# underscore.js

- # underscore.js

- ¶

      Underscore.js 1.5.2
      http://underscorejs.org
      (c) 2009-2013 Jeremy Ashkenas, DocumentCloud and Investigative Reporters & Editors
      Underscore may be freely distributed under the MIT license.

      (function() {

- ¶

  ## Baseline setup

- ¶

  Establish the root object, `window` in the browser, or `exports` 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 = '1.5.2';

- ¶

  ## 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, length = obj.length; i < length; i++) {
              if (iterator.call(context, obj[i], i, obj) === breaker) return;
            }
          } else {
            var keys = _.keys(obj);
            for (var i = 0, length = keys.length; i < length; i++) {
              if (iterator.call(context, obj[keys[i]], keys[i], 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.push(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.push(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 ? void 0 : [];
          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 WebKit Bug 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, using the modern version of the Fisher-Yates shuffle.

        _.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;
        };

- ¶

  Sample n random values from an array. If n is not specified, returns a single random element from the array. The internal `guard` argument allows it to work with `map`.

        _.sample = function(obj, n, guard) {
          if (arguments.length < 2 || guard) {
            return obj[_.random(obj.length - 1)];
          }
          return _.shuffle(obj).slice(0, Math.max(0, n));
        };

- ¶

  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;
          }), 'value');
        };

- ¶

  An internal function used for aggregate "group by" operations.

        var group = function(behavior) {
          return function(obj, value, context) {
            var result = {};
            var iterator = value == null ? _.identity : lookupIterator(value);
            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 = group(function(result, key, value) {
          (_.has(result, key) ? result[key] : (result[key] = [])).push(value);
        });

- ¶

  Indexes the object's values by a criterion, similar to `groupBy`, but for when you know that your index values will be unique.

        _.indexBy = group(function(result, key, value) {
          result[key] = 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 = group(function(result, key) {
          _.has(result, key) ? result[key]++ : result[key] = 1;
        });

- ¶

  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 create a real, live array from anything iterable.

        _.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 ? array[0] : slice.call(array, 0, n);
        };

- ¶

  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 array[array.length - 1];
          } else {
            return slice.call(array, Math.max(array.length - n, 0));
          }
        };

- ¶

  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) {
          if (shallow && _.every(input, _.isArray)) {
            return concat.apply(output, input);
          }
          each(input, function(value) {
            if (_.isArray(value) || _.isArguments(value)) {
              shallow ? push.apply(output, value) : flatten(value, shallow, output);
            } else {
              output.push(value);
            }
          });
          return output;
        };

- ¶

  Flatten out an array, either recursively (by default), or just one level.

        _.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(_.flatten(arguments, true));
        };

- ¶

  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 length = _.max(_.pluck(arguments, "length").concat(0));
          var results = new Array(length);
          for (var i = 0; i < length; i++) {
            results[i] = _.pluck(arguments, '' + 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, length = list.length; i < length; 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, length = array.length;
          if (isSorted) {
            if (typeof isSorted == 'number') {
              i = (isSorted < 0 ? Math.max(0, length + 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 < length; 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 length = Math.max(Math.ceil((stop - start) / step), 0);
          var idx = 0;
          var range = new Array(length);

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

          return range;
        };

- ¶

  ## Function (ahem) Functions

- ¶

  Reusable constructor function for prototype setting.

        var ctor = function(){};

- ¶

  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) {
          var args, bound;
          if (nativeBind && func.bind === nativeBind) return nativeBind.apply(func, slice.call(arguments, 1));
          if (!_.isFunction(func)) throw new TypeError;
          args = slice.call(arguments, 2);
          return bound = function() {
            if (!(this instanceof bound)) return func.apply(context, args.concat(slice.call(arguments)));
            ctor.prototype = func.prototype;
            var self = new ctor;
            ctor.prototype = null;
            var result = func.apply(self, args.concat(slice.call(arguments)));
            if (Object(result) === result) return result;
            return self;
          };
        };

- ¶

  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) throw new Error("bindAll must be passed function names");
          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. Normally, the throttled function will run as much as it can, without ever going more than once per `wait` duration; but if you'd like to disable the execution on the leading edge, pass `{leading: false}`. To disable execution on the trailing edge, ditto.

        _.throttle = function(func, wait, options) {
          var context, args, result;
          var timeout = null;
          var previous = 0;
          options || (options = {});
          var later = function() {
            previous = options.leading === false ? 0 : new Date;
            timeout = null;
            result = func.apply(context, args);
          };
          return function() {
            var now = new Date;
            if (!previous && options.leading === false) previous = now;
            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 && options.trailing !== false) {
              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, args, context, timestamp, result;
          return function() {
            context = this;
            args = arguments;
            timestamp = new Date();
            var later = function() {
              var last = (new Date()) - timestamp;
              if (last < wait) {
                timeout = setTimeout(later, wait - last);
              } else {
                timeout = null;
                if (!immediate) result = func.apply(context, args);
              }
            };
            var callNow = immediate && !timeout;
            if (!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) {
          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.push(key);
          return keys;
        };

- ¶

  Retrieve the values of an object's properties.

        _.values = function(obj) {
          var keys = _.keys(obj);
          var length = keys.length;
          var values = new Array(length);
          for (var i = 0; i < length; i++) {
            values[i] = obj[keys[i]];
          }
          return values;
        };

- ¶

  Convert an object into a list of `[key, value]` pairs.

        _.pairs = function(obj) {
          var keys = _.keys(obj);
          var length = keys.length;
          var pairs = new Array(length);
          for (var i = 0; i < length; i++) {
            pairs[i] = [keys[i], obj[keys[i]]];
          }
          return pairs;
        };

- ¶

  Invert the keys and values of an object. The values must be serializable.

        _.invert = function(obj) {
          var result = {};
          var keys = _.keys(obj);
          for (var i = 0, length = keys.length; i < length; i++) {
            result[obj[keys[i]]] = keys[i];
          }
          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] === void 0) 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.

          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;
          }

- ¶

  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;
          }

- ¶

  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 {

- ¶

  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(Math.max(0, 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;'
          }
        };
        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 with the `object` as context; otherwise, return it.

        _.result = function(object, property) {
          if (object == null) return void 0;
          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);