String.prototype.padStart()

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.

const str = "5";

console.log(str.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

Browser compatibility

See also

• Polyfill of String.prototype.padStart in core-js
• es-shims polyfill of String.prototype.padStart
• padEnd()