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JavaScript. W3Schools in English. Lessons for beginners


JavaScript Forms

JavaScript Form Validation

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

<form name="myForm" action="/action_page.html" onsubmit="return validateForm()" method="post">
Name: <input type="text" name="fname">
<input type="submit" value="Submit">
Try it Yourself »

JavaScript Can Validate Numeric Input

JavaScript is often used to validate numeric input:

Please input a number between 1 and 10

Try it Yourself »

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="/action_page.html" method="post">
  <input type="text" name="fname" required>
  <input type="submit" value="Submit">
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 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 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

For a full list, go to CSS Pseudo Classes.