JavaScript String toLowerCase() Method

What You’ll Learn

String.prototype.toLowerCase() returns a new string with every character lowercased using Unicode default rules (same API as MDN String.prototype.toLowerCase()). Learn the no-argument API, case-insensitive compares, when to prefer toLocaleLowerCase(), five examples, and try-it labs.

Introduction

Need every letter in lowercase — for display, search, or a fair comparison? toLowerCase() is the everyday tool.

It uses Unicode default case mappings (not a locale argument). For most English and ASCII text that is exactly what you want. When the UI language has special rules (Turkish dotted/dotless i), reach for toLocaleLowerCase() instead.

This page is part of JavaScript String Methods. Related topics include toLocaleLowerCase() and toLocaleUpperCase().

Understanding the toLowerCase() Method

str.toLowerCase() builds a new string where each character is converted to its lowercase form under Unicode default mappings.

• Returns a new string — the original is unchanged.
• Takes no parameters.
• Great for ASCII / English normalization and case-insensitive checks.
• For language-specific casing, use toLocaleLowerCase(locales).

📝 Syntax

General form of String.prototype.toLowerCase:

str.toLowerCase()

Parameters

None.

Return value

A new string representing the calling string converted to lower case.

Exceptions

None for normal string values. (Calling on null/undefined without boxing throws in the usual way.)

Common patterns

"ALPHABET".toLowerCase();                    // "alphabet"
"The Quick Brown Fox".toLowerCase();         // "the quick brown fox"
"Hello".toLowerCase() === "HELLO".toLowerCase(); // true
"MiXeD".toLowerCase();                       // "mixed"

Examples Gallery

Examples follow MDN String.toLowerCase() patterns plus practical beginner tips. Use View Output or Try It Yourself for each case.

const sentence = "The quick brown fox jumps over the lazy dog.";

console.log(sentence.toLowerCase());
// Expected output: "the quick brown fox jumps over the lazy dog."

the quick brown fox jumps over the lazy dog.

console.log("ALPHABET".toLowerCase());
// 'alphabet'

alphabet

const a = "Hello";
const b = "HELLO";

console.log(a === b);                         // false
console.log(a.toLowerCase() === b.toLowerCase()); // true

false
true

const dottedI = "\u0130"; // İ

console.log(dottedI.toLowerCase() === "i");              // false
console.log(dottedI.toLocaleLowerCase("tr") === "i");    // true

false
true

const fruits = ["Apple", "BANANA", "Cherry"];
const query = "BaNaNa";

const matches = fruits.filter(function (name) {
  return name.toLowerCase() === query.toLowerCase();
});

console.log(matches[0]);
// "BANANA"

console.log("MiXeD CaSe".toLowerCase());
// "mixed case"

BANANA
mixed case

🎯 Common Use Cases

• Display normalization — show labels in consistent lowercase.
• Case-insensitive compare — emails, usernames, tags, codes.
• Search / filter prep — lowercase query and candidates first.
• Not for Turkish UI — use toLocaleLowerCase("tr") when language rules matter.
• Not for sorting — use localeCompare / Intl.Collator.

📝 Notes

• The method does not change str itself — assign the result if you need it.
• There is no locales argument; use toLocaleLowerCase for that.
• For fair compares, lowercase both values the same way.
• Some characters may expand when case-converted (rare in English, common in other scripts).
• Pair with toUpperCase() when you need the uppercase mirror.

💡 Best Practices