Lodash _.toUpper Across Different Locales

Lodash's _.toUpper method converts string inputs to uppercase, but its behavior across different locales is strictly limited by its internal implementation. This article examines how _.toUpper processes strings, why it does not provide custom locale-sensitive transformations, how it behaves with regional edge cases like Turkish or German characters, and what alternatives should be used when locale-aware uppercase conversions are required.

How Lodash _.toUpper Works Internally

The _.toUpper function in the Lodash JavaScript library is designed to safely convert any provided value to an uppercase string. Internally, the method performs two primary steps:

1. It converts the incoming input to a string using Lodash's internal toString conversion logic, safely handling null, undefined, symbols, and other primitives.
2. It delegates the casing transformation directly to JavaScript’s native String.prototype.toUpperCase() method.

Because _.toUpper directly invokes native toUpperCase(), it does not accept a locale argument or configure custom internationalization rules.

Default Unicode Mapping vs. Locale Sensitivity

JavaScript's standard String.prototype.toUpperCase() method applies the default Unicode standard case mappings. It does not take the user's environment, language settings, or explicit locale arguments into account.

Consequently, _.toUpper applies the exact same universal Unicode casing rules regardless of the target locale:

• Standard Latin Characters: Letters such as a-z map uniformly to A-Z.
• Accented Characters: Accented glyphs with universal mappings, such as é or ñ, become É or Ñ.
• Length-Changing Characters: Standard Unicode mappings that expand character counts are honored. For example, the German lowercase sharp s (ß) converts to SS.

However, _.toUpper fails to respect language-specific orthographic conventions that deviate from standard Unicode defaults.

The Turkish and Azerbaijani Dotless "I" Issue

The most notable consequence of lacking locale awareness occurs with Turkic languages, such as Turkish and Azerbaijani, which distinguish between dotted and dotless versions of the letter "I":

• In Turkish:
  • Lowercase i (dotted) should become uppercase İ (dotted).
  • Lowercase ı (dotless) should become uppercase I (dotless).

When using _.toUpper:

_.toUpper('i'); // Returns "I" instead of "İ"
_.toUpper('ı'); // Returns "I"

Because _.toUpper lacks locale parameters, it cannot produce the grammatically correct uppercase İ for Turkish text, leading to potential data corruption or improper text display in localized applications.

Locale-Aware Alternative: toLocaleUpperCase

If an application requires case conversions that respect specific linguistic rules, _.toUpper is not the appropriate tool. Instead, native JavaScript provides String.prototype.toLocaleUpperCase(), which accepts a BCP 47 language tag:

const text = 'istanbul';

// Standard Lodash (Non-locale-aware)
_.toUpper(text); 
// Output: "ISTANBUL" (Incorrect in Turkish)

// Native JavaScript (Locale-aware)
text.toLocaleUpperCase('tr-TR'); 
// Output: "İSTANBUL" (Correct in Turkish)

Summary

_.toUpper provides a safe, null-tolerant wrapper around JavaScript's default uppercase functionality, but it is entirely locale-agnostic. Across all environments and intended locales, it applies standard Unicode mappings. For applications requiring accurate language-specific conversions, developers should bypass _.toUpper in favor of native toLocaleUpperCase(locale).