나의 풀이

function solution(phone_number) {
    let privacyNum = [];
    let backNumber = [...phone_number].splice(-4);

    for (let i = 0; i < phone_number.length - 4; i++) {
         privacyNum.push("*");
    }

    let answer = [...privacyNum, ...backNumber];
    
    return answer.join("");
}