JSFiddle - React, Tailwind, and code Playground

HTML

<form name="f1" action="" method="post">
<input type='text' name='qty' id='qty' value='1' onkeypress="return isNumber(event)" />
<input type='button' name='subtract' onclick='javascript: subtractQty();' value='-'/>
<input type='button' name='add' onclick='javascript: document.getElementById("qty").value++;' value='+'/>
<input type='submit' name='buy' value="buy">
<input type='hidden' name='product code' value="3">

</form>
    
<span>The Cost($): <span id="total">3000000</span></span>

JavaScript

function subtractQty(){
if(document.getElementById("qty").value - 1 < 1)
return;
else
document.getElementById("qty").value--;
}

function isNumber(evt) {
    evt = (evt) ? evt : window.event;
    var charCode = (evt.which) ? evt.which : evt.keyCode;
    if (charCode > 31 && (charCode < 48 || charCode > 57)) {
        return false;
    }
    return true;
}