Javascript Padding

Pads a string left or right to a certain length. If the length of the string is bigger than the value of n, the string is truncated.

str is the value to pad;
n is the total length. Positive for right padding and negative for left padding.
ch is optional and denotes the pad character. It defaults to a space, when str is a string. When str is a number, ch defaults to a zero.

function pad(str,n,ch) {
  if(typeof str=='number') {
    str=String(str);
    if(arguments.length<3) ch='0';
  }
  var N=Math.abs(n);
  if(str.length>=N) {
    return n>=0 ? str.substr(0, n) : str.substr(str.length-N);
  }
  return (n>=0 ? str : '') + new Array(N-str.length+1).join((ch||' ').charAt(0)) + (n<0 ? str : '');
}

Some examples used as unit-test, inspired by Python's doctest.

function tst(a,b) {
  if(arguments.length==1) console.log("Starting: "+a);
  else console[a==b ? "info" : "error"]("[%s|%s]", a, b);
}
tst("PAD test");
tst("doe", pad('doeke', 3));
tst("doeke", pad('doeke', 5));
tst("doeke  ", pad('doeke', 7));
tst("eke", pad('doeke', -3));
tst("doeke", pad('doeke', -5));
tst("  doeke", pad('doeke', -7));
tst("0123", pad("123", -4, "0"));
tst("0123", pad(123, -4));

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