Javascript subtracting from large integers

large_int_subtraction.js

function LargeInt(str){
  this.length = 0;
  if (str){
    Array.prototype.push.apply(this, String(str).split(''));
  }
}
LargeInt.prototype = {
  splice: Array.prototype.splice,
  minus: function(subtrahend){
    subtrahend = new LargeInt(subtrahend);
    if (subtrahend.greaterThan(this)){
      return "-" + subtrahend._minus(this);
    } else {
      return this._minus(subtrahend);
    }
  },
  _minus: function(subtrahend){
    var difference = [];
    for(var i=0,l=this.length; i<l; i++){
      var digit = this.digitFromEnd(i);
      var subDigit = subtrahend.digitFromEnd(i);
      if (digit >= subDigit){
        difference[i] = digit - subDigit;
      } else {
        difference[i] = digit + 10 - subDigit;
        subtrahend.setDigitFromEnd(i+1, subtrahend.digitFromEnd(i+1) + 1);
      }
    }
    var result = difference.reverse().join('').replace(/^0+/,'');
    if (result === '')
      result = '0';
    return result;
  },
  greaterThan: function(compareTo){
    if (compareTo.length != this.length) {
      return this.length > compareTo.length;
    }
    for(var i=this.length-1; i>=0; i--){
      var x = this.digitFromEnd(i), y = compareTo.digitFromEnd(i);
      if (x == y) {
        continue;
      } else {
        return x > y;
      }
    }
    return false;
  },
  _fromEnd: function(revIndex){
    return this.length - 1 - revIndex;
  },
  digitFromEnd: function(revIndex){
    return parseInt(this[this._fromEnd(revIndex)] || 0, 10);
  },
  setDigitFromEnd: function(revIndex, val){
    this[this._fromEnd(revIndex)] = val;
  }
};

function subtractFromLargeNumber(num, subtract){
  num = new LargeInt(num);
  return num.minus(subtract);
}

var testCases = [
{
  largeNumber: "199000000000000000",
  expected:    "198999999999999995",
  subtract: 5
},
{
  largeNumber: "432467823478368774",
  expected:    "432467823478368765",
  subtract: 9
},
{
  largeNumber: "200",
  expected: "-101",
  subtract: 301
},
{
  largeNumber: "4958",
  expected: "0",
  subtract: "4958"
},
{
  largeNumber: "432467823478368774",
  expected:    "-123205909923154",
  subtract:    "432591029388291928"
}
];

testCases.forEach(function(test){
  var result = subtractFromLargeNumber(test.largeNumber, test.subtract);
  if (result != test.expected) {
    console.log("fail! expected "+test.expected+", got "+result);
  } else {
    console.log("success!");
  }
});