String .padStart() / padEnd() / repeat() Since: ES2017（ECMAScript 2017）

// Pads the beginning of a string until it reaches the target length.
string.padStart(targetLength)
string.padStart(targetLength, "padString")

// Pads the end of a string until it reaches the target length.
string.padEnd(targetLength)
string.padEnd(targetLength, "padString")

// Repeats a string a specified number of times.
string.repeat(count)

// Use padStart to zero-pad a number.
var num = "5";
console.log(num.padStart(3, "0")); // Outputs "005".

// Useful for formatting dates.
var month = "3";
var day = "7";
var date = "2026-" + month.padStart(2, "0") + "-" + day.padStart(2, "0");
console.log(date); // Outputs "2026-03-07".

// Use padEnd to left-align strings.
console.log("Name".padEnd(10, ".") + "Kuu");  // Outputs "Name......Kuu".
console.log("Price".padEnd(10, ".") + "500");  // Outputs "Price.....500".

// If the string is already at or beyond the target length, it is returned as-is.
console.log("Hello".padStart(3, "0")); // Outputs "Hello".

// Use repeat to repeat a string.
console.log("Ha".repeat(3)); // Outputs "HaHaHa".
console.log("-".repeat(20)); // Outputs "--------------------".

// Passing 0 to repeat returns an empty string.
console.log("abc".repeat(0)); // Outputs an empty string "".

padStart() and padEnd() pad the beginning or end of a string with characters until it reaches the specified length. If you omit the second argument, the string is padded with spaces. The most common use case is zero-padding numbers, which is handy for formatting dates and times.

repeat() returns a new string consisting of the original string repeated the specified number of times. It is useful whenever you need to repeat the same string multiple times, such as generating separator lines or building indentation strings. Passing 0 returns an empty string, while passing a negative number or Infinity throws an error.

All of these methods leave the original string unchanged and return a new string.