JavaScript Forms
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:
Try it Yourself »
JavaScript Can Validate Numeric Input
JavaScript is often used to validate numeric input:
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
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
| 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
| :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.
Forms are ubiquitous in web applications. Some apps use forms to collect data to sign up users and provide an email address. Others use them to fulfill online transactions to facilitate a shopping experience.
You might use some web forms to apply for a new car loan, whereas you'll use others to order pizza for dinner. So it's important that the data collected from those forms is cleaned, formatted correctly, and devoid of any malicious code. This process is called form validation.
We need form validation anytime we are accepting user input. We must ensure that the data entered is in the correct format, lies within a valid range of data [such as for date fields], and does not contain malicious code that could lead to SQL injections. Malformed or missing data can also cause the API to throw errors.
What are the different types of form validations?
Form validation can happen on the client side and the server side.
Client side validation occurs using HTML5 attributes and client side JavaScript.
You may have noticed that in some forms, as soon as you enter an invalid email address, the form gives an error "Please enter a valid email". This immediate type of validation is usually done via client side JavaScript.
In other cases, you may have noticed that when you fill out a form and enter details such as a credit card, it may show a loading screen and then show an error "This credit card is invalid".
Here, the form made a call to its server side code, and returned a validation error after performing additional credit card checks. This validation case where a server-side call is made is called server side validation.
What data should be validated?
Form validation is needed anytime you accept data from a user. This may include:
- Validating the format of fields such as email address, phone number, zip code, name, password.
- Validating mandatory fields
- Checking the type of data such as string vs number for fields such as social security number.
- Ensuring that the value entered is a valid value such as country, date, and so on.
How to set up client side validation
On the client side, validation can be done in two ways:
- Using HTML5 functionality
- Using JavaScript
How to set up validation with HTML5 functionality
HTML5 provides a bunch of attributes to help validate data. Here are some common validation cases:
- Making fields required using
required - Constraining the length of data:
minlength,maxlength: for text dataminandmaxfor max value of num type
- Restricting the type of data using
type: