On keyup is not satisfying the condition

<label> Telugu</label>         
    <input type="text" onkeyup="return isNumber(event)" name="telugu"  id="telugu" maxlength="3"/> <br> <br>


JS 

<!DOCTYPE html>
<html>
<head>
<script>
function isNumber(event){
    var k= event.keyCode;
    console.log(k);
    if((k>47 && k<58))   /// THIS IS NOT WORKING
{
        console.log("entered");
        var s1 = document.getElementById("telugu").value;
        var s2= document.getElementById("hindi").value;
        var s3= document.getElementById("english").value;
        var s4= document.getElementById("maths").value;
        var s5= document.getElementById("science").value;
        if(s1<0 || s1>100){
            console.log("tel")
            document.getElementById("telugu").value = 0;      
        }

I want to input only numbers in a textbox. the condition is not working. If the value in the textbox is less than 0 or greater than 100. then I am resetting the value to 0. Resetting is working but the characters are also entering.

Solution

You could use a regex to remove everything that is not a digit. I also change to the input event which fires whenever the input changes.

If you want to force numbers you could also just set the type to type="number". The benefit for this is that it will automatically show the number keyboard on phones and tablets, though you can show this as well with the inputmode="numeric" attribute

// Get the textbox
const telugu = document.getElementById("telugu");

// Add event that fires whenever the input changes
telugu.addEventListener("input", () => {
    // Replace everything that is not a digit with nothing
  const stripped = telugu.value.replace(/[^\d]/g, "");
  
  // If the value is below 0 or above 100 set it to 0, else enter the stripped value
  stripped < 0 || stripped > 100
    ? telugu.value = 0
    : telugu.value = stripped;
});

<label for="telugu">Telugu</label>
<input type="text" name="telugu" id="telugu" maxlength="3"/>

Without comments:

const telugu = document.getElementById("telugu");

telugu.addEventListener("input", () => {
  const stripped = telugu.value.replace(/[^\d]/g, "");
  
  stripped < 0 || stripped > 100
    ? telugu.value = 0
    : telugu.value = stripped;
});

<label for="telugu">Telugu</label>
<input type="text" name="telugu" id="telugu" maxlength="3"/>

Simplified:

function validateValue(event) {
  var input = event.target;
  var stripped = input.value.replace(/[^0-9]/g, ""); /* Everthing that is not (^) in the range of 0 through 9 */
  
  if(stripped < 0 || stripped > 100) {
    input.value = 0;
  } else {
    input.value = stripped;
  }
}

<label for="telugu">Telugu</label>
<input type="text" oninput="validateValue(event)" name="telugu" id="telugu" maxlength="3"/>