JavaScript - test if letter is uppercase

In this article, we would like to show you how to test if the letter is uppercase using JavaScript.

Quick solution:

// ONLINE-RUNNER:browser;

console.log('A' === 'A'.toUpperCase());    // true
console.log('a' === 'a'.toUpperCase());    // false

1. Practical example

In this example, we use toUpperCase() method to compare a given letter with the letter in uppercase (expected).

// ONLINE-RUNNER:browser;

const isCapitalLetter = (letter) => {
    if (letter && letter.length === 1) {
        const expected = letter.toUpperCase();
        return expected === letter;
    }
    return false;
};


// Usage example:

console.log(isCapitalLetter('A'));    // true
console.log(isCapitalLetter('a'));    // false
console.log(isCapitalLetter('abc'));  // false
console.log(isCapitalLetter('ABC'));  // false
console.log(isCapitalLetter('123'));  // false

2. With specified locale

In this example, we use toLocaleUpperCase() method to compare a given letter with the letter in uppercase with specified locale (expected).

// ONLINE-RUNNER:browser;

const isCapitalLetter = (letter, locale) => {
    if (letter && letter.length === 1) {
        const expected = letter.toLocaleUpperCase(locale);
        return expected === letter;
    }
    return false;
};


// Usage example:

console.log(isCapitalLetter('П', 'ru-RU'));    // true
console.log(isCapitalLetter('п', 'ru-RU'));    // false
console.log(isCapitalLetter('пож', 'ru-RU'));  // false
console.log(isCapitalLetter('ПОЖ', 'ru-RU'));  // false
console.log(isCapitalLetter('123', 'en-US'));  // false

console.log(isCapitalLetter('П'));                      // true
console.log(isCapitalLetter('П', 'ru-RU'));             // true
console.log(isCapitalLetter('П', ['ru-RU', 'en-US']));  // true

See also

1. JavaScript - test if letter is lowercase

References

1. String.prototype.toUpperCase() - JavaScript | MDN
2. String.prototype.toLocaleUpperCase() - JavaScript | MDN