String.prototype.padStart()

BaselineWidely available

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

ThepadStart() method ofString 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 currentstr has been padded. If the value is less than or equal tostr.length, thenstr is returned as-is.
padStringOptional: The string to pad the currentstr with. IfpadString is too long to stay withintargetLength, it will be truncated from the end. The default value is the space character (U+0020).

Return value

AString of the specifiedtargetLength withpadString 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 numfunction leftFillNum(num, targetLength) {  return num.toString().padStart(targetLength, "0");}const num = 123;console.log(leftFillNum(num, 5)); // "00123"