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function reverseString(value: string): string {
return value.split("").reverse().join("");
}This reverses UTF-16 code units, which is fine for many ordinary strings but can split some Unicode characters. If that matters, first decide whether you need to reverse code points or user-perceived characters.
What the code does
split("") turns the string into an array of UTF-16 code units, reverse() reverses the array, and join("") combines the units into a new string. The TypeScript handbook’s Basic Types page demonstrates this pattern as an official example.
TypeScript checks the code, but the reversal is performed by standard JavaScript string and array methods at runtime. TypeScript is a static typechecker for JavaScript, not a language feature that supplies a separate string-reversal operation; see the Handbook introduction.
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Why the basic method can break some Unicode characters
Some characters are encoded as multiple UTF-16 code units. For example, TypeScript 1.5’s notes explain that 𠮷 is represented by the surrogate pair 0xD842 and 0xDFB7. Reversing the individual units can separate that pair, producing text that looks corrupted rather than reversing the character intact. See the TypeScript 1.5 release notes.
There are different possible meanings of “reverse”:
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- UTF-16 code units: what the basic
split("").reverse().join("")implementation reverses. - Unicode code points: a code-point iteration approach can keep surrogate pairs together, but may still separate marks from their base letters or split multi-code-point emoji.
- User-perceived grapheme clusters: the visible characters a person may regard as single units. For this requirement, choose and verify a grapheme-aware approach suited to your runtime; the basic snippet and code-point iteration alone do not guarantee this behavior.
Use the right TypeScript type
Declare parameters and return values with the primitive string type, as in the example. The handbook’s Basic Types page contrasts it with the String wrapper type and recommends the lowercase primitive.
Check compatibility when using newer APIs
If you choose an alternative that relies on newer JavaScript APIs, check both the project’s TypeScript library declarations and the JavaScript runtime where the code will execute. The TSConfig reference for lib explains how library declarations correspond to platform APIs. A declaration lets TypeScript understand an API; it does not add that API to the runtime or provide a polyfill.
Runtime reversal is different from a type-level transformation
Template literal type intrinsics are for type-level string transformations, not for reversing a runtime string value. Their documented manipulations use JavaScript runtime string functions and are not locale-aware; see the template literal types documentation.
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