How does javascript validate

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. 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.

HTML5 provides a bunch of attributes to help validate data. Here are some common validation cases:. When the input value matches the above HTML5 validation, it gets assigned a psuedo-class :valid , and :invalid if it doesn't.

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. 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. The error message is provided in-context thereby making the user experience intuitive. Interview Questions. Company Questions. Artificial Intelligence. Cloud Computing. Data Science. Angular 7. Machine Learning. Data Structures. Operating System.

Computer Network. Compiler Design. Computer Organization. Discrete Mathematics. Ethical Hacking. Computer Graphics. Your code must include appropriate logic to test correctness of data. We will take an example to understand the process of validation. Here is a simple form in html format. First let us see how to do a basic form validation. In the above form, we are calling validate to validate data when onsubmit event is occurring.

The following code shows the implementation of this validate function. Now we will see how we can validate our entered form data before submitting it to the web server.