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Editorial Team
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Asked: 2026-05-13T17:40:49+00:00

I have written a set of javascript functions that allow me to validate user

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I have written a set of javascript functions that allow me to validate user input on a form. I only want to accept valid input, and impose the following behaviour:

When a user enters an invalid form, I display an alert and inform them that the value entered is incorrect. Crucially, the original (valid) value in the form is not changed.

The value is only changed when the value has been validated.

For example, suppose I want to accept only positive integers in a field.

This is the sequence of events that describes the desired behaviour.

Scenario 1 (valid input)

  1. Form loads with valid default in the input field
  2. User types in valid number
  3. Input field value is updated (as per normal form behaviour)

Scenario 2 (INvalid input)

  1. Form loads with valid default in the input field
  2. User types in INvalid number
  3. Alert box is shown alert(‘Invalid value’)
  4. Input field value is NOT CHANGED (i.e. the value is the same as BEFORE the user typed in the invalid number)

[Edit]

The only problem I am facing at the moment (i.e. what this question is seeking an answer for), is Scenario 2, action point 4. More specifically put, the question degenerates to the following question:

How do I stop the value of a field from changing, if I (somehow) determine that the value being entered by the user is invalid. This is really, all I’m trying to answer.

I am also doing server side checks, this question is just about the front end – i.e. refusing to change a field (form text input) value if I determine that the value is incorrect.

BTW, I am using jQuery, and would like to implement this in a manner that separates behaviour from display (I think this is what is meant by the term ‘unobtrusive’ ?)

Any suggestions on how to implement this behaviour as described above, would be very much appreciated.

PS: I dont want to use yet another jQuery plugin for this. I should be able to use jQuery + the simple javascript validation functions I have already written.

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1 Answer

  1. Editorial Team

    When loading the page, couldn’t you create a hidden form value or js variable in which you store the initial/default value for the field? When they change the form field, validate it, and if it passes, update the hidden field or js variable to match that in the field they updated.

    When the input given by the user fails validation, show the invalid entry along with the error message and then update the form field back to the value you have saved which would be either the
    default value or the last valid value that they entered.

    EDIT:
    Note that this is only a quick and (very) dirty example of doing what I explained in my answer above. If you have a lot of inputs, you will probably want to store the values in an associative array instead of in hidden form values, but this should give you a good handle on what I am suggesting. I would also strongly encourage you to NOT use alert boxes for notification of invalid answers.

    <html>
<head>
<script type="text/javascript">
    function validate()
    {
        var field1 = document.getElementById("field1");
        var saved  = document.getElementById("field1_save");
        if (field1.value < 0 || field1.value > 10)
        {
            alert("Field1 value of " + field1.value + " is invalid");

            // Change the value back to the previous valid answer
            field1.value = saved.value;
            return false;
        }

        // Save the valid input
        saved.value = field1.value;
        return true;
    }
</script>
</head>

<body>
Test User Input

<form name="form1"   id="form1" method="post">
<input name="field1" id="field1" type="text" value="2" onblur="validate();"/>
<input name="field1_save" id="field1_save" type="hidden" value="2" />

<input name="btnSubmit" type="submit" value="Submit" />                             
</form>
</body>
</html>
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