# underscore.js |
--- | ---
¶
    Underscore.js 1.4.4
    http://underscorejs.org
    (c) 2009-2013 Jeremy Ashkenas, DocumentCloud Inc.
    Underscore may be freely distributed under the MIT license.
    (function() {
¶ ## Baseline setup |
¶ Establish the root object, `window` in the browser, or `global` on the server. |
      var root = this;
¶ Save the previous value of the `_` variable. |
      var previousUnderscore = root._;
¶ Establish the object that gets returned to break out of a loop iteration. |
      var breaker = {};
¶ Save bytes in the minified (but not gzipped) version: |
      var ArrayProto = Array.prototype, ObjProto = Object.prototype, FuncProto = Function.prototype;
¶ Create quick reference variables for speed access to core prototypes. |
      var push             = ArrayProto.push,
          slice            = ArrayProto.slice,
          concat           = ArrayProto.concat,
          toString         = ObjProto.toString,
          hasOwnProperty   = ObjProto.hasOwnProperty;
¶ All ECMAScript 5 native function implementations that we hope to use are declared here. |
      var
        nativeForEach      = ArrayProto.forEach,
        nativeMap          = ArrayProto.map,
        nativeReduce       = ArrayProto.reduce,
        nativeReduceRight  = ArrayProto.reduceRight,
        nativeFilter       = ArrayProto.filter,
        nativeEvery        = ArrayProto.every,
        nativeSome         = ArrayProto.some,
        nativeIndexOf      = ArrayProto.indexOf,
        nativeLastIndexOf  = ArrayProto.lastIndexOf,
        nativeIsArray      = Array.isArray,
        nativeKeys         = Object.keys,
        nativeBind         = FuncProto.bind;
¶ Create a safe reference to the Underscore object for use below. |
      var _ = function(obj) {
        if (obj instanceof _) return obj;
        if (!(this instanceof _)) return new _(obj);
        this._wrapped = obj;
      };
¶ Export the Underscore object for Node.js, with backwards-compatibility for the old `require()` API. If we're in the browser, add `_` as a global object via a string identifier, for Closure Compiler "advanced" mode. |
      if (typeof exports !== 'undefined') {
        if (typeof module !== 'undefined' && module.exports) {
          exports = module.exports = _;
        }
        exports._ = _;
      } else {
        root._ = _;
      }
¶ Current version. |
      _.VERSION = '1.4.4';
¶ ## Collection Functions |
¶ The cornerstone, an `each` implementation, aka `forEach`. Handles objects with the built-in `forEach`, arrays, and raw objects. Delegates to ECMAScript 5's native `forEach` if available. |
      var each = _.each = _.forEach = function(obj, iterator, context) {
        if (obj == null) return;
        if (nativeForEach && obj.forEach === nativeForEach) {
          obj.forEach(iterator, context);
        } else if (obj.length === +obj.length) {
          for (var i = 0, l = obj.length; i < l; i++) {
            if (iterator.call(context, obj[i], i, obj) === breaker) return;
          }
        } else {
          for (var key in obj) {
            if (_.has(obj, key)) {
              if (iterator.call(context, obj[key], key, obj) === breaker) return;
            }
          }
        }
      };
¶ Return the results of applying the iterator to each element. Delegates to ECMAScript 5's native `map` if available. |
      _.map = _.collect = function(obj, iterator, context) {
        var results = [];
        if (obj == null) return results;
        if (nativeMap && obj.map === nativeMap) return obj.map(iterator, context);
        each(obj, function(value, index, list) {
          results[results.length] = iterator.call(context, value, index, list);
        });
        return results;
      };

      var reduceError = 'Reduce of empty array with no initial value';
¶ Reduce builds up a single result from a list of values, aka `inject`, or `foldl`. Delegates to ECMAScript 5's native `reduce` if available. |
      _.reduce = _.foldl = _.inject = function(obj, iterator, memo, context) {
        var initial = arguments.length > 2;
        if (obj == null) obj = [];
        if (nativeReduce && obj.reduce === nativeReduce) {
          if (context) iterator = _.bind(iterator, context);
          return initial ? obj.reduce(iterator, memo) : obj.reduce(iterator);
        }
        each(obj, function(value, index, list) {
          if (!initial) {
            memo = value;
            initial = true;
          } else {
            memo = iterator.call(context, memo, value, index, list);
          }
        });
        if (!initial) throw new TypeError(reduceError);
        return memo;
      };
¶ The right-associative version of reduce, also known as `foldr`. Delegates to ECMAScript 5's native `reduceRight` if available. |
      _.reduceRight = _.foldr = function(obj, iterator, memo, context) {
        var initial = arguments.length > 2;
        if (obj == null) obj = [];
        if (nativeReduceRight && obj.reduceRight === nativeReduceRight) {
          if (context) iterator = _.bind(iterator, context);
          return initial ? obj.reduceRight(iterator, memo) : obj.reduceRight(iterator);
        }
        var length = obj.length;
        if (length !== +length) {
          var keys = _.keys(obj);
          length = keys.length;
        }
        each(obj, function(value, index, list) {
          index = keys ? keys[--length] : --length;
          if (!initial) {
            memo = obj[index];
            initial = true;
          } else {
            memo = iterator.call(context, memo, obj[index], index, list);
          }
        });
        if (!initial) throw new TypeError(reduceError);
        return memo;
      };
¶ Return the first value which passes a truth test. Aliased as `detect`. |
      _.find = _.detect = function(obj, iterator, context) {
        var result;
        any(obj, function(value, index, list) {
          if (iterator.call(context, value, index, list)) {
            result = value;
            return true;
          }
        });
        return result;
      };
¶ Return all the elements that pass a truth test. Delegates to ECMAScript 5's native `filter` if available. Aliased as `select`. |
      _.filter = _.select = function(obj, iterator, context) {
        var results = [];
        if (obj == null) return results;
        if (nativeFilter && obj.filter === nativeFilter) return obj.filter(iterator, context);
        each(obj, function(value, index, list) {
          if (iterator.call(context, value, index, list)) results[results.length] = value;
        });
        return results;
      };
¶ Return all the elements for which a truth test fails. |
      _.reject = function(obj, iterator, context) {
        return _.filter(obj, function(value, index, list) {
          return !iterator.call(context, value, index, list);
        }, context);
      };
¶ Determine whether all of the elements match a truth test. Delegates to ECMAScript 5's native `every` if available. Aliased as `all`. |
      _.every = _.all = function(obj, iterator, context) {
        iterator || (iterator = _.identity);
        var result = true;
        if (obj == null) return result;
        if (nativeEvery && obj.every === nativeEvery) return obj.every(iterator, context);
        each(obj, function(value, index, list) {
          if (!(result = result && iterator.call(context, value, index, list))) return breaker;
        });
        return !!result;
      };
¶ Determine if at least one element in the object matches a truth test. Delegates to ECMAScript 5's native `some` if available. Aliased as `any`. |
      var any = _.some = _.any = function(obj, iterator, context) {
        iterator || (iterator = _.identity);
        var result = false;
        if (obj == null) return result;
        if (nativeSome && obj.some === nativeSome) return obj.some(iterator, context);
        each(obj, function(value, index, list) {
          if (result || (result = iterator.call(context, value, index, list))) return breaker;
        });
        return !!result;
      };
¶ Determine if the array or object contains a given value (using `===`). Aliased as `include`. |
      _.contains = _.include = function(obj, target) {
        if (obj == null) return false;
        if (nativeIndexOf && obj.indexOf === nativeIndexOf) return obj.indexOf(target) != -1;
        return any(obj, function(value) {
          return value === target;
        });
      };
¶ Invoke a method (with arguments) on every item in a collection. |
      _.invoke = function(obj, method) {
        var args = slice.call(arguments, 2);
        var isFunc = _.isFunction(method);
        return _.map(obj, function(value) {
          return (isFunc ? method : value[method]).apply(value, args);
        });
      };
¶ Convenience version of a common use case of `map`: fetching a property. |
      _.pluck = function(obj, key) {
        return _.map(obj, function(value){ return value[key]; });
      };
¶ Convenience version of a common use case of `filter`: selecting only objects containing specific `key:value` pairs. |
      _.where = function(obj, attrs, first) {
        if (_.isEmpty(attrs)) return first ? null : [];
        return _[first ? 'find' : 'filter'](obj, function(value) {
          for (var key in attrs) {
            if (attrs[key] !== value[key]) return false;
          }
          return true;
        });
      };
¶ Convenience version of a common use case of `find`: getting the first object containing specific `key:value` pairs. |
      _.findWhere = function(obj, attrs) {
        return _.where(obj, attrs, true);
      };
¶ Return the maximum element or (element-based computation). Can't optimize arrays of integers longer than 65,535 elements. See: https://bugs.webkit.org/show_bug.cgi?id=80797 |
      _.max = function(obj, iterator, context) {
        if (!iterator && _.isArray(obj) && obj[0] === +obj[0] && obj.length < 65535) {
          return Math.max.apply(Math, obj);
        }
        if (!iterator && _.isEmpty(obj)) return -Infinity;
        var result = {computed : -Infinity, value: -Infinity};
        each(obj, function(value, index, list) {
          var computed = iterator ? iterator.call(context, value, index, list) : value;
          computed >= result.computed && (result = {value : value, computed : computed});
        });
        return result.value;
      };
¶ Return the minimum element (or element-based computation). |
      _.min = function(obj, iterator, context) {
        if (!iterator && _.isArray(obj) && obj[0] === +obj[0] && obj.length < 65535) {
          return Math.min.apply(Math, obj);
        }
        if (!iterator && _.isEmpty(obj)) return Infinity;
        var result = {computed : Infinity, value: Infinity};
        each(obj, function(value, index, list) {
          var computed = iterator ? iterator.call(context, value, index, list) : value;
          computed < result.computed && (result = {value : value, computed : computed});
        });
        return result.value;
      };
¶ Shuffle an array. |
      _.shuffle = function(obj) {
        var rand;
        var index = 0;
        var shuffled = [];
        each(obj, function(value) {
          rand = _.random(index++);
          shuffled[index - 1] = shuffled[rand];
          shuffled[rand] = value;
        });
        return shuffled;
      };
¶ An internal function to generate lookup iterators. |
      var lookupIterator = function(value) {
        return _.isFunction(value) ? value : function(obj){ return obj[value]; };
      };
¶ Sort the object's values by a criterion produced by an iterator. |
      _.sortBy = function(obj, value, context) {
        var iterator = lookupIterator(value);
        return _.pluck(_.map(obj, function(value, index, list) {
          return {
            value : value,
            index : index,
            criteria : iterator.call(context, value, index, list)
          };
        }).sort(function(left, right) {
          var a = left.criteria;
          var b = right.criteria;
          if (a !== b) {
            if (a > b || a === void 0) return 1;
            if (a < b || b === void 0) return -1;
          }
          return left.index < right.index ? -1 : 1;
        }), 'value');
      };
¶ An internal function used for aggregate "group by" operations. |
      var group = function(obj, value, context, behavior) {
        var result = {};
        var iterator = lookupIterator(value || _.identity);
        each(obj, function(value, index) {
          var key = iterator.call(context, value, index, obj);
          behavior(result, key, value);
        });
        return result;
      };
¶ Groups the object's values by a criterion. Pass either a string attribute to group by, or a function that returns the criterion. |
      _.groupBy = function(obj, value, context) {
        return group(obj, value, context, function(result, key, value) {
          (_.has(result, key) ? result[key] : (result[key] = [])).push(value);
        });
      };
¶ Counts instances of an object that group by a certain criterion. Pass either a string attribute to count by, or a function that returns the criterion. |
      _.countBy = function(obj, value, context) {
        return group(obj, value, context, function(result, key) {
          if (!_.has(result, key)) result[key] = 0;
          result[key]++;
        });
      };
¶ Use a comparator function to figure out the smallest index at which an object should be inserted so as to maintain order. Uses binary search. |
      _.sortedIndex = function(array, obj, iterator, context) {
        iterator = iterator == null ? _.identity : lookupIterator(iterator);
        var value = iterator.call(context, obj);
        var low = 0, high = array.length;
        while (low < high) {
          var mid = (low + high) >>> 1;
          iterator.call(context, array[mid]) < value ? low = mid + 1 : high = mid;
        }
        return low;
      };
¶ Safely convert anything iterable into a real, live array. |
      _.toArray = function(obj) {
        if (!obj) return [];
        if (_.isArray(obj)) return slice.call(obj);
        if (obj.length === +obj.length) return _.map(obj, _.identity);
        return _.values(obj);
      };
¶ Return the number of elements in an object. |
      _.size = function(obj) {
        if (obj == null) return 0;
        return (obj.length === +obj.length) ? obj.length : _.keys(obj).length;
      };
¶ ## Array Functions |
¶ Get the first element of an array. Passing n will return the first N values in the array. Aliased as `head` and `take`. The guard check allows it to work with `_.map`. |
      _.first = _.head = _.take = function(array, n, guard) {
        if (array == null) return void 0;
        return (n != null) && !guard ? slice.call(array, 0, n) : array[0];
      };
¶ Returns everything but the last entry of the array. Especially useful on the arguments object. Passing n will return all the values in the array, excluding the last N. The guard check allows it to work with `_.map`. |
      _.initial = function(array, n, guard) {
        return slice.call(array, 0, array.length - ((n == null) || guard ? 1 : n));
      };
¶ Get the last element of an array. Passing n will return the last N values in the array. The guard check allows it to work with `_.map`. |
      _.last = function(array, n, guard) {
        if (array == null) return void 0;
        if ((n != null) && !guard) {
          return slice.call(array, Math.max(array.length - n, 0));
        } else {
          return array[array.length - 1];
        }
      };
¶ Returns everything but the first entry of the array. Aliased as `tail` and `drop`. Especially useful on the arguments object. Passing an n will return the rest N values in the array. The guard check allows it to work with `_.map`. |
      _.rest = _.tail = _.drop = function(array, n, guard) {
        return slice.call(array, (n == null) || guard ? 1 : n);
      };
¶ Trim out all falsy values from an array. |
      _.compact = function(array) {
        return _.filter(array, _.identity);
      };
¶ Internal implementation of a recursive `flatten` function. |
      var flatten = function(input, shallow, output) {
        each(input, function(value) {
          if (_.isArray(value)) {
            shallow ? push.apply(output, value) : flatten(value, shallow, output);
          } else {
            output.push(value);
          }
        });
        return output;
      };
¶ Return a completely flattened version of an array. |
      _.flatten = function(array, shallow) {
        return flatten(array, shallow, []);
      };
¶ Return a version of the array that does not contain the specified value(s). |
      _.without = function(array) {
        return _.difference(array, slice.call(arguments, 1));
      };
¶ Produce a duplicate-free version of the array. If the array has already been sorted, you have the option of using a faster algorithm. Aliased as `unique`. |
      _.uniq = _.unique = function(array, isSorted, iterator, context) {
        if (_.isFunction(isSorted)) {
          context = iterator;
          iterator = isSorted;
          isSorted = false;
        }
        var initial = iterator ? _.map(array, iterator, context) : array;
        var results = [];
        var seen = [];
        each(initial, function(value, index) {
          if (isSorted ? (!index || seen[seen.length - 1] !== value) : !_.contains(seen, value)) {
            seen.push(value);
            results.push(array[index]);
          }
        });
        return results;
      };
¶ Produce an array that contains the union: each distinct element from all of the passed-in arrays. |
      _.union = function() {
        return _.uniq(concat.apply(ArrayProto, arguments));
      };
¶ Produce an array that contains every item shared between all the passed-in arrays. |
      _.intersection = function(array) {
        var rest = slice.call(arguments, 1);
        return _.filter(_.uniq(array), function(item) {
          return _.every(rest, function(other) {
            return _.indexOf(other, item) >= 0;
          });
        });
      };
¶ Take the difference between one array and a number of other arrays. Only the elements present in just the first array will remain. |
      _.difference = function(array) {
        var rest = concat.apply(ArrayProto, slice.call(arguments, 1));
        return _.filter(array, function(value){ return !_.contains(rest, value); });
      };
¶ Zip together multiple lists into a single array -- elements that share an index go together. |
      _.zip = function() {
        var args = slice.call(arguments);
        var length = _.max(_.pluck(args, 'length'));
        var results = new Array(length);
        for (var i = 0; i < length; i++) {
          results[i] = _.pluck(args, "" + i);
        }
        return results;
      };
¶ Converts lists into objects. Pass either a single array of `[key, value]` pairs, or two parallel arrays of the same length -- one of keys, and one of the corresponding values. |
      _.object = function(list, values) {
        if (list == null) return {};
        var result = {};
        for (var i = 0, l = list.length; i < l; i++) {
          if (values) {
            result[list[i]] = values[i];
          } else {
            result[list[i][0]] = list[i][1];
          }
        }
        return result;
      };
¶ If the browser doesn't supply us with indexOf (I'm looking at you, MSIE), we need this function. Return the position of the first occurrence of an item in an array, or -1 if the item is not included in the array. Delegates to ECMAScript 5's native `indexOf` if available. If the array is large and already in sort order, pass `true` for isSorted to use binary search. |
      _.indexOf = function(array, item, isSorted) {
        if (array == null) return -1;
        var i = 0, l = array.length;
        if (isSorted) {
          if (typeof isSorted == 'number') {
            i = (isSorted < 0 ? Math.max(0, l + isSorted) : isSorted);
          } else {
            i = _.sortedIndex(array, item);
            return array[i] === item ? i : -1;
          }
        }
        if (nativeIndexOf && array.indexOf === nativeIndexOf) return array.indexOf(item, isSorted);
        for (; i < l; i++) if (array[i] === item) return i;
        return -1;
      };
¶ Delegates to ECMAScript 5's native `lastIndexOf` if available. |
      _.lastIndexOf = function(array, item, from) {
        if (array == null) return -1;
        var hasIndex = from != null;
        if (nativeLastIndexOf && array.lastIndexOf === nativeLastIndexOf) {
          return hasIndex ? array.lastIndexOf(item, from) : array.lastIndexOf(item);
        }
        var i = (hasIndex ? from : array.length);
        while (i--) if (array[i] === item) return i;
        return -1;
      };
¶ Generate an integer Array containing an arithmetic progression. A port of the native Python `range()` function. See the Python documentation. |
      _.range = function(start, stop, step) {
        if (arguments.length <= 1) {
          stop = start || 0;
          start = 0;
        }
        step = arguments[2] || 1;

        var len = Math.max(Math.ceil((stop - start) / step), 0);
        var idx = 0;
        var range = new Array(len);

        while(idx < len) {
          range[idx++] = start;
          start += step;
        }

        return range;
      };
¶ ## Function (ahem) Functions |
¶ Create a function bound to a given object (assigning `this`, and arguments, optionally). Delegates to ECMAScript 5's native `Function.bind` if available. |
      _.bind = function(func, context) {
        if (func.bind === nativeBind && nativeBind) return nativeBind.apply(func, slice.call(arguments, 1));
        var args = slice.call(arguments, 2);
        return function() {
          return func.apply(context, args.concat(slice.call(arguments)));
        };
      };
¶ Partially apply a function by creating a version that has had some of its arguments pre-filled, without changing its dynamic `this` context. |
      _.partial = function(func) {
        var args = slice.call(arguments, 1);
        return function() {
          return func.apply(this, args.concat(slice.call(arguments)));
        };
      };
¶ Bind all of an object's methods to that object. Useful for ensuring that all callbacks defined on an object belong to it. |
      _.bindAll = function(obj) {
        var funcs = slice.call(arguments, 1);
        if (funcs.length === 0) funcs = _.functions(obj);
        each(funcs, function(f) { obj[f] = _.bind(obj[f], obj); });
        return obj;
      };
¶ Memoize an expensive function by storing its results. |
      _.memoize = function(func, hasher) {
        var memo = {};
        hasher || (hasher = _.identity);
        return function() {
          var key = hasher.apply(this, arguments);
          return _.has(memo, key) ? memo[key] : (memo[key] = func.apply(this, arguments));
        };
      };
¶ Delays a function for the given number of milliseconds, and then calls it with the arguments supplied. |
      _.delay = function(func, wait) {
        var args = slice.call(arguments, 2);
        return setTimeout(function(){ return func.apply(null, args); }, wait);
      };
¶ Defers a function, scheduling it to run after the current call stack has cleared. |
      _.defer = function(func) {
        return _.delay.apply(_, [func, 1].concat(slice.call(arguments, 1)));
      };
¶ Returns a function, that, when invoked, will only be triggered at most once during a given window of time. |
      _.throttle = function(func, wait) {
        var context, args, timeout, result;
        var previous = 0;
        var later = function() {
          previous = new Date;
          timeout = null;
          result = func.apply(context, args);
        };
        return function() {
          var now = new Date;
          var remaining = wait - (now - previous);
          context = this;
          args = arguments;
          if (remaining <= 0) {
            clearTimeout(timeout);
            timeout = null;
            previous = now;
            result = func.apply(context, args);
          } else if (!timeout) {
            timeout = setTimeout(later, remaining);
          }
          return result;
        };
      };
¶ Returns a function, that, as long as it continues to be invoked, will not be triggered. The function will be called after it stops being called for N milliseconds. If `immediate` is passed, trigger the function on the leading edge, instead of the trailing. |
      _.debounce = function(func, wait, immediate) {
        var timeout, result;
        return function() {
          var context = this, args = arguments;
          var later = function() {
            timeout = null;
            if (!immediate) result = func.apply(context, args);
          };
          var callNow = immediate && !timeout;
          clearTimeout(timeout);
          timeout = setTimeout(later, wait);
          if (callNow) result = func.apply(context, args);
          return result;
        };
      };
¶ Returns a function that will be executed at most one time, no matter how often you call it. Useful for lazy initialization. |
      _.once = function(func) {
        var ran = false, memo;
        return function() {
          if (ran) return memo;
          ran = true;
          memo = func.apply(this, arguments);
          func = null;
          return memo;
        };
      };
¶ Returns the first function passed as an argument to the second, allowing you to adjust arguments, run code before and after, and conditionally execute the original function. |
      _.wrap = function(func, wrapper) {
        return function() {
          var args = [func];
          push.apply(args, arguments);
          return wrapper.apply(this, args);
        };
      };
¶ Returns a function that is the composition of a list of functions, each consuming the return value of the function that follows. |
      _.compose = function() {
        var funcs = arguments;
        return function() {
          var args = arguments;
          for (var i = funcs.length - 1; i >= 0; i--) {
            args = [funcs[i].apply(this, args)];
          }
          return args[0];
        };
      };
¶ Returns a function that will only be executed after being called N times. |
      _.after = function(times, func) {
        if (times <= 0) return func();
        return function() {
          if (--times < 1) {
            return func.apply(this, arguments);
          }
        };
      };
¶ ## Object Functions |
¶ Retrieve the names of an object's properties. Delegates to ECMAScript 5's native `Object.keys` |
      _.keys = nativeKeys || function(obj) {
        if (obj !== Object(obj)) throw new TypeError('Invalid object');
        var keys = [];
        for (var key in obj) if (_.has(obj, key)) keys[keys.length] = key;
        return keys;
      };
¶ Retrieve the values of an object's properties. |
      _.values = function(obj) {
        var values = [];
        for (var key in obj) if (_.has(obj, key)) values.push(obj[key]);
        return values;
      };
¶ Convert an object into a list of `[key, value]` pairs. |
      _.pairs = function(obj) {
        var pairs = [];
        for (var key in obj) if (_.has(obj, key)) pairs.push([key, obj[key]]);
        return pairs;
      };
¶ Invert the keys and values of an object. The values must be serializable. |
      _.invert = function(obj) {
        var result = {};
        for (var key in obj) if (_.has(obj, key)) result[obj[key]] = key;
        return result;
      };
¶ Return a sorted list of the function names available on the object. Aliased as `methods` |
      _.functions = _.methods = function(obj) {
        var names = [];
        for (var key in obj) {
          if (_.isFunction(obj[key])) names.push(key);
        }
        return names.sort();
      };
¶ Extend a given object with all the properties in passed-in object(s). |
      _.extend = function(obj) {
        each(slice.call(arguments, 1), function(source) {
          if (source) {
            for (var prop in source) {
              obj[prop] = source[prop];
            }
          }
        });
        return obj;
      };
¶ Return a copy of the object only containing the whitelisted properties. |
      _.pick = function(obj) {
        var copy = {};
        var keys = concat.apply(ArrayProto, slice.call(arguments, 1));
        each(keys, function(key) {
          if (key in obj) copy[key] = obj[key];
        });
        return copy;
      };
¶ Return a copy of the object without the blacklisted properties. |
      _.omit = function(obj) {
        var copy = {};
        var keys = concat.apply(ArrayProto, slice.call(arguments, 1));
        for (var key in obj) {
          if (!_.contains(keys, key)) copy[key] = obj[key];
        }
        return copy;
      };
¶ Fill in a given object with default properties. |
      _.defaults = function(obj) {
        each(slice.call(arguments, 1), function(source) {
          if (source) {
            for (var prop in source) {
              if (obj[prop] == null) obj[prop] = source[prop];
            }
          }
        });
        return obj;
      };
¶ Create a (shallow-cloned) duplicate of an object. |
      _.clone = function(obj) {
        if (!_.isObject(obj)) return obj;
        return _.isArray(obj) ? obj.slice() : _.extend({}, obj);
      };
¶ Invokes interceptor with the obj, and then returns obj. The primary purpose of this method is to "tap into" a method chain, in order to perform operations on intermediate results within the chain. |
      _.tap = function(obj, interceptor) {
        interceptor(obj);
        return obj;
      };
¶ Internal recursive comparison function for `isEqual`. |
      var eq = function(a, b, aStack, bStack) {
¶ Identical objects are equal. `0 === -0`, but they aren't identical. See the Harmony `egal` proposal: http://wiki.ecmascript.org/doku.php?id=harmony:egal. |
        if (a === b) return a !== 0 || 1 / a == 1 / b;
¶ A strict comparison is necessary because `null == undefined`. |
        if (a == null || b == null) return a === b;
¶ Unwrap any wrapped objects. |
        if (a instanceof _) a = a._wrapped;
        if (b instanceof _) b = b._wrapped;
¶ Compare `[[Class]]` names. |
        var className = toString.call(a);
        if (className != toString.call(b)) return false;
        switch (className) {
¶ Strings, numbers, dates, and booleans are compared by value. |
          case '[object String]':
¶ Primitives and their corresponding object wrappers are equivalent; thus, `"5"` is equivalent to `new String("5")`. |
            return a == String(b);
          case '[object Number]':
¶ `NaN`s are equivalent, but non-reflexive. An `egal` comparison is performed for other numeric values. |
            return a != +a ? b != +b : (a == 0 ? 1 / a == 1 / b : a == +b);
          case '[object Date]':
          case '[object Boolean]':
¶ Coerce dates and booleans to numeric primitive values. Dates are compared by their millisecond representations. Note that invalid dates with millisecond representations of `NaN` are not equivalent. |
            return +a == +b;
¶ RegExps are compared by their source patterns and flags. |
          case '[object RegExp]':
            return a.source == b.source &&
                   a.global == b.global &&
                   a.multiline == b.multiline &&
                   a.ignoreCase == b.ignoreCase;
        }
        if (typeof a != 'object' || typeof b != 'object') return false;
¶ Assume equality for cyclic structures. The algorithm for detecting cyclic structures is adapted from ES 5.1 section 15.12.3, abstract operation `JO`. |
        var length = aStack.length;
        while (length--) {
¶ Linear search. Performance is inversely proportional to the number of unique nested structures. |
          if (aStack[length] == a) return bStack[length] == b;
        }
¶ Add the first object to the stack of traversed objects. |
        aStack.push(a);
        bStack.push(b);
        var size = 0, result = true;
¶ Recursively compare objects and arrays. |
        if (className == '[object Array]') {
¶ Compare array lengths to determine if a deep comparison is necessary. |
          size = a.length;
          result = size == b.length;
          if (result) {
¶ Deep compare the contents, ignoring non-numeric properties. |
            while (size--) {
              if (!(result = eq(a[size], b[size], aStack, bStack))) break;
            }
          }
        } else {
¶ Objects with different constructors are not equivalent, but `Object`s from different frames are. |
          var aCtor = a.constructor, bCtor = b.constructor;
          if (aCtor !== bCtor && !(_.isFunction(aCtor) && (aCtor instanceof aCtor) &&
                                   _.isFunction(bCtor) && (bCtor instanceof bCtor))) {
            return false;
          }
¶ Deep compare objects. |
          for (var key in a) {
            if (_.has(a, key)) {
¶ Count the expected number of properties. |
              size++;
¶ Deep compare each member. |
              if (!(result = _.has(b, key) && eq(a[key], b[key], aStack, bStack))) break;
            }
          }
¶ Ensure that both objects contain the same number of properties. |
          if (result) {
            for (key in b) {
              if (_.has(b, key) && !(size--)) break;
            }
            result = !size;
          }
        }
¶ Remove the first object from the stack of traversed objects. |
        aStack.pop();
        bStack.pop();
        return result;
      };
¶ Perform a deep comparison to check if two objects are equal. |
      _.isEqual = function(a, b) {
        return eq(a, b, [], []);
      };
¶ Is a given array, string, or object empty? An "empty" object has no enumerable own-properties. |
      _.isEmpty = function(obj) {
        if (obj == null) return true;
        if (_.isArray(obj) || _.isString(obj)) return obj.length === 0;
        for (var key in obj) if (_.has(obj, key)) return false;
        return true;
      };
¶ Is a given value a DOM element? |
      _.isElement = function(obj) {
        return !!(obj && obj.nodeType === 1);
      };
¶ Is a given value an array? Delegates to ECMA5's native Array.isArray |
      _.isArray = nativeIsArray || function(obj) {
        return toString.call(obj) == '[object Array]';
      };
¶ Is a given variable an object? |
      _.isObject = function(obj) {
        return obj === Object(obj);
      };
¶ Add some isType methods: isArguments, isFunction, isString, isNumber, isDate, isRegExp. |
      each(['Arguments', 'Function', 'String', 'Number', 'Date', 'RegExp'], function(name) {
        _['is' + name] = function(obj) {
          return toString.call(obj) == '[object ' + name + ']';
        };
      });
¶ Define a fallback version of the method in browsers (ahem, IE), where there isn't any inspectable "Arguments" type. |
      if (!_.isArguments(arguments)) {
        _.isArguments = function(obj) {
          return !!(obj && _.has(obj, 'callee'));
        };
      }
¶ Optimize `isFunction` if appropriate. |
      if (typeof (/./) !== 'function') {
        _.isFunction = function(obj) {
          return typeof obj === 'function';
        };
      }
¶ Is a given object a finite number? |
      _.isFinite = function(obj) {
        return isFinite(obj) && !isNaN(parseFloat(obj));
      };
¶ Is the given value `NaN`? (NaN is the only number which does not equal itself). |
      _.isNaN = function(obj) {
        return _.isNumber(obj) && obj != +obj;
      };
¶ Is a given value a boolean? |
      _.isBoolean = function(obj) {
        return obj === true || obj === false || toString.call(obj) == '[object Boolean]';
      };
¶ Is a given value equal to null? |
      _.isNull = function(obj) {
        return obj === null;
      };
¶ Is a given variable undefined? |
      _.isUndefined = function(obj) {
        return obj === void 0;
      };
¶ Shortcut function for checking if an object has a given property directly on itself (in other words, not on a prototype). |
      _.has = function(obj, key) {
        return hasOwnProperty.call(obj, key);
      };
¶ ## Utility Functions |
¶ Run Underscore.js in noConflict mode, returning the `_` variable to its previous owner. Returns a reference to the Underscore object. |
      _.noConflict = function() {
        root._ = previousUnderscore;
        return this;
      };
¶ Keep the identity function around for default iterators. |
      _.identity = function(value) {
        return value;
      };
¶ Run a function n times. |
      _.times = function(n, iterator, context) {
        var accum = Array(n);
        for (var i = 0; i < n; i++) accum[i] = iterator.call(context, i);
        return accum;
      };
¶ Return a random integer between min and max (inclusive). |
      _.random = function(min, max) {
        if (max == null) {
          max = min;
          min = 0;
        }
        return min + Math.floor(Math.random() * (max - min + 1));
      };
¶ List of HTML entities for escaping. |
      var entityMap = {
        escape: {
          '&': '&amp;',
          '<': '&lt;',
          '>': '&gt;',
          '"': '&quot;',
          "'": '&#x27;',
          '/': '&#x2F;'
        }
      };
      entityMap.unescape = _.invert(entityMap.escape);
¶ Regexes containing the keys and values listed immediately above. |
      var entityRegexes = {
        escape:   new RegExp('[' + _.keys(entityMap.escape).join('') + ']', 'g'),
        unescape: new RegExp('(' + _.keys(entityMap.unescape).join('|') + ')', 'g')
      };
¶ Functions for escaping and unescaping strings to/from HTML interpolation. |
      _.each(['escape', 'unescape'], function(method) {
        _[method] = function(string) {
          if (string == null) return '';
          return ('' + string).replace(entityRegexes[method], function(match) {
            return entityMap[method][match];
          });
        };
      });
¶ If the value of the named property is a function then invoke it; otherwise, return it. |
      _.result = function(object, property) {
        if (object == null) return null;
        var value = object[property];
        return _.isFunction(value) ? value.call(object) : value;
      };
¶ Add your own custom functions to the Underscore object. |
      _.mixin = function(obj) {
        each(_.functions(obj), function(name){
          var func = _[name] = obj[name];
          _.prototype[name] = function() {
            var args = [this._wrapped];
            push.apply(args, arguments);
            return result.call(this, func.apply(_, args));
          };
        });
      };
¶ Generate a unique integer id (unique within the entire client session). Useful for temporary DOM ids. |
      var idCounter = 0;
      _.uniqueId = function(prefix) {
        var id = ++idCounter + '';
        return prefix ? prefix + id : id;
      };
¶ By default, Underscore uses ERB-style template delimiters, change the following template settings to use alternative delimiters. |
      _.templateSettings = {
        evaluate    : /<%([\s\S]+?)%>/g,
        interpolate : /<%=([\s\S]+?)%>/g,
        escape      : /<%-([\s\S]+?)%>/g
      };
¶ When customizing `templateSettings`, if you don't want to define an interpolation, evaluation or escaping regex, we need one that is guaranteed not to match. |
      var noMatch = /(.)^/;
¶ Certain characters need to be escaped so that they can be put into a string literal. |
      var escapes = {
        "'":      "'",
        '\\':     '\\',
        '\r':     'r',
        '\n':     'n',
        '\t':     't',
        '\u2028': 'u2028',
        '\u2029': 'u2029'
      };

      var escaper = /\\|'|\r|\n|\t|\u2028|\u2029/g;
¶ JavaScript micro-templating, similar to John Resig's implementation. Underscore templating handles arbitrary delimiters, preserves whitespace, and correctly escapes quotes within interpolated code. |
      _.template = function(text, data, settings) {
        var render;
        settings = _.defaults({}, settings, _.templateSettings);
¶ Combine delimiters into one regular expression via alternation. |
        var matcher = new RegExp([
          (settings.escape || noMatch).source,
          (settings.interpolate || noMatch).source,
          (settings.evaluate || noMatch).source
        ].join('|') + '|$', 'g');
¶ Compile the template source, escaping string literals appropriately. |
        var index = 0;
        var source = "__p+='";
        text.replace(matcher, function(match, escape, interpolate, evaluate, offset) {
          source += text.slice(index, offset)
            .replace(escaper, function(match) { return '\\' + escapes[match]; });

          if (escape) {
            source += "'+\n((__t=(" + escape + "))==null?'':_.escape(__t))+\n'";
          }
          if (interpolate) {
            source += "'+\n((__t=(" + interpolate + "))==null?'':__t)+\n'";
          }
          if (evaluate) {
            source += "';\n" + evaluate + "\n__p+='";
          }
          index = offset + match.length;
          return match;
        });
        source += "';\n";
¶ If a variable is not specified, place data values in local scope. |
        if (!settings.variable) source = 'with(obj||{}){\n' + source + '}\n';

        source = "var __t,__p='',__j=Array.prototype.join," +
          "print=function(){__p+=__j.call(arguments,'');};\n" +
          source + "return __p;\n";

        try {
          render = new Function(settings.variable || 'obj', '_', source);
        } catch (e) {
          e.source = source;
          throw e;
        }

        if (data) return render(data, _);
        var template = function(data) {
          return render.call(this, data, _);
        };
¶ Provide the compiled function source as a convenience for precompilation. |
        template.source = 'function(' + (settings.variable || 'obj') + '){\n' + source + '}';

        return template;
      };
¶ Add a "chain" function, which will delegate to the wrapper. |
      _.chain = function(obj) {
        return _(obj).chain();
      };
¶ ## OOP If Underscore is called as a function, it returns a wrapped object that can be used OO-style. This wrapper holds altered versions of all the underscore functions. Wrapped objects may be chained. |
¶ Helper function to continue chaining intermediate results. |
      var result = function(obj) {
        return this._chain ? _(obj).chain() : obj;
      };
¶ Add all of the Underscore functions to the wrapper object. |
      _.mixin(_);
¶ Add all mutator Array functions to the wrapper. |
      each(['pop', 'push', 'reverse', 'shift', 'sort', 'splice', 'unshift'], function(name) {
        var method = ArrayProto[name];
        _.prototype[name] = function() {
          var obj = this._wrapped;
          method.apply(obj, arguments);
          if ((name == 'shift' || name == 'splice') && obj.length === 0) delete obj[0];
          return result.call(this, obj);
        };
      });
¶ Add all accessor Array functions to the wrapper. |
      each(['concat', 'join', 'slice'], function(name) {
        var method = ArrayProto[name];
        _.prototype[name] = function() {
          return result.call(this, method.apply(this._wrapped, arguments));
        };
      });

      _.extend(_.prototype, {
¶ Start chaining a wrapped Underscore object. |
        chain: function() {
          this._chain = true;
          return this;
        },
¶ Extracts the result from a wrapped and chained object. |
        value: function() {
          return this._wrapped;
        }

      });

    }).call(this);