JavaScript Form Validation
HTML form validation can be done by a JavaScript.
If a form field (fname) is empty, this function alerts a message, and returns false, to prevent the form from being submitted:
JavaScript Example
function validateForm() {
var
x = document.forms["myForm"]["fname"].value;
if (x == null || x == "") {
alert("Name must be filled out");
return false;
}
}
The function can be called when the form is submitted:
HTML Form Example
<form name="myForm" action="demo_form.asp" onsubmit="return validateForm()"
method="post">
Name: <input type="text" name="fname">
<input type="submit" value="Submit">
</form>
Try it Yourself
JavaScript Can Validate Numeric Input
JavaScript is often used to validate numeric input:
Automatic HTML Form Validation
HTML form validation can be performed automatically by the browser:
If a form field (fname) is empty, the required attribute prevents this form from being submitted:
HTML Form Example
<form action="demo_form.asp" method="post">
<input type="text" name="fname" required>
<input type="submit" value="Submit">
</form>
Try it Yourself
Automatic HTML form validation does not work in Internet Explorer 9 or earlier.
Data Validation
Data validation is the process of ensuring that computer input is clean, correct, and useful.
Typical validation tasks are:
Most often, the purpose of data validation is to ensure correct input to a computer application.
Validation can be defined by many different methods, and deployed in many different ways.
Server side validation is performed by a web server, after input has been sent to the server.
Client side validation is performed by a web browser, before input is sent to a web server.
HTML Constraint Validation
HTML5 introduced a new HTML validation concept called constraint validation.
HTML constraint validation is based on:
Constraint Validation HTML Input Attributes
| Attribute | Description |
|---|---|
| disabled | Specifies that the input element should be disabled |
| max | Specifies the maximum value of an input element |
| min | Specifies the minimum value of an input element |
| pattern | Specifies the value pattern of an input element |
| required | Specifies that the input field requires an element |
| type | Specifies the type of an input element |
Constraint Validation CSS Pseudo Selectors
| Selector | Description |
|---|---|
| :disabled | Selects input elements with the "disabled" attribute specified |
| :invalid | Selects input elements with invalid values |
| :optional | Selects input elements with no "required" attribute specified |
| :required | Selects input elements with the "required" attribute specified |
| :valid | Selects input elements with valid values |
For a full list, got to CSS Pseudo Classes.