Repository: TypeScript
Issue: Capitalize does not work for characters which are 2 utf-16 code units
Labels: Suggestion,Awaiting More Feedback

### 🔎 Search Terms

Capitalize, utf-16, string, type

### 🕗 Version & Regression Information

- This is known to not work in 4.1.5 through 5.5.2 and current nightly (impacts all version with  `Capitalize`)


### ⏯ Playground Link

https://www.typescriptlang.org/play?#code/PQKhCgAIUhhBDADgSwC7wDbIF4FNLyQDOqATsgHYDmAdFCMOAGYCuFAxqsgPYWTtI0mHLgA8AZUi4AHqlwUAJkWJlKVAHwAKIgC5I4gJR6EKdFjwT1kAN7gAkMGCQAKtwIA3bsgXFEWVFzU-AAW8KTwnLikygDuwcjswQSk+AC28Ar43EyQqSwYXH74AKrOAGIAtACMAGz83JmQbGhEADT2jk1E+GhR8Fy8kJQkuBmQ2UOKMmqQqG7diGH9+KjB+EzI0aghS5GkdHbsvCRDcuFyPgC8xADa4gCeqQBG3Bg0vefcpAC6mgY0FBkqD+AG57MgcpoPssFDQFLx8JckbNSCxcAYbPYHE4AKKpRCoe4qchBATdA52FKoFikPgAIjpBGUJiE5jE4nUYLsAF9wPYqTS+FCzjCaO5MGiaHNiohEFEEN0-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

### 💻 Code

```ts
/**
 * Capitalize a string.
 */
function capitalize<S extends string>(s: S): Capitalize<S> {
	// To avoid splitting characters which are made of multiple UTF-16 code units,
	// use iteration instead of indexing to separate the first character.
	const iterated = s[Symbol.iterator]().next();
	if (iterated.done === true) {
		// Empty string case.
		return "" as Capitalize<S>;
	}

	return (iterated.value.toUpperCase() + s.slice(iterated.value.length)) as Capitalize<S>;
}

const data: [string, string][] = [
    ["", ""],
    ["a", "A"],
    ["aa", "Aa"],
    // Non-ascii
    ["ñx", "Ñx"],
    // Lowercase letter that is 2 UTF-16 code units:
    ["𞥃", "𞤡"],
    ["𞥃a", "𞤡a"],
];
for (const [input, expected] of data) {
    console.log(`input: "${input}", result: "${capitalize(input)}", expected: "${expected}"`);
}


type Test<Input extends string, Expected extends Capitalize<Input>> = Capitalize<Input>;


type _1 = Test<"", "">;
type _2 = Test<"a", "A">;
type _3 = Test<"aa", "Aa">;
type _4 = Test<"ñx", "Ñx">;

// These cases don't work.
type _5 = Test<"𞥃", "𞤡">;
type _6 = Test<"𞥃a", "𞤡a">;
```


### 🙁 Actual behavior

Capitalize appears to capitalize `theString[0]` which is not a correct implementation due to mishandling of characters which require two utf-16 code units and thus take up two indexes in the string.

For example, `Capitalize<"𞥃">` gives `"𞥃"`;

### 🙂 Expected behavior

When the first character in the string passed to Capitalize is in the Unicode "Lowercase Letter" category, that character should be replaced with its corresponding uppercase version.

For example, `Capitalize<"𞥃">` should give`"𞤡"`;

### Additional information about the issue

The included code provides a tested correct implementation for runtime use. This implementation is taken from my work here: https://github.com/microsoft/FluidFramework/blob/ddee06b429b26b571b7383d4acd95a62d3b9758c/experimental/dds/tree2/src/util/utils.ts#L394

That code is available under the MIT license and already copyright Microsoft, so feel free to use it in the TypeScript implementation if you find it useful to do so.

This will be a breaking change, but I highly doubt it will impact real use.