Resposta SOpt change mask

https://pt.stackoverflow.com/q/411307/35358

by Wurthmann

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery.mask/1.14.16/jquery.mask.js"></script>
<select id="docType" name="doc">
<option value="CPF">CPF</option>
<option value="CNPJ">CNPJ</option>
</select>

<input id="doc" type="text" name="doc" />

JavaScript

$("#doc").mask("999.999.999-99");

$('#docType').change(function () {
    if ($('#docType option:selected').val() == 'CNPJ') {
       $("#doc").mask("99.999.999/9999-99");
    } else {
        $("#doc").mask("999.999.999-99");
    }
});