change case

changeLetterCase.js

function changeLetterCase(str, letterCase, firstLetterReversed) {
    if (typeof str !== "string") {
        throw Error(`First parameter - ${typeof str} but must be string`);
    }

    if (letterCase != "upper" && letterCase != "lower") {
        throw Error(
            `Invalid parameter - letterCase: "${letterCase}". Two valid options: lower and upper`
        );
    }

    const isLowerCase = code => 97 <= code && code <= 122;
    const isUpperCase = code => 65 <= code && code <= 90;

    const caseChangers = {
        upper: code => (isLowerCase(code) ? code - 32 : code),
        lower: code => (isUpperCase(code) ? code + 32 : code)
    };

    const caseChanger = caseChangers[letterCase];
    const caseChangerReverse =
        letterCase == "upper" ? caseChangers.lower : caseChangers.upper;

    let newStr = "";
    for (let i = 0; i < str.length; i++) {
        let code = str[i].charCodeAt(0);

        if (firstLetterReversed && i == 0) {
            code = caseChangerReverse(code);
        } else {
            code = caseChanger(code);
        }
        newStr += String.fromCharCode(code);
    }

    return newStr;
}