padStart(길이값, 빈길이만큼채울값); padEnd(길이값, 빈길이만큼채울값);

const one = 1;
const newOne = one.toString().padStart(2, 0);
console.log(newOne); // '01'

const id = "abcdefgh";
const sliced = id.slice(0, 4);
const bluredId = sliced.padEnd(id.length, "*");
console.log(bluredId); // 'abcd****'

참고: padStart | mdn web docs