String.prototype.padStart()

const str = "5";
console.log(str.padStart(2, "0"));
// 予想される結果: "05"
const fullNumber = "2034399002125581";
const last4Digits = fullNumber.slice(-4);
const maskedNumber = last4Digits.padStart(fullNumber.length, "*");
console.log(maskedNumber);
// 予想される結果: "************5581"

padStart(targetLength)
padStart(targetLength, padString)

"abc".padStart(10); // "       abc"
"abc".padStart(10, "foo"); // "foofoofabc"
"abc".padStart(6, "123465"); // "123abc"
"abc".padStart(8, "0"); // "00000abc"
"abc".padStart(1); // "abc"

// JavaScript version of: (unsigned)
// printf "%0*d" width num
function leftFillNum(num, targetLength) {
  return num.toString().padStart(targetLength, "0");
}
const num = 123;
console.log(leftFillNum(num, 5)); // "00123"