URIError: malformed URI sequence

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#

{{jsxref("URIError")}}

What went wrong?#

URI encoding or decoding wasn't successful. An argument given to either the {{jsxref("decodeURI")}}, {{jsxref("encodeURI")}}, {{jsxref("encodeURIComponent")}}, or {{jsxref("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 {{jsxref("URIError")}} will be thrown if there is an attempt to encode a surrogate which is not part of a high-low pair, for example:

```js example-bad encodeURI("\uD800"); // "URIError: malformed URI sequence"

encodeURI("\uDFFF"); // "URIError: malformed URI sequence"

A high-low pair is OK. For example:

```js example-good
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:

```js example-bad decodeURIComponent("%E0%A4%A"); // "URIError: malformed URI sequence"

With proper input, this should usually look like something like this:

```js example-good
decodeURIComponent("JavaScript_%D1%88%D0%B5%D0%BB%D0%BB%D1%8B");
// "JavaScript_шеллы"

See also#

• {{jsxref("URIError")}}
• {{jsxref("decodeURI")}}
• {{jsxref("encodeURI")}}
• {{jsxref("encodeURIComponent")}}
• {{jsxref("decodeURIComponent")}}