Pad an integer with X number of zeroes

add-leading-zeroes.js

/**
 * Add leading zeroes to an integer.
 * @param   {int} num      The number to which to add leading zeros.
 * @param   {ing} zeros    The length of the resulting string.
 * @returns {str}
 */
function addLeadingZeros(num, width) {
    return String(Math.pow(10, width) + num).slice(-width) ;
}

// Examples

console.log( addLeadingZeros( 21, 3 ); // returns "021"
console.log( addLeadingZeros( 7, 2 );  // returns "07"
console.log( addLeadingZeros( 4, 6);   // returns "000004"