Alternate casing a string in JavaScript

We are required to write a JavaScript function that takes in a string and constructs a new string with all the uppercase characters from the original string converted to lowercase and all the lowercase characters converted to uppercase from the original string.

For example: If the string is ?

const str = 'The Case OF tHis StrinG Will Be FLiPped';

Expected Output

Then the output should be ?

const output = 'tHE cASE of ThIS sTRINg wILL bE flIpPED';

Using Manual Character Code Manipulation

This approach manually checks character codes and applies the ASCII difference between uppercase and lowercase letters (32):

const str = 'The Case OF tHis StrinG Will Be FLiPped';

const isUpperCase = char => char.charCodeAt(0) >= 65 && char.charCodeAt(0) <= 90;
const isLowerCase = char => char.charCodeAt(0) >= 97 && char.charCodeAt(0) <= 122;

const flipCase = str => {
    let newStr = '';
    const margin = 32;
    for(let i = 0; i < str.length; i++){
        const curr = str[i];
        if(isLowerCase(curr)){
            newStr += String.fromCharCode(curr.charCodeAt(0) - margin);
        }else if(isUpperCase(curr)){
            newStr += String.fromCharCode(curr.charCodeAt(0) + margin);
        }else{
            newStr += curr;
        };
    };
    return newStr;
};

console.log(flipCase(str));

tHE cASE of ThIS sTRINg wILL bE flIpPED

Using Built-in String Methods (Simpler Approach)

A more readable approach using JavaScript's built-in toUpperCase() and toLowerCase() methods:

const flipCaseSimple = str => {
    return str.split('').map(char => {
        if (char === char.toUpperCase() && char !== char.toLowerCase()) {
            return char.toLowerCase();
        } else if (char === char.toLowerCase() && char !== char.toUpperCase()) {
            return char.toUpperCase();
        } else {
            return char; // Non-alphabetic characters remain unchanged
        }
    }).join('');
};

const testStr = 'Hello World 123!';
console.log(flipCaseSimple(testStr));

hELLO wORLD 123!

Comparison of Methods

Key Points

• ASCII codes for uppercase letters: 65-90 (A-Z)
• ASCII codes for lowercase letters: 97-122 (a-z)
• The difference between corresponding cases is always 32
• Non-alphabetic characters remain unchanged in both approaches

Conclusion

Both methods effectively flip the case of alphabetic characters. Use the built-in method approach for better readability and Unicode support, or the manual approach for slightly better performance with ASCII-only text.