0埋め　padStart padEnd slice

文字列.padStart(長さ,[追加文字列]) // 追加文字列は省略可

console.log('10'.padStart(5, '*'));  // ***10
console.log('12345678'.padStart(5, '*'));  // 12345678
console.log('10'.padStart(5, '*-+'));  // *-+10
console.log('10'.padEnd(5, '*-+'));  // 10*-+
console.log('10'.padEnd(5));  // '10

const fromTime = Date.now();
let nowTime;
let count = 0;
showTime();

function showTime() {
  const timeoutId = setTimeout(() => {
    const pastMs = new Date(Date.now() - fromTime).getMilliseconds();
    const zeroPad = String(pastMs).padStart(4, '0');
    console.log(zeroPad);
    count++;
    showTime();
  }, 100);

  if (count > 10) {
    clearTimeout(timeoutId);
  }
}

const num = 1234;  // 0詰めする数字
const result = ("00000" + num).slice(-5);  // できた文字列
console.log(result);  // 01234

0埋め padStart padEnd slice

0埋め　padStart padEnd slice