SOURCE

function caseConvert(str) {
    return str.replace(/([a-z]*)([A-Z]*)/g,(m,s1,s2)=>{
        return `${s1.toUpperCase()}${s2.toLowerCase()}`
    })
}

function upperStr(str){
    let strArr = [...str];
    let arrCase = ""
    for(var key in strArr) {
        if(/[a-z]/.test(strArr[key])) {
            arrCase += strArr[key].toUpperCase();
        } else {
            arrCase += strArr[key].toLowerCase();
        }
    }
    console.log(arrCase)
}

upperStr('aBcDeFJ')
console.log(caseConvert('aBcDsf1w33e34wefeFJ'))