An idea for unit conversions in javascript

ConversionStrategy.js

var ConversionStrategy = (function () {

  function ConversionStrategy() {
  }

  ConversionStrategy.prototype.valueOf = function (value) {
    return -1;
  };

  return ConversionStrategy;

})();

var PeopleConversionStrategy = (function () {

  function PeopleConversionStrategy() {
    this.units = "people";
  }

  PeopleConversionStrategy.prototype.valueOf = function (value) {
    if (value > 100000000) { // 100,000,000 => "0.1 B"
      return (value / 1000000000).toFixed(1) + " B";
    } else if (value > 100000) { // 100,000 =? "0.1 M"
      return (value / 1000000).toFixed(1) + " M";
    }
    return value;
  };

  return PeopleConversionStrategy;

})();


var TonsConversionStrategy = (function () {

  function PeopleConversionStrategy() {
    this.units = "tons";
  }

  TonsConversionStrategy.prototype.valueOf = function (value) {
    if (value > 100000) { // 100,000 => "0.5 kton"
      return (value / 2000000).toFixed(1) + " kton";
    } else if (value > 1000) { // 1,000 => "0.5 ton"
      return (value / 2000).toFixed(1) + " ton";
    } else {
      return value.toFixed(0) + " lbs";
    }
  };

  return TonsConversionStrategy;

})();

var UnitConverter = (function () {

  var strategies = {
    "people": PeopleConversionStrategy,
    "tons": TonsConversionStrategy,
    "*": DefaultConverstionStrategy
  }

  function UnitConverter() {
  }

  UnitConverter.prototype.units = function (units) {
    return strategies[units];
  };

  return UnitConverter;

})();

var convert = new UnitConverter();

// usage:
// convert.units("people").valueOf(7200000000)  => "7.2 B"
// convert.units("tons").valueOf(5200000)       => "2.6 kton"