Is uppercase used correctly JavaScript

In JavaScript, we can determine whether a string follows correct uppercase usage based on three specific rules. A string is considered to have correct uppercase usage if it meets any of these criteria:

• All letters in a word are capitals, like "INDIA".
• All letters in a word are not capitals, like "example".
• Only the first letter in a word is capital, like "Ramesh".

We need to write a JavaScript function that takes a string and determines whether it complies with any of these three rules, returning true if it does, false otherwise.

Using Loop-Based Approach

Here's a function that checks each character to validate the uppercase usage:

const detectCapitalUse = (word = '') => {
    let allCap = true;
    for (let i = 0; i  1)
                return false;
            else allCap = false;
        };
    };
    return true;
};

console.log(detectCapitalUse('INDIA'));    // All uppercase
console.log(detectCapitalUse('jdsdS'));    // Mixed case (invalid)
console.log(detectCapitalUse('dsdsdsd'));  // All lowercase
console.log(detectCapitalUse('Ramesh'));   // First letter capital

true
false
true
true

Using Regular Expressions

A more concise approach using regular expressions to match the three valid patterns:

const detectCapitalUseRegex = (word) => {
    // Pattern 1: All uppercase
    const allUpper = /^[A-Z]+$/;
    // Pattern 2: All lowercase  
    const allLower = /^[a-z]+$/;
    // Pattern 3: First letter uppercase, rest lowercase
    const firstUpper = /^[A-Z][a-z]*$/;
    
    return allUpper.test(word) || allLower.test(word) || firstUpper.test(word);
};

console.log(detectCapitalUseRegex('HELLO'));   // All uppercase
console.log(detectCapitalUseRegex('world'));   // All lowercase
console.log(detectCapitalUseRegex('JavaScript')); // First capital
console.log(detectCapitalUseRegex('javaScript')); // Invalid pattern

true
true
true
false

How It Works

The loop-based approach tracks whether all characters seen so far are uppercase using the allCap flag. When it encounters a lowercase letter, it checks if this violates the rules. The regex approach directly matches against the three valid patterns using separate regular expressions.

Comparison

Conclusion

Both approaches effectively validate uppercase usage in strings. The regular expression method offers cleaner, more readable code, while the loop-based approach provides more granular control over the validation logic.