You can use JavaScript to check if the value entered in an input field is a number. Here’s an example of how to do it:

<form>
  Number: <input type="text" id="number">
  <input type="button" value="Submit" onclick="validate()">
</form>
<script>
  function validate() {
    var number = document.getElementById("number").value;
    if (isNaN(number)) {
      alert("Not a number.");
    } else {
      alert("Number.");
    }
  }
</script>

In this example, when the user clicks the “Submit” button, the validate() function is called. This function gets the value of the “number” field, and check if it’s a number or not by using the isNaN() function. If the value is not a number, an alert is displayed to the user indicating that the input is not a number. Otherwise, it will show “Number”.

You can also use parseFloat() or parseInt() function to check if the input is a number

if(!parseInt(number) && !parseFloat(number)){
    alert("Not a number.");
}else{
    alert("Number.");
}

It is also a good practice to validate the input on the server side as well, to ensure that the validation is done even if the user has disabled JavaScript.

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