You have two ids with the same name – “adults”.
You need to have only one id with this name.
still, you can access the input with this selector:
$('#search-box-adults #adults').val("YOUR VALUE")
so your function call should be:
only_arrows_number("#search-box-adults #adults", "#adults_number_up", "#adults_number_down");
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solved .val() don’t edit input