The JavaScript exception "malformed URI sequence" occurs when URI encoding or decoding wasn't successful.
Message
URIError: URI malformed (V8-based)
URIError: malformed URI sequence (Firefox)
URIError: String contained an illegal UTF-16 sequence. (Safari)
Error type
What went wrong?
URI encoding or decoding wasn't successful. An argument given to either the decodeURI, encodeURI, encodeURIComponent, or decodeURIComponent function was not valid, so that the function was unable encode or decode properly.
Examples
Encoding
Encoding replaces each instance of certain characters by one, two, three, or four escape sequences representing the UTF-8 encoding of the character. An URIError will be thrown if there is an attempt to encode a surrogate which is not part of a high-low pair, for example:
encodeURI("\uD800");
// "URIError: malformed URI sequence"
encodeURI("\uDFFF");
// "URIError: malformed URI sequence"
A high-low pair is OK. For example:
encodeURI("\uD800\uDFFF");
// "%F0%90%8F%BF"
Decoding
Decoding replaces each escape sequence in the encoded URI component with the character that it represents. If there isn't such a character, an error will be thrown:
decodeURIComponent("%E0%A4%A");
// "URIError: malformed URI sequence"
With proper input, this should usually look like something like this:
decodeURIComponent("JavaScript_%D1%88%D0%B5%D0%BB%D0%BB%D1%8B");
// "JavaScript_шеллы"