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How to customize the "multiple" attribute of a select tag in Symfony

I'm using Symfony 4 and Virtual Select, "A javascript plugin for dropdown with virtual scroll".

By default, when Symfony renders a multiple select, the HTML code looks like this :

<select multiple="multiple">

However, Virtual Select seems very picky, and only behaves properly if the multiple attribute doesn't have a value, like this :

<select multiple>

(Which, as far as I know, is the "official" syntax, multiple="multiple" isn't mentioned by Mozilla for example.)

Is there there a way to force Symfony to use the second syntax instead of the first ?

I thought of a few workaround which wouldn't work :

  • Writing the whole HTML myself wouldn't be practical as it is very long (hence Virtual Select).
  • I tried passing the "multiple" option in Javascript as described here, but that has no effect if the the multiple attribute with the wrong syntax is still present in the HTML.
  • I could replace Virtual Select, but I would rather not as it is already used in many other places.
Solution

  • In the end I used a workaround in javascript :

    document.getElementById('select#whatever').setAttribute('multiple','');

    Which seems to satisfy Virtual Select.

    (And then I had to pile up more workarounds for other issues between Virtual Select and Symfony. All in all, you probably should avoid using those two together.)