URL Encoder/Decoder

%20 is percent-encoding and means a space anywhere in a URL. + means a space only in query strings encoded as application/x-www-form-urlencoded, the format of HTML forms and URLSearchParams; in a path, + is a literal plus sign. Decode text that comes from a form or a query string with Form (+), and write a literal plus as %2B.

Use encodeURIComponent (Component) for a piece of a URL, such as one query value or one path segment: it also encodes / ? & = and #, so they cannot change the structure of the URL. Use encodeURI (Full URL) on a complete address: it leaves those characters alone and encodes spaces, non-ASCII letters, the % sign and characters such as " < > { } that URLs do not allow. Running encodeURI on a single value is a common bug, because an & inside it then starts a new parameter.

Either a % is not followed by two hexadecimal digits, as in 100% or %zz, or the %-encoded bytes are not valid UTF-8: a character cut in half such as %E2%82, or text encoded in the old Latin-1 charset, where é is %E9 instead of %C3%A9. The decoder only reads UTF-8 and shows which sequence failed and where. A literal % must be written %25.

It is a space that was encoded twice: the first pass turned the space into %20, and the second encoded the % of %20 as %25. Decoding it once gives %20, so decode the result again to get the space. Double encoding usually happens when code encodes a value that was already encoded.

The protocol, host, path and fragment are kept as they are, except that browsers write international domain names in punycode (xn--…). The query is rebuilt from the table with URLSearchParams, so spaces come out as + and characters are encoded the standard way: the rebuilt query can look different from the original while meaning the same. Everything runs in your browser and nothing is sent to a server.