not be shown among the fields to index, which is kind of strange but that is how it is as many other properties and fields of content are there by default.

Not many devs use try catch in custom code, especially in drupal 7. Thing is that it can save you from bigger problems if you wrap your (bad) code into that block, so lets just make simple example how to use it.

If you are using file_scan_directory to scan files in some directory and then loop over them to use them for some action, you probably also want to sort them, otherwise you will get them in order that is set in filesystem. So to do that, you need to set key in options to "filename" to use filename as key, so you can sort that associative array per key.

To flush some image style, you can install module for that with UI, but you can also do it in lean way, with just drush php command or if locally with devel/php window. If using drush use this command

 drush php-eval '$style = image_style_load("optimize_original"); image_style_flush($style);'

First get current language

$language =  \Drupal::languageManager()->getCurrentLanguage(\Drupal\Core\Language\LanguageInterface::TYPE_CONTENT)->getId();

then load term with providing term id