UPDATE: Small one-liner function using the ES2017 String.prototype.padStart method:

const zeroPad = (num, places) => String(num).padStart(places, '0')

console.log(zeroPad(5, 2)); // "05"
console.log(zeroPad(5, 4)); // "0005"
console.log(zeroPad(5, 6)); // "000005"
console.log(zeroPad(1234, 2)); // "1234"

Another ES5 approach:

function zeroPad(num, places) {
  var zero = places - num.toString().length + 1;
  return Array(+(zero > 0 && zero)).join("0") + num;
}

zeroPad(5, 2); // "05"
zeroPad(5, 4); // "0005"
zeroPad(5, 6); // "000005"
zeroPad(1234, 2); // "1234" :)

Just for fun, here's my version of a pad function:

function pad(num, len) {
  return Array(len + 1 - num.toString().length).join('0') + num;
}

It also won't truncate numbers longer than the padding length

The parseInt() and parseFloat() functions parse a string until they reach a character that isn't valid for the specified number format, then return the number parsed up to that point. However the "+" operator simply converts the string to NaN if there is an invalid character contained within it. Just try parsing the string "10.2abc" with each method by yourself in the console and you'll understand the differences better.

+"42"; // 42
+"010"; // 10
+"0x10"; // 16

a = "0042";
alert(typeof +a); // number
alert(+a); // 42