JavaScript does not have a built-in isUpperCase function, but checking whether a string is uppercase is a common task during string manipulation. In this guide we’ll explain how to determine if a character or a string is in uppercase using JS.

Understanding character codes

Each character in a string has an associated Unicode value which can be used to determine its case. For standard English letters, uppercase characters have code points from 65 ('A') to 90 ('Z').

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const charCode = 'A'.charCodeAt(0);
console.log(charCode); // 65

Using a regular expression

A regular expression can check if a string consists solely of uppercase characters:

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const isUpperCase = (str) => /^[A-Z]+$/.test(str);
console.log(isUpperCase("HELLO")); // true
console.log(isUpperCase("Hello")); // false

Checking individual characters

If you want to check individual characters instead of the whole string:

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const isCharUpperCase = (char) => char === char.toUpperCase();
console.log(isCharUpperCase("H")); // true
console.log(isCharUpperCase("h")); // false

Iterating over strings

To determine if every character in a string is uppercase, iterate over them:

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const isFullyUpperCase = (str) => {
  for (let i = 0; i < str.length; i++) {
    if (str[i] !== str[i].toUpperCase()) {
      return false;
    }
  }
  return true;
};

Handling non-English characters

Be cautious with non-English characters. toUpperCase might not behave as expected with characters that don't have a distinct uppercase form.

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console.log(isCharUpperCase("ß")); // false, even though "ß".toUpperCase() is "SS"

Using locale-specific methods

For locale-aware case conversions, use toLocaleUpperCase:

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const isCharUpperCaseLocale = (char, locale) => char === char.toLocaleUpperCase(locale);
console.log(isCharUpperCaseLocale("i", "tr-TR")); // true for Turkish locale

Edge cases to consider

• Non-alphabetic characters: Decide if you need to treat them as uppercase, lowercase, or neither.
• Empty strings: Define what the return value should be for an empty string input.
• Case conversion exceptions: Some characters don't convert neatly between cases, such as the German "ß".

Performance considerations

Regular expressions are convenient but might not be as fast as a loop-based approach for long strings. Always consider the context and the size of the data when choosing your method.