I'd say this example doesn't fully show off what immutable data can do--it tends to help as things scale up to much larger code--but here's how I might do it in JS.

function generate_class_name( display_type, size, bordered, class_name_prop ) 
{
  classes = [
      'btn',
      ( display_type ? display_type : [] ),
      ( size ? size : [] ),
      ( bordered ? bordered : [] ),
      ( class_name_prop ? class_name_prop : [] ),
  ];

  return classes.flat().join( " " );
}

console.log( "<"
    + generate_class_name( "mobile", "big", null, null )
    + ">" );
console.log( "<"
    + generate_class_name( "desktop", "small", "solid", "my-class" ) 
    + ">" );
console.log( "<"
    + generate_class_name( null, "medium", null, null ) 
    + ">" );

Results:

<btn mobile big>
<btn desktop small solid my-class>
<btn medium>

Notice that JavaScript has a bit of the immutability idea built in here. The Array.flat() returns a new array with flattened elements. That means we can chain the call to Array.join( " " ). The classes array is never modified, and we could keep using it as it was. Unfortunately, JavaScript doesn't always do that; push() and pop() modify the array in place.

This particular example would show off its power a little more if there wasn't that initial btn class always there. Then you would end up with a leading space in your example, but handling it as an array this way avoids the problem.