Javascript "".charCodeAt(0) stuck at 55357?

Question

the following doesn't seem correct

"".charCodeAt(0);  // returns 55357 in both Firefox and Chrome

that's a Unicode character named ROCKET (U+1F680), the decimal should be 128640.

this is for a unicode app am writing. Seems most but not ALL chars from unicode 6 all stuck at 55357.

how can I fix it? Thanks.

https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/String/charCodeAt — Josh Lee, Mar 03 '13 at 02:38
@XahLee, it was a comment, not an answer. Please mark the best *answer* as accepted. — Jukka K. Korpela, Mar 03 '13 at 11:18

Daniel · Accepted Answer · 2013-03-04T15:04:43.220

10

JavaScript is using UTF-16 encoding; see this article for details:

Characters outside the BMP, e.g. U+1D306 tetragram for centre (), can only be encoded in UTF-16 using two 16-bit code units: 0xD834 0xDF06. This is called a surrogate pair. Note that a surrogate pair only represents a single character.

The first code unit of a surrogate pair is always in the range from 0xD800 to 0xDBFF, and is called a high surrogate or a lead surrogate.

The second code unit of a surrogate pair is always in the range from 0xDC00 to 0xDFFF, and is called a low surrogate or a trail surrogate.

You can decode the surrogate pair like this:

codePoint = (text.charCodeAt(0) - 0xD800) * 0x400 + text.charCodeAt(1) - 0xDC00 + 0x10000

Complete code can be found can be found in the Mozilla documentation for charCodeAt.

edited Mar 04 '13 at 15:04

answered Mar 03 '13 at 02:40

Daniel

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all great answers. Josh Lee's link to https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/String/charCodeAt , which contain code to fix the problem. – Xah Lee Mar 04 '13 at 03:16
Daniel, would you consider adding that Mozilla link? as it contains working code. thanks. – Xah Lee Mar 04 '13 at 03:18

score 5 · Answer 2 · edited May 23 '17 at 12:07

5

Tried this out:

> "".charCodeAt(0);
55357

> "".charCodeAt(1);
56960

Related questions on SO:

You might want to take a look at this too:

Getting it to work with higher values

edited May 23 '17 at 12:07

Community

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answered Mar 03 '13 at 02:36

Samuel Liew

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score 0 · Answer 3 · answered Mar 03 '13 at 02:34

I think it's because they're returning you the first code unit UTF-16 encoding of that character. I'm not sure there's much you can do, because they're returning a 16-bit value -- I would probably try manually decoding the character from the first two code units and then encoding it in UTF-32, which seems to be what you want.

Javascript "".charCodeAt(0) stuck at 55357?

3 Answers3