JavaScript injection is a cyberattack where malicious JavaScript code is inserted into a web application. This code runs in the user's browser, often without their knowledge, and can be used to steal sensitive information, hijack user sessions, or manipulate website content. Attackers exploit vulnerabilities in websites that do not properly validate or sanitize user input, allowing them to inject harmful scripts. Preventing JavaScript injection involves secure coding practices, such as input validation and output encoding, to ensure only safe data is processed and displayed.