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Does a browser case converter apply JavaScript's Unicode rules? Ours does, byte for byte - and the sharp s is where it shows

I work on OnePassApps, the team behind our own browser apps. We ship a browser Text Studio, and this one is about the letters underneath the eight case styles - specifically the character where a text transform usually lies to you.

I measured it today, 11 November 2026: signed out, one build, one browser, one line of input.

```
straße mcdonald's o'brien hello-world foo_bar ı İ
```

Eight styles, the same line through each of them, read back verbatim out of the result box:

| Style | Output |
| --- | --- |
| UPPERCASE | STRASSE MCDONALD'S O'BRIEN HELLO-WORLD FOO_BAR I İ |
| lowercase | straße mcdonald's o'brien hello-world foo_bar ı i̇ |
| Title Case | Straße Mcdonald's O'brien Hello-World Foo_Bar I İ |
| Sentence case | Straße mcdonald's o'brien hello-world foo_bar ı i̇ |
| camelCase | straßeMcdonaldSOBrienHelloWorldFooBarIİ |
| PascalCase | StraßeMcdonaldSOBrienHelloWorldFooBarIİ |
| snake_case | straße_mcdonald_s_o_brien_hello_world_foo_bar_ı_i̇ |
| kebab-case | straße-mcdonald-s-o-brien-hello-world-foo-bar-ı-i̇ |

Two of those outputs carry the story.

**UPPERCASE expands the sharp s.** The string starts with the German sharp s (U+00DF), and UPPERCASE turns it into two letters, SS - so the word comes out as STRASSE. A transform that only knows ASCII leaves the sharp s alone, because U+00DF is not in the ASCII range at all. Ours does not leave it alone. The sharp s is also the only letter in this line where UPPERCASE changes the length of the word.

**lowercase does the same thing in reverse on the dotted capital I.** The line ends with the dotless i (U+0131) and the capital I with dot (U+0130). lowercase leaves the dotless i alone and turns U+0130 into a plain i followed by a combining dot above (U+0307) - two code points, not one, so the lowercased output is not the same length in characters as the uppercase one. (I have written about that lowercasing behaviour in the Data Studio's column-name rule before, in our DEV Community article - the same rule, a different feature.)

Both of those are exactly what JavaScript does. I ran toUpperCase() and toLowerCase() on the same string in the same browser session and compared the two results: our UPPERCASE output is identical to toUpperCase() and our lowercase output is identical to toLowerCase(), character for character. I am not claiming what the app calls internally - I am claiming the two strings match, and that they match on the sharp s as well as on the plain ASCII letters.

The identifier styles inherit all of it, because they are built on the lowercased string. The camelCase output ends with the same two characters, with the U+0307 still attached, and the snake_case output carries the dotless i as its own segment.

Earlier on Substack I listed what the eight styles are called and what the panel looks like; this post is about the letter-level behaviour underneath those names.

The styles also disagree with each other about one more thing: the apostrophe. Title Case keeps mcdonald's as a single token, while camelCase, PascalCase, snake_case and kebab-case split it, so the same input gives Mcdonald's in one mode and McdonaldS in another. I measured that layer separately and it is not what this post is about.

Limits, so you can judge how far this goes: one build, one browser, signed out, one line of input, measured today. I did not test multi-line input, an empty input, or a line that is only punctuation, and the claim that the identifier styles match the plain ones is a comparison of these eight outputs, not a reading of the source.

The page itself says the work happens on your device - "Nothing leaves this tab." and "Text transformed locally." are on the panel.

If you have built a case converter, I would like to hear how you handle the sharp s, because getting that one right is the difference between a slug that round-trips and one that quietly changes.

Our profile lists the nine apps we ship and carries the link to the tool.