How to render template for custom module in drupal 8?

It is easy way to render/call template(.html.twig) file for custom module in drupal 8, But here concept is slightly different from drupal 7.
First we will go to the Controller file:
Like our controller name is ContactController.php so we will write there code like.
//Go to the ContactController.php file..
/**
 * @file
 * Contains \Drupal\contact\Controller\ContactController.
 */

namespace Drupal\contact\Controller;
use Drupal\node\Entity\Node;
use Drupal\Core\Controller\ControllerBase;


class Contactv4Controller extends ControllerBase {  
    public function indexv2() {
     
      $data = "<div>Testing</div>";
     
      return [
       '#theme'  => 'specific_contact',
       '#contact'   => $data,
    ];
  }
}

// Then go to the contact.module file.
/**
* Implements hook_theme() to add the template definition..
**/
function contact_theme($existing, $type, $theme, $path) {
  return array(
       'specific_contact' => [
        'variables' => [
            'contact' => NULL,
       ]
    ]
  );
}
 
// Now in the create template folder and the file with name of 'specific_contact.html.twig'.
And render the variable there like: <p> {{ contact }} </p>
And clear the cache after this.
Now Front end result will be 'Testing' on new template page.


Note: If have any suggestions or issue regarding 'How to render template for custom module in drupal 8?' then you can ask by comments.   

How to add JS file using theme file in Drupal 8 ?

Sometimes we add Jquery file by libraries.yml but if we want to add JS file by `.theme` file in drupal 8 so we will add <script> code in `hook_page_attachments_alter`.
There we will create a variable and put the script code and then add in $page['#attached']['html_head'][].
We will add #tag is script and pass the script code variable using this code of line..

\Drupal\Core\Render\Markup::create($javascript_header)
 
here $javascript_header is our script code variable.

Example:
<?php
function hook_page_attachments_alter(array &$page) {   
   
$javascript_header = 'var hmt = hmt || [];
(function()
{
var hm = document.createElement("script");
hm.src = "https://hm.baidu.com/hm.js?1236547890";
var s = document.getElementsByTagName("script")[0]; s.parentNode.insertBefore(hm, s);
}
 )();';
        $page['#attached']['html_head'][] = [       
            [       
              '#tag' => 'script',         
              '#value' => \Drupal\Core\Render\Markup::create($javascript_header),       
              '#weight' => -1,
            ],       
            'key'
        ];
   
}
?>
So this is simple way to add JS file in header of project of Drupal 8.

Note: If have any suggestions or issue regarding 'How to add JS file using theme file in Drupal 8 ?' then you can ask by comments.  

How to remove or unset META tag from web page?

If we want to hide or unset or remove META Tag from our web page then we have to use `hook_page_attachments_alter` in `.theme` file.
we have to create an array and pass the name of `META name` which we want to remove or unset from web page.
like..
$unset_meta = [
    'title', // Meta name "title"
    'description', // Meta name "description"
    'keywords', // Meta name "keywords"
    'Generator'     // Meta name "Generator"
];

then with the help of $page['#attached']['html_head'], page attachments html_head we have to check that meta name if exist then we have to unset this.
we are giving here an example for this..

Example:
function moldev_page_attachments_alter(array &$page) { // here moldev is my theme name.
 // Define an array of META tags to remove.
   $unset_meta = [
    'title', // Meta name "title" 
    'description', // Meta name "description"
    'keywords', // Meta name "keywords"
    'Generator'     // Meta name "Generator"   
   ];
  // Check values and delete.
   foreach ($page['#attached']['html_head'] as $key => $value) {
     if (in_array($value[1], $unset_meta)) unset($page['#attached']['html_head'][$key]);
   }
}

if you want to delete/remove/unset any link rel="alternate" and hreflang="en" or any language then follow the below given code.
foreach ($page['#attached']['html_head_link'] as $key => $attachment) {   
          if ($attachment[0]['rel'] == 'alternate' && $attachment[0]['hreflang'] == 'en') {
               unset($page['#attached']['html_head_link'][$key]);
          }
}

Note: If have any suggestions or issue regarding 'How to remove or unset META tag from web page?' then you can ask by comments.  

Programmatically add canonical, shortlink, viewport and robots in site header.

For add Programmatically, first we will work on .theme file, we will create alter attachments in .theme file like 'theme_name_page_attachments_alter(array &$page).
Code for add 'viewport' in html head is..
<?php
function theme name/module name_page_attachments_alter(array &$page) {
    $viewport = [
      '#type' => 'html_tag',
      '#tag' => 'meta',
      '#attributes' => [
        'name' => 'viewport',
        'content' => 'width=device-width, initial-scale=1, maximum-scale=1, user-scalable=no',
      ],
    ];
    $page['#attached']['html_head'][] = [$viewport, 'viewport'];

global $base_url;
$current_page = \Drupal::request()->getpathInfo();   
$canon_url = $base_url.''.$current_page;  
  
Code for add 'canonical' in html head is..
    $viewport_canonical = [
      '#type' => 'html_tag',
      '#tag' => 'link',
      '#attributes' => [
        'rel' => 'canonical',
        'href' => $canon_url,
      ],
    ];
    $page['#attached']['html_head'][] = [$viewport_canonical, 'canonical'];   

Code for add 'shortlink' in html head is..
    $viewport_shortlink = [
      '#type' => 'html_tag',
      '#tag' => 'link',
      '#attributes' => [
        'rel' => 'shortlink',
        'href' => $canon_url,
      ],
    ];
    $page['#attached']['html_head'][] = [$viewport_shortlink, 'shortlink'];       
       

For add robots programmatically on site header, we have to work on 'theme_name_preprocess_html(&$variables).
Code for add 'robots' in html head is..
    $noindex = [
        '#tag' => 'meta',
        '#attributes' => [
            'name' => 'robots',
            'content' => 'noindex, nofollow',
        ],
    ];
    $variables['page']['#attached']['html_head'][] = [$noindex, 'noindex'];     

?>

Note: If have any suggestions or issue regarding 'Programmatically add canonical, shortlink, viewport and robots in site header.' then you can ask by comments.