js num to string with leading 0

function padLeadingZeros(num, size) {
    var s = num+"";
    while (s.length < size) s = "0" + s;
    return s;
}
padLeadingZeros(57, 3);// "057"
padLeadingZeros(57, 4); //"0057"String(<number>).padStart(<places>, '0')

String(42).padStart(5, '0'); 	// '00042'