jschardet is a JavaScript library that automatically detects the character encoding of a given text or file. It is a port of Python's chardet library and is useful for developers working with text data in various encodings, particularly when the encoding is unknown or mixed. The library supports a wide range of encodings, including UTF-8, Big5, Shift_JIS, and many others, making it valuable for web applications, text processing tools, and any project that needs to handle text data from diverse sources.