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 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
strhas been padded. If the value is less than or equal tostr.length, thenstris returned as-is. padStringOptional-
The string to pad the current
strwith. IfpadStringis too long to stay withintargetLength, 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 |
Browser compatibility
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