underscore.js

var underscore = require('underscore'); 

var _ = exports;

var map = function(list, results, transform) {
  if (list.length === 0){
    return results;
  } else {
    return map(list.slice(1), results.concat(transform(list[0], results.length)), transform);
  }
};

exports.map = function(list, transform) {
  return map(list, [], transform);
};

var filter = function(list, results, condition) {
  if (list.length === 0){
    return results;
  } else {
    return filter(list.slice(1), (condition(list[0]) ? results.concat(list[0]) : results), condition);
  }
}

exports.filter = function(list, condition) {
  return filter(list, [], condition);
};

exports.reduce = function(list, iterator, result, context) {
  if (arguments.length < 3) {
    return exports.reduce(list.slice(1), iterator, list[0]);
  } else if (list.length === 0) {
    return result;
  } else {
    return exports.reduce(list.slice(1), iterator, iterator(result, list[0]), context)
  }
}

var range = function(num, stop, step) {
  if (num >= stop) {
    return [];
  } else {
    return [num].concat(range(num + step, stop, step));
  }

};
  //rewrite with def!!
exports.range = function() {
  var start = 0, stop, step = 1, list = [];
  if (arguments.length === 1) {
    stop = arguments[0];
  } else if (arguments.length === 2) {
    start = arguments[0];
    stop = arguments[1];
  } else {
    start = arguments[0];
    stop = arguments[1];
    step = arguments[2];
  }
  return range(start, stop, step);
}

exports.union = function(){
  return underscore.uniq(Array.prototype.concat.apply([], arguments));
}

var intersection = function(a, b) {
  var results = [];
  for (i = 0; i < a.length; i += 1) {
   if (b.indexOf(a[i]) >= 0) {
    results.push(a[i]);
   }
  }
  return results;
};

var intersection = function(a, b) {
  return exports.filter(a, function(val) {
    return b.indexOf(val) >= 0;
  });
};

exports.intersection = function() {
  var lists = Array.prototype.slice.apply(arguments);
  return underscore.uniq(exports.reduce(lists, intersection));
}

var difference = function(a, b) {
  return exports.filter(a, function(val) { return b.indexOf(val) < 0; });
};

exports.difference = function() {
  var lists = Array.prototype.slice.apply(arguments);
  return exports.reduce(lists, difference);
}

exports.uniq = function(array, iterator, isSorted) {
  if (array.length === 0){
    return [];
  } else {
    var first = array[0];
    var rest = array.slice(1);
    return [first].concat(exports.uniq(exports.filter(rest, function(val){
      return val !== first;
    })));
  }
};


exports.isArray = function(val) {
  return Object.prototype.toString.call(val) === '[object Array]';
};


var shallow = function(){
  if (array.length === 0){
    return [];
  } else {
  var first = array[0];
  var rest = array.slice(1);
    if (first = []){
      return first.concat(rest)}
  }
};

_.flatten = function(array, shallow) {
  if (shallow) {
    return Array.prototype.concat.apply([], array);
  } else if (array.length === 0){
    return [];
  } else {
    var first = array[0];
    return (_.isArray(first) ? _.flatten(first) : [first])
           .concat(_.flatten(array.slice(1)));
  }
};

_.pluck = function(list, propertyName){
  return _.map(list, function(obj){
    return obj[propertyName];
  });
};

_.each = function(list, iterator) {
  var type = Object.prototype.toString.call(list);
  if (type === '[object Array]') {
    for (var i = 0; i < list.length; i += 1) {
      iterator(list[i], i, list);
    }
  } else if (type === '[object Object]') {
    for (var prop in list) {
      iterator(list[value], prop, list);
    } 
  }
};

_.invoke = function(list, methodName) {
  _.each(list, function(element) {
    element[methodName]();
  })
};
//console.log(_.invoke(['5', '7', '9', 'bob'], 'toUpperCase'));

_.contains = function(list, value) {
 return list.indexOf(value) >= 0 
};
//console.log(_.contains([5, 8, 9, 3], 8));

_.find = function(list, predicate) { 
  for (i = 0; i < list.length; i += 1) {
    if (predicate(list[i])) {
      return list[i];
    } 
  }
};
//console.log(_.find([5, 8, 9, 10],function(num){ return num % 2 == 0; }));

var list = [
  {a: 1, b: 2},
  {a: 2, b: 2},
  {a: 1, b: 3},
  {a: 1, b: 4},
  {a: 2, b: 4}
];

_.findWhere = function(list, properties) {
  for (var i = 0; i < list.length; i += 1) {
    if (_.isMatch(list[i], properties)) {
      return list[i];
    }
  }
 };
//console.log(_.findWhere(list, {a: 1, b: 3}));
_.isMatch = function(obj, properties) {
  for (var key in properties) {
    if (obj[key] !== properties[key]) {
      return false;
    }
  }
  return true;
}

_.where = function(list, properties) {
  return _.filter(list, function(obj) {
    return _.isMatch(obj, properties);
  });
};