Validation form for javascript

JavaScript Forms

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

JavaScript Can Validate Numeric Input

JavaScript is often used to validate numeric input:

Please input a number between 1 and 10

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 HTMLInput 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

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

Источник

Data Validation – How to Check User Input on HTML Forms with Example JavaScript Code

Shruti Kapoor

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:

1. Validating the format of fields such as email address, phone number, zip code, name, password.
2. Validating mandatory fields
3. Checking the type of data such as string vs number for fields such as social security number.
4. 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:

How to set up validation with HTML5 functionality

HTML5 provides a bunch of attributes to help validate data. Here are some common validation cases:

Link to JSFiddle Here we have two required fields — First Name and Last Name. Try this example in JSFidle. If you skip either of these fields and press submit, you’ll get a message, «Please fill out this field». This is validation using in-built HTML5.

How to set up validation using JavaScript

1. What is defined as «valid» data? This helps you answer questions about the format, length, required fields, and type of data.
2. What happens when invalid data is entered? This will help you define the user experience of the validation — whether to show an error message inline or at the top of the form, how detailed should the error message be, should the form be sumitted anyways, should there be analytics to track invalid format of data? And so on.

You can perform JavaScript validation in two ways:

Inline validation using JavaScript

 
 First Name* Submit Please enter First Name 
 

const submit = document.getElementById("submit"); submit.addEventListener("click", validate); function validate(e) < e.preventDefault(); const firstNameField = document.getElementById("firstname"); let valid = true; if (!firstNameField.value) < const nameError = document.getElementById("nameError"); nameError.classList.add("visible"); firstNameField.classList.add("invalid"); nameError.setAttribute("aria-hidden", false); nameError.setAttribute("aria-invalid", true); >return valid; > 

#nameError < display: none; font-size: 0.8em; >#nameError.visible < display: block; >input.invalid

In this example, we check for required fields using JavaScript. If a required field is not present, we use CSS to show the error message.

Aria labels are modified accordingly to signal an error. By using CSS to show / hide an error, we are reducing the number of DOM manipulations we need to make. The error message is provided in-context thereby making the user experience intuitive.

HTML5 Constraint validation API

The required and pattern HTML attributes can help perform basic validation. But if you want more complex validation or want to provide detailed error messaging, you can use the Constraint Validation API.

Some methods provided by this API are:

The following properties are useful:

In this example, we will validate using HTML5 inbuilt methods such as required and length in conjunction with the Constraint Validation API to provide detailed error messages.

const nameField = document.querySelector("input"); nameField.addEventListener("input", () => < nameField.setCustomValidity(""); nameField.checkValidity(); console.log(nameField.checkValidity()); >); nameField.addEventListener("invalid", () => < nameField.setCustomValidity("Please fill in your First Name."); >); 

Don’t forget server side validation

Client side validation is not the only validation check you should do. You must also validate the data received from your client on the server side code to ensure that the data matches what you expect it to be.

You can also use server-side validation to perform business logic verifications that should not live on the client side.

Form Validation best practices

1. Always have server side validation, since malicious actors can bypass client side validation.
2. Provide detailed error messages in-context with the field that produced the error.
3. Provide an example of what the data should look like in case of an error message, such as — «Email did not match format — test@example.com»
4. Avoid using single error pages that involve redirection. This is bad user experience and forces the user to go back to a previous page to fix the form and lose context.
5. Always mark required fields.

Источник