Why doesn't decodeURI("a+b") == "a b"?

Question

I'm trying to encode URLs in Ruby and decode them with Javascript. However, the plus character is giving me weird behavior.

In Ruby:

[Dev]> CGI.escape "a b"
=> "a+b"
[Dev]> CGI.unescape "a+b"
=> "a b"

So far so good. But what about Javascript?

>>> encodeURI("a b")
"a%20b"
>>> decodeURI("a+b")
"a+b"

Basically I need a method of encoding / decoding URLs that works the same way in Javascript and Ruby.

Edit: decodeURIComponent is no better:

>>> encodeURIComponent("a b")
"a%20b"
>>> decodeURIComponent("a+b")
"a+b"

Answer 1

+ is not considered a space. One workaround is to replace + with %20 and then call decodeURIComponent

Taken from php.js' urldecode:

decodeURIComponent((str+'').replace(/\+/g, '%20'));