Does + always mean a space?

No. Only in the query string, which is the part after the question mark. In the path a plus is a literal plus sign.

Why does decodeURIComponent leave my plus signs alone?

Because it follows the URL standard, where a plus has no special meaning. The plus-means-space rule comes from HTML form encoding, which is a separate specification. If you are decoding a query string, convert plus to space yourself first, or use URLSearchParams, which does it for you.

Should I use %20 or + for spaces?

%20 works everywhere in a URL. Plus only works in the query string. If you are unsure, use %20.

%2B is a plus sign

%2B is a literal plus sign.

Character	"+"
Name	Plus Sign
Encoded	%2B
Also seen as	%20
Reserved	Yes — it has a special meaning in a URL

Why it breaks things

The plus sign is the most confusing character in a URL because it means two different things depending on where it sits. In a query string — the part after the question mark — a bare plus means a space. That rule comes from HTML forms, which have encoded spaces as plus signs since the early web. In the path, a plus means an actual plus sign. So the same character reads differently either side of the question mark.

This is why a real plus sign has to be written as %2B in a query string. Phone numbers and email addresses are where it causes the most trouble: a search for +64 21 555 will arrive as a space followed by 64, and an address like [email protected] will lose its tag.

Note that decodeURIComponent in JavaScript never converts a plus to a space, so code that relies on it alone will get query strings wrong.

Real examples

Without encoding

https://example.com/search?q=+64 21 555

With encoding

https://example.com/search?q=%2B64%2021%20555

Without encoding, the leading plus is read as a space and the phone number loses its country code marker.

Without encoding

https://example.com/[email protected]

With encoding

https://example.com/signup?email=user%2Btag%40example.com

Email addresses using plus-addressing lose the tag unless the plus is encoded.

Decode something

Paste a URL or text

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Common questions

Does + always mean a space?: No. Only in the query string, which is the part after the question mark. In the path a plus is a literal plus sign.
Why does decodeURIComponent leave my plus signs alone?: Because it follows the URL standard, where a plus has no special meaning. The plus-means-space rule comes from HTML form encoding, which is a separate specification. If you are decoding a query string, convert plus to space yourself first, or use URLSearchParams, which does it for you.
Should I use %20 or + for spaces?: %20 works everywhere in a URL. Plus only works in the query string. If you are unsure, use %20.