Drupal 8 Custom Block how to attach JavaScript ?

Question

I have created a custom block successfully and just want to call java script alert function on the block. I have created a .js file .The issue is how to call the function declared in the javascript throgh array render in build function of the BLOCK PHP

Answer 1

Please elaborate the question. As per my understanding try this one. Reference

Attaching js to a render array of a Block Plugin

To give another example of attaching a library to a render array, If you are building a block plugin in your module, you can attach the libraries to the render array in the build() function of your class extending the BlockBase class (as of Drupal 8 beta 6).

return [
  '#theme' => 'your_module_theme_id',
  '#someVariable' => $some_variable,
  '#attached' => array(
    'library' => array(
      'your_module/library_name',
    ),
  ),
];

Answer 2

You can attach a library to a Block in a twig file:
1) supposing the block name is: block--foobar.html.twig
2) and you created a library in THEME.libraries.yml file called: contact-js
3) => you can attach the library to the Block by calling this in block--foobar.html.twig :
{{ attach_library('THEME/contact-js') }}