In JavaScript, to pad a number with leading zeros, we can use the padStart() method. The padStart() method pads the current string with another string until the resulting string reaches the given length. The padding is applied from the start of the current string.

Syntax

str.padStart(targetLength, <padString>)

targetLength is the length of the string to be returned, padString is the source string, by default is empty string (” “).

Examples

const toBePadded = '99';
console.log(toBePadded.padStart(5, '0')); //00099

You can mask a few characters in a string like:

const creditCardNumber = '2034399002125581';
const ccNumberWithoutFirstNumber = creditCardNumber.slice(4,creditCardNumber.length);
const maskedNumber = ccNumberWithoutFirstNumber.padStart(creditCardNumber.length, '*');
console.log(maskedNumber); //****399002125581