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


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:

HTML Form Example

<form action="/action_page.php" method="post">
  <input type="text" name="fname" required>
 
<input type="submit" value="Submit">
</form>
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 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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Reference: https://www.w3schools.com/js/js_validation.asp