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14.6 DOM 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:

Example 1: JS Validation - HTML Form Example part 1

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

JavaScript Can Validate Numeric Input

JavaScript is often used to validate numeric input:

Please input a number between 1 and 10
Submit

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:

Example 2: JS Validation - HTML Form Example part 2

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

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

For a full list, go to CSS Pseudo Classes.

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Module 14. JS HTML DOM

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Example files created in this module:
HTML DOM Methods JS HTML DOM - Finding HTML Element by Id JS HTML DOM - Finding HTML Elements by Tag Name part 1 JS HTML DOM - Finding HTML Elements by Tag Name part 2 JS HTML DOM - Finding HTML Elements by Class Name JS HTML DOM - Finding HTML Elements by CSS Selectors JS HTML DOM - Finding HTML Elements by HTML document.forms JS Changing HTML Content part 1 JS Changing HTML Content part 2 JS HTML DOM - Changing the Value of an Attribute JS HTML DOM - Dynamic HTML content JS HTML DOM - document.write() JS Validation - HTML Form Example part 1 JS Validation - HTML Form Example part 2 JS HTML DOM - Changing the HTML style JS HTML DOM - Changing the HTML style Using Events My first JS animation part 1 My first JS animation part 2 JS HTML Events - The onclick Attribute part 1 JS HTML Events - The onclick Attribute part 2 JS HTML Events - The onclick Attribute part 3 JS HTML Events - The onclick Attribute part 4 JS HTML Events - The onload Attribute JS addEventListener() part 1 JS addEventListener() part 2 JS addEventListener() part 3 JS addEventListener() part 4 JS addEventListener() part 5 JS addEventListener() part 6 JS addEventListener() part 7 JS addEventListener() part 8 JS removeEventListener() JS HTML Child Nodes and Node Values part 1 JS HTML Child Nodes and Node Values part 2 JS HTML Child Nodes and Node Values part 3 JS HTMLDOM - DOM Root Nodes part 1 JS HTMLDOM - DOM Root Nodes part 2 JS HTMLDOM - The nodeName Property JS HTMLDOM - The nodeType Property JS HTML DOM - Creating New HTML Elements (Nodes) JS HTML DOM - Creating new HTML Elements - insertBefore() JS HTML DOM - Removing Existing HTML Elements JS HTML DOM - Remove Child Element JS HTML DOM - Replacing an HTML Elements JS HTML DOM Collections JS HTML DOM Collections - HTMLCollection Length par 1 JS HTML DOM Collections - HTMLCollection Length part 2 JS HTML DOM - DOM Node Lists JS HTML DOM - DOM Node List Length part 1 JS HTML DOM - DOM Node List Length part 2