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JavaScript - test if letter is lowercase

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Created by:

Mariam-Barron

811

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

Quick solution:

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

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1. Practical example

Edit

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

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const isCapitalLetter = (letter) => {
    if (letter && letter.length === 1) {
        const expected = letter.toLowerCase();
        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

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2. With specified locale

Edit

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

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const isCapitalLetter = (letter, locale) => {
    if (letter && letter.length === 1) {
        const expected = letter.toLocaleLowerCase(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