How to Check if a Letter is Uppercase in JavaScript

To check if a letter is uppercase in JavaScript, you can use the toUpperCase() method and compare it to the original letter.

function isUppercase(letter) {
    return letter === letter.toUpperCase();
}

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

Determining if a letter is uppercase is a common task in JavaScript, useful in form validation, text processing, and other applications. This guide will cover various methods to check if a letter is uppercase with detailed explanations and code examples.

Checking if a letter is uppercase is often required in various programming scenarios, such as validating user input, processing text files, or implementing case-sensitive logic. JavaScript offers multiple ways to achieve this, each with its own advantages.

Using the toUpperCase() Method

The toUpperCase() method converts a string to uppercase. By comparing the original letter with its uppercase version, you can determine if it is uppercase.

Example 1: Using toUpperCase()

function isUppercase(letter) {
    return letter === letter.toUpperCase();
}

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

Using Regular Expressions

Regular expressions (regex) provide a powerful way to match patterns in strings. You can use regex to check if a letter is uppercase.

Example 2: Using Regular Expressions

function isUppercase(letter) {
    return /^[A-Z]$/.test(letter);
}

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

Using ASCII Values

Every character in JavaScript has an associated ASCII value. Uppercase letters have ASCII values between 65 (A) and 90 (Z). You can use this to check if a letter is uppercase.

Example 3: Using ASCII Values

function isUppercase(letter) {
    let ascii = letter.charCodeAt(0);
    return ascii >= 65 && ascii <= 90;
}

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

Creating a Custom Function

You can encapsulate the logic for checking if a letter is uppercase in a custom function, making your code reusable and easier to maintain.

Example 4: Custom Function

function isUppercase(letter) {
    return letter === letter.toUpperCase() && /^[A-Z]$/.test(letter);
}

let letters = ['A', 'a', 'B', 'b', 'C', 'c'];
letters.forEach(letter => {
    console.log(`${letter} is uppercase: ${isUppercase(letter)}`);
});
// A is uppercase: true
// a is uppercase: false
// B is uppercase: true
// b is uppercase: false
// C is uppercase: true
// c is uppercase: false

Handling Edge Cases

When checking if a letter is uppercase, consider handling edge cases such as non-alphabetic characters, empty strings, and null or undefined values.

Example 5: Handling Edge Cases

function isUppercase(letter) {
    if (typeof letter !== 'string' || letter.length !== 1) {
        return false;
    }
    return /^[A-Z]$/.test(letter);
}

console.log(isUppercase('A')); // true
console.log(isUppercase('a')); // false
console.log(isUppercase('1')); // false
console.log(isUppercase('')); // false
console.log(isUppercase(null)); // false
console.log(isUppercase(undefined)); // false

Conclusion

Checking if a letter is uppercase in JavaScript can be accomplished using various methods, including the toUpperCase() method, regular expressions, ASCII values, and custom functions. Each approach has its benefits, and choosing the right one depends on your specific needs and the context in which you’re working.

By mastering these techniques, you can handle text processing tasks with confidence, making your web development projects more robust and maintainable.