How to swap first and last letter case in each word after using reverse() in JavaScript

Question

I wanna try swapping cases for the first and last letters in each word that are already reversed using the reverse function.

Let's say I have a string like this.

I am Firdaus

And I wanna make it just like this.

I ma Suadrif

Here is how the script looks like

let reverseWords = str => {
    console.log(`Input: ${str}`)
    let words = str.split(" ").map(word => {
        let chars = word.split("").reverse();
        let result = chars.map((char, index) => {
            if (index == chars.length-1 && chars.length != 0 && char == char.toUpperCase() && index != 0) {
                return char.toLowerCase();
            }
            return char;
        });
        return result.join('');
    });
    return words.join(' ');
}
console.log(`Output: ${reverseWords("I am Firdaus")}`);

If you run the code above, the result is not like what I expected

The uppercase F successfully becomes lowercase f, but the lowercase s failed to become uppercase S. I am confused about how to swap the s become an uppercase S.

Is there a way and the best way to make the string just like what I expected?

Thanks in advance.

Answer 1

It look like you need to compare against the original string's casing to determine whether the returned character should be upper-cased or lower-cased. Use the index of the (reversed) character you're mapping over to access the original character at the same index in the original word.

const reverseWords = str => str
    .split(' ')
    .map(word => [...word].reverse()
        .map((char, i) => (
            word[i].toLowerCase() === word[i]
            ? char.toLowerCase()
            : char.toUpperCase()
        ))
        .join('')
    )
    .join(' ');
console.log(`Output: ${reverseWords("I am Firdaus")}`);