Call toLowerCase() on a runtime string: const lowercase = value.toLowerCase();. It returns a new string and leaves the original unchanged. Use TypeScript’s Lowercase<T> type instead only when you need to transform a string literal type at compile time.
Lowercase a runtime string with toLowerCase()
TypeScript uses JavaScript’s built-in string method to convert a value while the program runs:
const input: string = "Hello, TypeScript!";
const lowercase = input.toLowerCase();
console.log(lowercase); // "hello, typescript!"
console.log(input); // "Hello, TypeScript!"
The method returns a new string; it does not modify the original. See the MDN documentation for String.prototype.toLowerCase().
Choose between runtime conversion and Lowercase<T>
These forms operate at different stages and are not interchangeable:
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| Form | What it does | Example |
|---|---|---|
value.toLowerCase() |
Converts a string value at runtime. | "HELLO".toLowerCase() produces "hello". |
Lowercase<T> |
Transforms a string literal type at compile time; it does not change a runtime variable. | type Result = Lowercase<"HELLO">; makes Result the type "hello". |
The TypeScript Handbook explains that the intrinsic string manipulation types use JavaScript’s string functions and are not locale-aware. This is useful for type-level patterns such as constructing template-literal types, but it does not replace a runtime method call. Read the TypeScript Handbook section on template literal types.
Use locale-aware lowercasing when language rules matter
For text that should follow a particular language’s casing rules, call toLocaleLowerCase() and provide the intended locale when known. For example, Turkish casing can differ from default lowercasing:
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const city = "İstanbul";
const lowercaseForTurkish = city.toLocaleLowerCase("tr");
console.log(lowercaseForTurkish); // "istanbul"
This method also returns a new string. Choose the locale deliberately rather than assuming the default casing rules match the language of the text. See MDN’s toLocaleLowerCase() reference.
For case-insensitive comparison, choose comparison semantics deliberately
Lowercasing both strings before comparing them is convenient in some cases, but it is not a universal definition of linguistic equality. If comparison needs to respect a locale, consider Intl.Collator or localeCompare() with an appropriate sensitivity option. Select the locale and behavior that match your product’s requirements; the right choice depends on what distinctions the comparison should treat as equal. MDN discusses these options in its guide to JavaScript internationalization.
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