const price = 9.9;
price.toString().padStart(5, '0'); // "09.9"

String(null).padStart(2, '0'); // "null"

// 金額分轉(zhuǎn)元并補(bǔ)零
function formatCurrency(cents) {
  const yuan = (cents / 100).toFixed(2);
  return yuan.padStart(8, ' '); // 對(duì)齊到8位
}

console.log(formatCurrency(12345)); // "  123.45"

// 10進(jìn)制轉(zhuǎn)8位二進(jìn)制
function toBinary(num) {
  return num.toString(2).padStart(8, '0');
}

console.log(toBinary(42)); // "00101010"

const logLevels = ['DEBUG', 'INFO', 'WARN'];
const messages = ['Starting app', 'User logged in', 'Memory low'];

// 生成對(duì)齊的日志輸出
logLevels.forEach((level, i) => {
  console.log(
    `[${level.padStart(5)}] ${messages[i].padEnd(20)}`
  );
});
/*
[DEBUG] Starting app        
[ INFO] User logged in      
[ WARN] Memory low          
*/

pie
    title 各方法CPU耗時(shí)占比
    "手動(dòng)循環(huán)填充" : 42
    "Array.join填充" : 29
    "padStart" : 29

// 生成銀行賬號(hào)格式：****-****-1234
const lastFour = '1234';
const masked = lastFour
  .padStart(12, '*')      // "********1234"
  .replace(/(.{4})/g, '$1-') // 每4位加分隔符
  .slice(0, -1);          // 移除末尾多余分隔符

console.log(masked); // "****-****-1234"

// 創(chuàng)建文本裝飾線
const title = " CHAPTER 1 ";
console.log(
  title.padStart(30, '═').padEnd(40, '═')
);
// "══════════ CHAPTER 1 ══════════"

'overflow'.padStart(3); // "overflow" 

'1'.padStart(3, true); // "tr1" 

'A'.padStart(5, 'XYZ'); // "XYXYA" 

// 安全墊片實(shí)現(xiàn)
if (!String.prototype.padStart) {
  String.prototype.padStart = function(targetLen, padStr) {
    targetLen = Math.floor(targetLen) || 0;
    if (targetLen <= this.length) return String(this);
    
    padStr = padStr ? String(padStr) : ' ';
    let repeatCnt = Math.ceil((targetLen - this.length) / padStr.length);
    
    return padStr.repeat(repeatCnt).slice(0, targetLen - this.length) 
           + String(this);
  };
}