SOURCE

function transform(str) {
    let res = "";
    for(let i =0;i<str.length;i++) {
        if(str[i] >= "A" && str[i] <="Z") {
            res +='_' +str[i].toLowerCase();
        } else {
            res +=str[i]
        }
    }
    return res
}
console.log(transform("strAbc"))