String.prototype.padStart() | MDNconst 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"
