How to add Javascript in sugarcrm?

Suppose you want to add cases_custom.js in cases module,

Open
custom/modules/Cases/metadata/editviewdefs.php file,

Add following code at right place in above file,

  'includes'=> array(
    array('file'=>'custom/modules/Cases/cases_custom.js'),
  ),

Now create file cases_custom.js inside custom/modules/Cases/metadata/cases_custom.js

That's it. Now write javascript inside,  cases_custom.js