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HTML form validation can be done by JavaScript.
If a form field (fname) is empty, this function alerts a message, and returns false, to prevent the form from being submitted.
<html>
<body>
<form name="myForm" action="#" onsubmit="return validateForm()" method="get">
Name: <input type="text" name="fname">
<input type="submit" value="Submit">
</form>
<script>
function validateForm() {
let x = document.forms["myForm"]["fname"].value;
if (x == "") {
alert("Name must be filled out");
return false;
}
}
</script>
</body>
</html>
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.
<form action="#" method="get">
Name: <input type="text" name="fname" required>
<input type="submit" value="Submit">
</form>
Data validation is the process of ensuring that user input is clean, correct, and useful.
Typical validation tasks are:
Most often, the purpose of data validation is to ensure correct user input.
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.
HTML5 introduced a new HTML validation concept called constraint validation.
HTML constraint validation is based on:
| 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 |
| maxlength | Specifies the maximum character length of an input element |
| minlength | Specifies the minimum character length of an input element |
| type | Specifies the type of an input element |
| 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 |
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