klientsidevalidering

Klientsidevalidering refers to validation of user input performed in the user's web browser before data is sent to a server. It commonly uses HTML5 form attributes and JavaScript to check values such as required fields, formats (email, URL), length, and numerical ranges.

In practice, developers use the constraint validation API (for example, checkValidity and reportValidity) and event handlers on forms or inputs, or implement custom validation logic in JavaScript. Progressive enhancement allows forms to function without JavaScript while providing enhanced feedback when JavaScript is available.

Benefits of klientsidevalidering include immediate feedback to users, reduced server load from invalid submissions, and a more responsive user experience. However, there are important limitations: data on the client can be manipulated, and validation can be bypassed, so client-side checks are not a security boundary. Server-side validation remains mandatory to ensure data integrity and protect against attacks.

Best practices emphasize using klientsidevalidering to catch obvious issues early and improve UX, while validating and sanitizing all input on the server. Use accessible error messages and summaries, support localization, and apply progressive enhancement to accommodate users with differing capabilities and devices. Also consider security controls such as HTTPS, input sanitization, and robust server-side validation to complement client-side checks.