仅允许文本字段中的特定数字

Question

我有一个带有HTML表单的脚本，里面有一个文本字段，现在这个文本字段只允许数字。

虽然我希望文本字段只允许数字，并且只允许变量+1的当前值。 例如，假设变量（currentValue）的当前值为1，则可以放入文本字​​段的唯一数字是2。

这是我到目前为止的代码：

HTML：

<form id="value-form" action="value.php" method="POST" name="value-form">
<input onkeypress="return isNumberKey(event)" type="text" name="amount" id="amount" /> <!-- This is the text field I would like to only allow the value+1 -->
<input type="image"  src="Button1.png" id="customButton" value="submit" alt="but"/>
</form>

使用Javascript：

function isNumberKey(evt) //This is the function I use to only allow numbers in the text field
{
var charCode = (evt.which) ? evt.which : event.keyCode
if (charCode > 31 && (charCode < 48 || charCode > 57))
return false;

return true;
}

var request = new XMLHttpRequest();
request.open('GET', 'currentValueFile.txt', false);
request.send();

var currentValue = request.responseText //this is the variable value I want to only allow +1 in the text field
document.getElementById("currentValue").innerHTML = currentValue;

Answer 1

<form id="value-form" action="value.php" method="POST" name="value-form">
<input onkeypress="return isNumberKey(event)" type="text" name="amount" id="amount" /> <!-- This is the text field I would like to only allow the value+1 -->
<input type="image"  src="Button1.png" id="customButton" value="submit" alt="but"/>
</form>

<script type="text/javascript">
function toDec(code) {
  return code - 48;
}
function isNumberKey(evt) //This is the function I use to only allow numbers in the text field
{
    var charCode = (evt.which) ? evt.which : event.keyCode
    if (charCode > 31 && (charCode < 48 || charCode > 57 || toDec(charCode) != currentValue + 1))
      return false;

    return true;
}

</script>

<script type="text/javascript"> 
var request = new XMLHttpRequest();
request.open('GET', 'currentValueFile.txt', false);
request.send();

var currentValue = parseInt(request.responseText) //this is the variable value I want to only allow +1 in the text field

</script>