Create a custom function to convert a field data In Drupal 7, I can create a custom function in the template.php file. here is an example of custom function:

function myCustomFunction($arg) {
    $data_converted = $arg . 'XXXXX';
    return $data_converted;
}


...and in Drupal 7 and inside my template I can call this function with passing a specific data / field data.

In Drupal 8, it's different, and I don't see how to proceed. In my .theme file, I tried to create a custom function. In my twig template I tried to call this custom function but I get an error : function not found.

With drupal 8 how can I proceed to implement a custom function (with args) ?
Thanks for your help, and eventualy yours examples.
In Drupal 7, I can create a custom function in the template.php file


To cut a very long story short, in Drupal 7 you should ideally have been  calling that function in a preprocess function, to prepare variables for the template, rather than calling it from the template directly.

In Drupal 8 it's now Fine™ to call functions directly in the template. But now you can't use PHP directly, so you need to write a Twig extension.

It's very easy, core even has its own that you can copy from. The important thing is to register the service class with a twig.extension tag, so the system picks it up.

See this blog post for a full example.