I need a mask that always puts the negative sign in front of the typed value

Question

I need it this way: -20,00

Answer 1

A very simple implementation for this problem:

$('#mask').keyup(function(){
  if((this.value.search('-') == -1 && this.value.length > 0 )){
    $(this).val('-' + this.value);
  }
})

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<input type="text"  id="mask" >Máscara

Answer 2

Just concatenate the - (minus) signal to the returned value

$('input').change(function(){
 var input = "-" + ($('#numInput').val());

 console.log(input);

});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<input id="numInput">

To use in mathematical calculations requires turning the comma into a point

$('input').keyup(function(){
var input = "-" + $($(this)).val().replace(',','.');

 var result = ((100+parseFloat(input)).toFixed(2));

 console.log(result);

 var resultVirgula = result.replace('.',',');

 console.log(resultVirgula);

});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<input id="numInput">