JavaScript Forms
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() <
let x = document.forms[«myForm»][«fname»].value;
if (x == «») <
alert(«Name must be filled out»);
return false;
>
>
The function can be called when the form is submitted:
HTML Form Example
JavaScript Can Validate Numeric Input
JavaScript is often used to validate numeric input:
Please input a number between 1 and 10
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
Automatic HTML form validation does not work in Internet Explorer 9 or earlier.
Data Validation
Data validation is the process of ensuring that user input is clean, correct, and useful.
Typical validation tasks are:
- has the user filled in all required fields?
- has the user entered a valid date?
- has the user entered text in a numeric field?
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.
HTML Constraint Validation
HTML5 introduced a new HTML validation concept called constraint validation.
HTML constraint validation is based on:
- Constraint validation HTMLInput Attributes
- Constraint validation CSS Pseudo Selectors
- Constraint validation DOM Properties and Methods
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 |