String.prototype.padStart() - JavaScript | MDN
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
js
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()
js
"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
js
// 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® 2027 Language Specification # sec-string.prototype.padstart |