console.clear();

var toPrimitive = function(obj,preferredType){

  var APIs = {

    typeOf: function(obj){

      return Object.prototype.toString.call(obj).slice(8,-1);

    },

    isPrimitive: function(obj){

      var _this = this,

          types = ['Null','Undefined','String','Boolean','Number'];

      return types.indexOf(_this.typeOf(obj)) !== -1; 

    }

  };

  // if obj is primitive, return

  if(APIs.isPrimitive(obj)) {return obj;}

  preferredType = (preferredType === 'String' || APIs.typeOf(obj) === 'Date' ) ? 'String' : 'Number';

  if(preferredType==='Number'){

    if(APIs.isPrimitive(obj.valueOf())) { return obj.valueOf()};

    if(APIs.isPrimitive(obj.toString())) { return obj.toString()};

  }else{

    if(APIs.isPrimitive(obj.toString())) { return obj.toString()};

    if(APIs.isPrimitive(obj.valueOf())) { return obj.valueOf()};

  }

  throw new TypeError('TypeError');

}

​