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:
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
Name: JavaScript Can Validate Numeric Input JavaScript is often used to validate numeric input:Please input a number between 1 and 10
Submit ADVERTISEMENT 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 HTML Input 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 For a full list, go to HTML Input Attributes.
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
