JavaScript String toLocaleLowerCase() Method

What You’ll Learn

String.prototype.toLocaleLowerCase() returns a new lowercased string using locale-specific case rules (same API as MDN String.prototype.toLocaleLowerCase()). Learn the optional locales argument, why Turkish İ differs from English, how it compares to toLowerCase(), five examples, and try-it labs.

Introduction

Need lowercase text that respects a language’s alphabet rules — not only English Unicode defaults? toLocaleLowerCase() applies locale-specific case mappings.

For everyday English or ASCII, the result usually matches toLowerCase(). For languages like Turkish, the dotted/dotless i family differs, so passing the right locales tag matters for display and comparisons.

This page is part of JavaScript String Methods. Related topics include localeCompare() and repeat().

Understanding the toLocaleLowerCase() Method

str.toLocaleLowerCase(locales?) builds a new string where each character is lowercased according to the chosen locale’s rules (or the host default when you omit locales).

• Returns a new string — the original is unchanged.
• Optional locales: one BCP 47 tag or an array of tags.
• Unlike Intl.Collator, there is no locale matching — the first tag wins.
• Most locales match toLowerCase(); Turkish is the classic exception.

📝 Syntax

General form of String.prototype.toLocaleLowerCase:

str.toLocaleLowerCase()
str.toLocaleLowerCase(locales)

Parameters

• locales (optional) — a BCP 47 language tag (e.g. "en-US", "tr") or an array of tags. After validation, only the first locale is used (or the default locale if the list is empty). There is no fallback matching like other Intl APIs.

Return value

A new string representing the calling string converted to lower case according to locale-specific case mappings.

Exceptions

RangeError if a locale identifier is structurally invalid (same family of checks as other Intl locale arguments).

Common patterns

"ALPHABET".toLocaleLowerCase();           // "alphabet"
"İstanbul".toLocaleLowerCase("en-US");    // "i̇stanbul" (i + combining dot)
"İstanbul".toLocaleLowerCase("tr");       // "istanbul"
"\u0130".toLocaleLowerCase("tr") === "i"; // true
"HELLO".toLocaleLowerCase() === "HELLO".toLowerCase(); // usually true

Examples Gallery

Examples follow MDN String.toLocaleLowerCase() patterns. Use View Output or Try It Yourself for each case.

"ALPHABET".toLocaleLowerCase();
// "alphabet"

alphabet

const dotted = "İstanbul";

console.log(`EN-US: ${dotted.toLocaleLowerCase("en-US")}`);
// Expected output: "i̇stanbul"

console.log(`TR: ${dotted.toLocaleLowerCase("tr")}`);
// Expected output: "istanbul"

EN-US: i̇stanbul
TR: istanbul

"\u0130".toLocaleLowerCase("tr") === "i";
// true

"\u0130".toLocaleLowerCase("en-US") === "i";
// false

true
false

const locales = ["tr", "TR", "tr-TR", "tr-u-co-search", "tr-x-turkish"];

"\u0130".toLocaleLowerCase(locales) === "i";
// true

true

function displayLabel(text, locale) {
  return text.toLocaleLowerCase(locale);
}

const title = "İSTANBUL GUIDE";
const en = displayLabel(title, "en-US");
const tr = displayLabel(title, "tr");

console.log(en === tr);           // false — different mappings
console.log(tr.includes("istan")); // true
console.log("MIXED".toLocaleLowerCase() === "MIXED".toLowerCase()); // true

false
true
true

🎯 Common Use Cases

• Localized UI copy — headings and labels in the user’s language.
• Search / filter prep — normalize case before comparing Turkish (or similar) text.
• Multilingual apps — pair with localeCompare and the same locale tag.
• Not always needed — for ASCII-only codes and IDs, toLowerCase() is fine.
• Not for sorting — use localeCompare / Intl.Collator for order.

📝 Notes

• In most cases the result equals toLowerCase() — locale differences show up for specific languages.
• Locale arrays do not fall back: the first tag is always chosen after validation.
• Case conversion can change string length (one code point → multiple).
• For case-insensitive equality across locales, normalize both sides with the same locale.
• Pair with toLocaleUpperCase() when you need the matching uppercase path.

💡 Best Practices