String.prototype.padStart()

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since April 2017.

The padStart() method of String values pads this string with a given string (repeated and/or truncated, if needed) so that the resulting string has a given length. The padding is applied from the start of this string.

Try it

const str1 = "5";

console.log(str1.padStart(2, "0"));
// Expected output: "05"

const fullNumber = "2034399002125581";
const last4Digits = fullNumber.slice(-4);
const maskedNumber = last4Digits.padStart(fullNumber.length, "*");

console.log(maskedNumber);
// Expected output: "************5581"

Syntax

padStart(targetLength)
padStart(targetLength, padString)

Parameters

targetLength: The length of the resulting string once the current str has been padded. If the value is less than or equal to str.length, then str is returned as-is.
padString Optional: The string to pad the current str with. If padString is too long to stay within targetLength, it will be truncated from the end. The default value is the space character (U+0020).

Return value

A String of the specified targetLength with padString applied at the start.

Examples

Using String.prototype.padStart()

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

Fixed width string number conversion

// 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"

Specifications

Specification
ECMAScript® 2026 Language Specification # sec-string.prototype.padstart