- # underscore.js

- ¶

      Underscore.js 1.8.3
      http://underscorejs.org
      (c) 2009-2017 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._;

- ¶

  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,
          toString         = ObjProto.toString,
          hasOwnProperty   = ObjProto.hasOwnProperty;

- ¶

  All ECMAScript 5 native function implementations that we hope to use are declared here.

        var
          nativeIsArray      = Array.isArray,
          nativeKeys         = Object.keys,
          nativeBind         = FuncProto.bind,
          nativeCreate       = Object.create;

- ¶

  Naked function reference for surrogate-prototype-swapping.

        var Ctor = function(){};

- ¶

  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.

        if (typeof exports !== 'undefined') {
          if (typeof module !== 'undefined' && module.exports) {
            exports = module.exports = _;
          }
          exports._ = _;
        } else {
          root._ = _;
        }

- ¶

  Current version.

        _.VERSION = '1.8.3';

- ¶

  Internal function that returns an efficient (for current engines) version of the passed-in callback, to be repeatedly applied in other Underscore functions.

        var optimizeCb = function(func, context, argCount) {
          if (context === void 0) return func;
          switch (argCount == null ? 3 : argCount) {
            case 1: return function(value) {
              return func.call(context, value);
            };
            case 2: return function(value, other) {
              return func.call(context, value, other);
            };
            case 3: return function(value, index, collection) {
              return func.call(context, value, index, collection);
            };
            case 4: return function(accumulator, value, index, collection) {
              return func.call(context, accumulator, value, index, collection);
            };
          }
          return function() {
            return func.apply(context, arguments);
          };
        };

- ¶

  A mostly-internal function to generate callbacks that can be applied to each element in a collection, returning the desired result — either identity, an arbitrary callback, a property matcher, or a property accessor.

        var cb = function(value, context, argCount) {
          if (value == null) return _.identity;
          if (_.isFunction(value)) return optimizeCb(value, context, argCount);
          if (_.isObject(value)) return _.matcher(value);
          return _.property(value);
        };
        _.iteratee = function(value, context) {
          return cb(value, context, Infinity);
        };

- ¶

  An internal function for creating assigner functions.

        var createAssigner = function(keysFunc, undefinedOnly) {
          return function(obj) {
            var length = arguments.length;
            if (length < 2 || obj == null) return obj;
            for (var index = 1; index < length; index++) {
              var source = arguments[index],
                  keys = keysFunc(source),
                  l = keys.length;
              for (var i = 0; i < l; i++) {
                var key = keys[i];
                if (!undefinedOnly || obj[key] === void 0) obj[key] = source[key];
              }
            }
            return obj;
          };
        };

- ¶

  An internal function for creating a new object that inherits from another.

        var baseCreate = function(prototype) {
          if (!_.isObject(prototype)) return {};
          if (nativeCreate) return nativeCreate(prototype);
          Ctor.prototype = prototype;
          var result = new Ctor;
          Ctor.prototype = null;
          return result;
        };

        var property = function(key) {
          return function(obj) {
            return obj == null ? void 0 : obj[key];
          };
        };

- ¶

  Helper for collection methods to determine whether a collection should be iterated as an array or as an object Related: http://people.mozilla.org/~jorendorff/es6-draft.html#sec-tolength Avoids a very nasty iOS 8 JIT bug on ARM-64. #2094

        var MAX_ARRAY_INDEX = Math.pow(2, 53) - 1;
        var getLength = property('length');
        var isArrayLike = function(collection) {
          var length = getLength(collection);
          return typeof length == 'number' && length >= 0 && length <= MAX_ARRAY_INDEX;
        };

- ¶

  ## Collection Functions

- ¶
- ¶

  The cornerstone, an `each` implementation, aka `forEach`. Handles raw objects in addition to array-likes. Treats all sparse array-likes as if they were dense.

        _.each = _.forEach = function(obj, iteratee, context) {
          iteratee = optimizeCb(iteratee, context);
          var i, length;
          if (isArrayLike(obj)) {
            for (i = 0, length = obj.length; i < length; i++) {
              iteratee(obj[i], i, obj);
            }
          } else {
            var keys = _.keys(obj);
            for (i = 0, length = keys.length; i < length; i++) {
              iteratee(obj[keys[i]], keys[i], obj);
            }
          }
          return obj;
        };

- ¶

  Return the results of applying the iteratee to each element.

        _.map = _.collect = function(obj, iteratee, context) {
          iteratee = cb(iteratee, context);
          var keys = !isArrayLike(obj) && _.keys(obj),
              length = (keys || obj).length,
              results = Array(length);
          for (var index = 0; index < length; index++) {
            var currentKey = keys ? keys[index] : index;
            results[index] = iteratee(obj[currentKey], currentKey, obj);
          }
          return results;
        };

- ¶

  Create a reducing function iterating left or right.

        function createReduce(dir) {

- ¶

  Optimized iterator function as using arguments.length in the main function will deoptimize the, see #1991.

          function iterator(obj, iteratee, memo, keys, index, length) {
            for (; index >= 0 && index < length; index += dir) {
              var currentKey = keys ? keys[index] : index;
              memo = iteratee(memo, obj[currentKey], currentKey, obj);
            }
            return memo;
          }

          return function(obj, iteratee, memo, context) {
            iteratee = optimizeCb(iteratee, context, 4);
            var keys = !isArrayLike(obj) && _.keys(obj),
                length = (keys || obj).length,
                index = dir > 0 ? 0 : length - 1;

- ¶

  Determine the initial value if none is provided.

            if (arguments.length < 3) {
              memo = obj[keys ? keys[index] : index];
              index += dir;
            }
            return iterator(obj, iteratee, memo, keys, index, length);
          };
        }

- ¶

  Reduce builds up a single result from a list of values, aka `inject`, or `foldl`.

        _.reduce = _.foldl = _.inject = createReduce(1);

- ¶

  The right-associative version of reduce, also known as `foldr`.

        _.reduceRight = _.foldr = createReduce(-1);

- ¶

  Return the first value which passes a truth test. Aliased as `detect`.

        _.find = _.detect = function(obj, predicate, context) {
          var key;
          if (isArrayLike(obj)) {
            key = _.findIndex(obj, predicate, context);
          } else {
            key = _.findKey(obj, predicate, context);
          }
          if (key !== void 0 && key !== -1) return obj[key];
        };

- ¶

  Return all the elements that pass a truth test. Aliased as `select`.

        _.filter = _.select = function(obj, predicate, context) {
          var results = [];
          predicate = cb(predicate, context);
          _.each(obj, function(value, index, list) {
            if (predicate(value, index, list)) results.push(value);
          });
          return results;
        };

- ¶

  Return all the elements for which a truth test fails.

        _.reject = function(obj, predicate, context) {
          return _.filter(obj, _.negate(cb(predicate)), context);
        };

- ¶

  Determine whether all of the elements match a truth test. Aliased as `all`.

        _.every = _.all = function(obj, predicate, context) {
          predicate = cb(predicate, context);
          var keys = !isArrayLike(obj) && _.keys(obj),
              length = (keys || obj).length;
          for (var index = 0; index < length; index++) {
            var currentKey = keys ? keys[index] : index;
            if (!predicate(obj[currentKey], currentKey, obj)) return false;
          }
          return true;
        };

- ¶

  Determine if at least one element in the object matches a truth test. Aliased as `any`.

        _.some = _.any = function(obj, predicate, context) {
          predicate = cb(predicate, context);
          var keys = !isArrayLike(obj) && _.keys(obj),
              length = (keys || obj).length;
          for (var index = 0; index < length; index++) {
            var currentKey = keys ? keys[index] : index;
            if (predicate(obj[currentKey], currentKey, obj)) return true;
          }
          return false;
        };

- ¶

  Determine if the array or object contains a given item (using `===`). Aliased as `includes` and `include`.

        _.contains = _.includes = _.include = function(obj, item, fromIndex, guard) {
          if (!isArrayLike(obj)) obj = _.values(obj);
          if (typeof fromIndex != 'number' || guard) fromIndex = 0;
          return _.indexOf(obj, item, fromIndex) >= 0;
        };

- ¶

  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) {
            var func = isFunc ? method : value[method];
            return func == null ? func : func.apply(value, args);
          });
        };

- ¶

  Convenience version of a common use case of `map`: fetching a property.

        _.pluck = function(obj, key) {
          return _.map(obj, _.property(key));
        };

- ¶

  Convenience version of a common use case of `filter`: selecting only objects containing specific `key:value` pairs.

        _.where = function(obj, attrs) {
          return _.filter(obj, _.matcher(attrs));
        };

- ¶

  Convenience version of a common use case of `find`: getting the first object containing specific `key:value` pairs.

        _.findWhere = function(obj, attrs) {
          return _.find(obj, _.matcher(attrs));
        };

- ¶

  Return the maximum element (or element-based computation).

        _.max = function(obj, iteratee, context) {
          var result = -Infinity, lastComputed = -Infinity,
              value, computed;
          if (iteratee == null && obj != null) {
            obj = isArrayLike(obj) ? obj : _.values(obj);
            for (var i = 0, length = obj.length; i < length; i++) {
              value = obj[i];
              if (value > result) {
                result = value;
              }
            }
          } else {
            iteratee = cb(iteratee, context);
            _.each(obj, function(value, index, list) {
              computed = iteratee(value, index, list);
              if (computed > lastComputed || computed === -Infinity && result === -Infinity) {
                result = value;
                lastComputed = computed;
              }
            });
          }
          return result;
        };

- ¶

  Return the minimum element (or element-based computation).

        _.min = function(obj, iteratee, context) {
          var result = Infinity, lastComputed = Infinity,
              value, computed;
          if (iteratee == null && obj != null) {
            obj = isArrayLike(obj) ? obj : _.values(obj);
            for (var i = 0, length = obj.length; i < length; i++) {
              value = obj[i];
              if (value < result) {
                result = value;
              }
            }
          } else {
            iteratee = cb(iteratee, context);
            _.each(obj, function(value, index, list) {
              computed = iteratee(value, index, list);
              if (computed < lastComputed || computed === Infinity && result === Infinity) {
                result = value;
                lastComputed = computed;
              }
            });
          }
          return result;
        };

- ¶

  Shuffle a collection, using the modern version of the Fisher-Yates shuffle.

        _.shuffle = function(obj) {
          var set = isArrayLike(obj) ? obj : _.values(obj);
          var length = set.length;
          var shuffled = Array(length);
          for (var index = 0, rand; index < length; index++) {
            rand = _.random(0, index);
            if (rand !== index) shuffled[index] = shuffled[rand];
            shuffled[rand] = set[index];
          }
          return shuffled;
        };

- ¶

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

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

- ¶

  Sort the object’s values by a criterion produced by an iteratee.

        _.sortBy = function(obj, iteratee, context) {
          iteratee = cb(iteratee, context);
          return _.pluck(_.map(obj, function(value, index, list) {
            return {
              value: value,
              index: index,
              criteria: iteratee(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, iteratee, context) {
            var result = {};
            iteratee = cb(iteratee, context);
            _.each(obj, function(value, index) {
              var key = iteratee(value, index, obj);
              behavior(result, value, key);
            });
            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, value, key) {
          if (_.has(result, key)) result[key].push(value); else result[key] = [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, value, key) {
          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, value, key) {
          if (_.has(result, key)) result[key]++; else result[key] = 1;
        });

- ¶

  Safely create a real, live array from anything iterable.

        _.toArray = function(obj) {
          if (!obj) return [];
          if (_.isArray(obj)) return slice.call(obj);
          if (isArrayLike(obj)) return _.map(obj, _.identity);
          return _.values(obj);
        };

- ¶

  Return the number of elements in an object.

        _.size = function(obj) {
          if (obj == null) return 0;
          return isArrayLike(obj) ? obj.length : _.keys(obj).length;
        };

- ¶

  Split a collection into two arrays: one whose elements all satisfy the given predicate, and one whose elements all do not satisfy the predicate.

        _.partition = function(obj, predicate, context) {
          predicate = cb(predicate, context);
          var pass = [], fail = [];
          _.each(obj, function(value, key, obj) {
            (predicate(value, key, obj) ? pass : fail).push(value);
          });
          return [pass, fail];
        };

- ¶

  Trim out all falsy values from an object.

        _.compact = function(obj) {
          return _.filter(obj, _.identity);
        };

- ¶

  ## 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;
          if (n == null || guard) return array[0];
          return _.initial(array, array.length - 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.

        _.initial = function(array, n, guard) {
          return slice.call(array, 0, Math.max(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.

        _.last = function(array, n, guard) {
          if (array == null) return void 0;
          if (n == null || guard) return array[array.length - 1];
          return _.rest(array, Math.max(0, array.length - n));
        };

- ¶

  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.

        _.rest = _.tail = _.drop = function(array, n, guard) {
          return slice.call(array, n == null || guard ? 1 : n);
        };

- ¶

  Internal implementation of a recursive `flatten` function.

        var flatten = function(input, shallow, strict, startIndex) {
          var output = [], idx = 0;
          for (var i = startIndex || 0, length = getLength(input); i < length; i++) {
            var value = input[i];
            if (isArrayLike(value) && (_.isArray(value) || _.isArguments(value))) {

- ¶

  flatten current level of array or arguments object

              if (!shallow) value = flatten(value, shallow, strict);
              var j = 0, len = value.length;
              output.length += len;
              while (j < len) {
                output[idx++] = value[j++];
              }
            } else if (!strict) {
              output[idx++] = value;
            }
          }
          return output;
        };

- ¶

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

        _.flatten = function(array, shallow) {
          return flatten(array, shallow, false);
        };

- ¶

  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, iteratee, context) {
          if (!_.isBoolean(isSorted)) {
            context = iteratee;
            iteratee = isSorted;
            isSorted = false;
          }
          if (iteratee != null) iteratee = cb(iteratee, context);
          var result = [];
          var seen = [];
          for (var i = 0, length = getLength(array); i < length; i++) {
            var value = array[i],
                computed = iteratee ? iteratee(value, i, array) : value;
            if (isSorted) {
              if (!i || seen !== computed) result.push(value);
              seen = computed;
            } else if (iteratee) {
              if (!_.contains(seen, computed)) {
                seen.push(computed);
                result.push(value);
              }
            } else if (!_.contains(result, value)) {
              result.push(value);
            }
          }
          return result;
        };

- ¶

  Produce an array that contains the union: each distinct element from all of the passed-in arrays.

        _.union = function() {
          return _.uniq(flatten(arguments, true, true));
        };

- ¶

  Produce an array that contains every item shared between all the passed-in arrays.

        _.intersection = function(array) {
          var result = [];
          var argsLength = arguments.length;
          for (var i = 0, length = getLength(array); i < length; i++) {
            var item = array[i];
            if (_.contains(result, item)) continue;
            for (var j = 1; j < argsLength; j++) {
              if (!_.contains(arguments[j], item)) break;
            }
            if (j === argsLength) result.push(item);
          }
          return result;
        };

- ¶

  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 = flatten(arguments, true, true, 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() {
          return _.unzip(arguments);
        };

- ¶

  Complement of _.zip. Unzip accepts an array of arrays and groups each array’s elements on shared indices

        _.unzip = function(array) {
          var length = array && _.max(array, getLength).length || 0;
          var result = Array(length);

          for (var index = 0; index < length; index++) {
            result[index] = _.pluck(array, index);
          }
          return result;
        };

- ¶

  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) {
          var result = {};
          for (var i = 0, length = getLength(list); i < length; i++) {
            if (values) {
              result[list[i]] = values[i];
            } else {
              result[list[i][0]] = list[i][1];
            }
          }
          return result;
        };

- ¶

  Generator function to create the findIndex and findLastIndex functions

        function createPredicateIndexFinder(dir) {
          return function(array, predicate, context) {
            predicate = cb(predicate, context);
            var length = getLength(array);
            var index = dir > 0 ? 0 : length - 1;
            for (; index >= 0 && index < length; index += dir) {
              if (predicate(array[index], index, array)) return index;
            }
            return -1;
          };
        }

- ¶

  Returns the first index on an array-like that passes a predicate test

        _.findIndex = createPredicateIndexFinder(1);
        _.findLastIndex = createPredicateIndexFinder(-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, iteratee, context) {
          iteratee = cb(iteratee, context, 1);
          var value = iteratee(obj);
          var low = 0, high = getLength(array);
          while (low < high) {
            var mid = Math.floor((low + high) / 2);
            if (iteratee(array[mid]) < value) low = mid + 1; else high = mid;
          }
          return low;
        };

- ¶

  Generator function to create the indexOf and lastIndexOf functions

        function createIndexFinder(dir, predicateFind, sortedIndex) {
          return function(array, item, idx) {
            var i = 0, length = getLength(array);
            if (typeof idx == 'number') {
              if (dir > 0) {
                  i = idx >= 0 ? idx : Math.max(idx + length, i);
              } else {
                  length = idx >= 0 ? Math.min(idx + 1, length) : idx + length + 1;
              }
            } else if (sortedIndex && idx && length) {
              idx = sortedIndex(array, item);
              return array[idx] === item ? idx : -1;
            }
            if (item !== item) {
              idx = predicateFind(slice.call(array, i, length), _.isNaN);
              return idx >= 0 ? idx + i : -1;
            }
            for (idx = dir > 0 ? i : length - 1; idx >= 0 && idx < length; idx += dir) {
              if (array[idx] === item) return idx;
            }
            return -1;
          };
        }

- ¶

  Return the position of the first occurrence of an item in an array, or -1 if the item is not included in the array. If the array is large and already in sort order, pass `true` for isSorted to use binary search.

        _.indexOf = createIndexFinder(1, _.findIndex, _.sortedIndex);
        _.lastIndexOf = createIndexFinder(-1, _.findLastIndex);

- ¶

  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 (stop == null) {
            stop = start || 0;
            start = 0;
          }
          step = step || 1;

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

          for (var idx = 0; idx < length; idx++, start += step) {
            range[idx] = start;
          }

          return range;
        };

- ¶

  ## Function (ahem) Functions

- ¶
- ¶

  Determines whether to execute a function as a constructor or a normal function with the provided arguments

        var executeBound = function(sourceFunc, boundFunc, context, callingContext, args) {
          if (!(callingContext instanceof boundFunc)) return sourceFunc.apply(context, args);
          var self = baseCreate(sourceFunc.prototype);
          var result = sourceFunc.apply(self, args);
          if (_.isObject(result)) return result;
          return self;
        };

- ¶

  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 (nativeBind && func.bind === nativeBind) return nativeBind.apply(func, slice.call(arguments, 1));
          if (!_.isFunction(func)) throw new TypeError('Bind must be called on a function');
          var args = slice.call(arguments, 2);
          var bound = function() {
            return executeBound(func, bound, context, this, args.concat(slice.call(arguments)));
          };
          return bound;
        };

- ¶

  Partially apply a function by creating a version that has had some of its arguments pre-filled, without changing its dynamic `this` context. _ acts as a placeholder, allowing any combination of arguments to be pre-filled.

        _.partial = function(func) {
          var boundArgs = slice.call(arguments, 1);
          var bound = function() {
            var position = 0, length = boundArgs.length;
            var args = Array(length);
            for (var i = 0; i < length; i++) {
              args[i] = boundArgs[i] === _ ? arguments[position++] : boundArgs[i];
            }
            while (position < arguments.length) args.push(arguments[position++]);
            return executeBound(func, bound, this, this, args);
          };
          return bound;
        };

- ¶

  Bind a number of an object’s methods to that object. Remaining arguments are the method names to be bound. Useful for ensuring that all callbacks defined on an object belong to it.

        _.bindAll = function(obj) {
          var i, length = arguments.length, key;
          if (length <= 1) throw new Error('bindAll must be passed function names');
          for (i = 1; i < length; i++) {
            key = arguments[i];
            obj[key] = _.bind(obj[key], obj);
          }
          return obj;
        };

- ¶

  Memoize an expensive function by storing its results.

        _.memoize = function(func, hasher) {
          var memoize = function(key) {
            var cache = memoize.cache;
            var address = '' + (hasher ? hasher.apply(this, arguments) : key);
            if (!_.has(cache, address)) cache[address] = func.apply(this, arguments);
            return cache[address];
          };
          memoize.cache = {};
          return memoize;
        };

- ¶

  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 = _.partial(_.delay, _, 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;
          if (!options) options = {};
          var later = function() {
            previous = options.leading === false ? 0 : _.now();
            timeout = null;
            result = func.apply(context, args);
            if (!timeout) context = args = null;
          };
          return function() {
            var now = _.now();
            if (!previous && options.leading === false) previous = now;
            var remaining = wait - (now - previous);
            context = this;
            args = arguments;
            if (remaining <= 0 || remaining > wait) {
              if (timeout) {
                clearTimeout(timeout);
                timeout = null;
              }
              previous = now;
              result = func.apply(context, args);
              if (!timeout) context = args = null;
            } 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;

          var later = function() {
            var last = _.now() - timestamp;

            if (last < wait && last >= 0) {
              timeout = setTimeout(later, wait - last);
            } else {
              timeout = null;
              if (!immediate) {
                result = func.apply(context, args);
                if (!timeout) context = args = null;
              }
            }
          };

          return function() {
            context = this;
            args = arguments;
            timestamp = _.now();
            var callNow = immediate && !timeout;
            if (!timeout) timeout = setTimeout(later, wait);
            if (callNow) {
              result = func.apply(context, args);
              context = args = null;
            }

            return result;
          };
        };

- ¶

  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 _.partial(wrapper, func);
        };

- ¶

  Returns a negated version of the passed-in predicate.

        _.negate = function(predicate) {
          return function() {
            return !predicate.apply(this, arguments);
          };
        };

- ¶

  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 args = arguments;
          var start = args.length - 1;
          return function() {
            var i = start;
            var result = args[start].apply(this, arguments);
            while (i--) result = args[i].call(this, result);
            return result;
          };
        };

- ¶

  Returns a function that will only be executed on and after the Nth call.

        _.after = function(times, func) {
          return function() {
            if (--times < 1) {
              return func.apply(this, arguments);
            }
          };
        };

- ¶

  Returns a function that will only be executed up to (but not including) the Nth call.

        _.before = function(times, func) {
          var memo;
          return function() {
            if (--times > 0) {
              memo = func.apply(this, arguments);
            }
            if (times <= 1) func = null;
            return memo;
          };
        };

- ¶

  Returns a function that will be executed at most one time, no matter how often you call it. Useful for lazy initialization.

        _.once = _.partial(_.before, 2);

- ¶

  ## Object Functions

- ¶
- ¶

  Keys in IE < 9 that won’t be iterated by `for key in ...` and thus missed.

        var hasEnumBug = !{toString: null}.propertyIsEnumerable('toString');
        var nonEnumerableProps = ['valueOf', 'isPrototypeOf', 'toString',
                            'propertyIsEnumerable', 'hasOwnProperty', 'toLocaleString'];

        function collectNonEnumProps(obj, keys) {
          var nonEnumIdx = nonEnumerableProps.length;
          var constructor = obj.constructor;
          var proto = (_.isFunction(constructor) && constructor.prototype) || ObjProto;

- ¶

  Constructor is a special case.

          var prop = 'constructor';
          if (_.has(obj, prop) && !_.contains(keys, prop)) keys.push(prop);

          while (nonEnumIdx--) {
            prop = nonEnumerableProps[nonEnumIdx];
            if (prop in obj && obj[prop] !== proto[prop] && !_.contains(keys, prop)) {
              keys.push(prop);
            }
          }
        }

- ¶

  Retrieve the names of an object’s own properties. Delegates to ECMAScript 5‘s native `Object.keys`

        _.keys = function(obj) {
          if (!_.isObject(obj)) return [];
          if (nativeKeys) return nativeKeys(obj);
          var keys = [];
          for (var key in obj) if (_.has(obj, key)) keys.push(key);

- ¶

  Ahem, IE < 9.

          if (hasEnumBug) collectNonEnumProps(obj, keys);
          return keys;
        };

- ¶

  Retrieve all the property names of an object.

        _.allKeys = function(obj) {
          if (!_.isObject(obj)) return [];
          var keys = [];
          for (var key in obj) keys.push(key);

- ¶

  Ahem, IE < 9.

          if (hasEnumBug) collectNonEnumProps(obj, keys);
          return keys;
        };

- ¶

  Retrieve the values of an object’s properties.

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

- ¶

  Returns the results of applying the iteratee to each element of the object In contrast to _.map it returns an object

        _.mapObject = function(obj, iteratee, context) {
          iteratee = cb(iteratee, context);
          var keys =  _.keys(obj),
                length = keys.length,
                results = {},
                currentKey;
            for (var index = 0; index < length; index++) {
              currentKey = keys[index];
              results[currentKey] = iteratee(obj[currentKey], currentKey, obj);
            }
            return results;
        };

- ¶

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

        _.pairs = function(obj) {
          var keys = _.keys(obj);
          var length = keys.length;
          var pairs = 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 = createAssigner(_.allKeys);

- ¶

  Assigns a given object with all the own properties in the passed-in object(s) (https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Object/assign)

        _.extendOwn = _.assign = createAssigner(_.keys);

- ¶

  Returns the first key on an object that passes a predicate test

        _.findKey = function(obj, predicate, context) {
          predicate = cb(predicate, context);
          var keys = _.keys(obj), key;
          for (var i = 0, length = keys.length; i < length; i++) {
            key = keys[i];
            if (predicate(obj[key], key, obj)) return key;
          }
        };

- ¶

  Return a copy of the object only containing the whitelisted properties.

        _.pick = function(object, oiteratee, context) {
          var result = {}, obj = object, iteratee, keys;
          if (obj == null) return result;
          if (_.isFunction(oiteratee)) {
            keys = _.allKeys(obj);
            iteratee = optimizeCb(oiteratee, context);
          } else {
            keys = flatten(arguments, false, false, 1);
            iteratee = function(value, key, obj) { return key in obj; };
            obj = Object(obj);
          }
          for (var i = 0, length = keys.length; i < length; i++) {
            var key = keys[i];
            var value = obj[key];
            if (iteratee(value, key, obj)) result[key] = value;
          }
          return result;
        };

- ¶

  Return a copy of the object without the blacklisted properties.

        _.omit = function(obj, iteratee, context) {
          if (_.isFunction(iteratee)) {
            iteratee = _.negate(iteratee);
          } else {
            var keys = _.map(flatten(arguments, false, false, 1), String);
            iteratee = function(value, key) {
              return !_.contains(keys, key);
            };
          }
          return _.pick(obj, iteratee, context);
        };

- ¶

  Fill in a given object with default properties.

        _.defaults = createAssigner(_.allKeys, true);

- ¶

  Creates an object that inherits from the given prototype object. If additional properties are provided then they will be added to the created object.

        _.create = function(prototype, props) {
          var result = baseCreate(prototype);
          if (props) _.extendOwn(result, props);
          return result;
        };

- ¶

  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;
        };

- ¶

  Returns whether an object has a given set of `key:value` pairs.

        _.isMatch = function(object, attrs) {
          var keys = _.keys(attrs), length = keys.length;
          if (object == null) return !length;
          var obj = Object(object);
          for (var i = 0; i < length; i++) {
            var key = keys[i];
            if (attrs[key] !== obj[key] || !(key in obj)) return false;
          }
          return true;
        };

- ¶

  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, regular expressions, dates, and booleans are compared by value.

            case '[object RegExp]':

- ¶

  RegExps are coerced to strings for comparison (Note: ‘’ + /a/i === ‘/a/i’)

            case '[object String]':

- ¶

  Primitives and their corresponding object wrappers are equivalent; thus, `"5"` is equivalent to `new String("5")`.

              return '' + a === '' + b;
            case '[object Number]':

- ¶

  `NaN`s are equivalent, but non-reflexive. Object(NaN) is equivalent to NaN

              if (+a !== +a) return +b !== +b;

- ¶

  An `egal` comparison is performed for other numeric values.

              return +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;
          }

          var areArrays = className === '[object Array]';
          if (!areArrays) {
            if (typeof a != 'object' || typeof b != 'object') return false;

- ¶

  Objects with different constructors are not equivalent, but `Object`s or `Array`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)
                                && ('constructor' in a && 'constructor' in b)) {
              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`.

- ¶

  Initializing stack of traversed objects. It’s done here since we only need them for objects and arrays comparison.

          aStack = aStack || [];
          bStack = bStack || [];
          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);

- ¶

  Recursively compare objects and arrays.

          if (areArrays) {

- ¶

  Compare array lengths to determine if a deep comparison is necessary.

            length = a.length;
            if (length !== b.length) return false;

- ¶

  Deep compare the contents, ignoring non-numeric properties.

            while (length--) {
              if (!eq(a[length], b[length], aStack, bStack)) return false;
            }
          } else {

- ¶

  Deep compare objects.

            var keys = _.keys(a), key;
            length = keys.length;

- ¶

  Ensure that both objects contain the same number of properties before comparing deep equality.

            if (_.keys(b).length !== length) return false;
            while (length--) {

- ¶

  Deep compare each member

              key = keys[length];
              if (!(_.has(b, key) && eq(a[key], b[key], aStack, bStack))) return false;
            }
          }

- ¶

  Remove the first object from the stack of traversed objects.

          aStack.pop();
          bStack.pop();
          return true;
        };

- ¶

  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 (isArrayLike(obj) && (_.isArray(obj) || _.isString(obj) || _.isArguments(obj))) return obj.length === 0;
          return _.keys(obj).length === 0;
        };

- ¶

  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) {
          var type = typeof obj;
          return type === 'function' || type === 'object' && !!obj;
        };

- ¶

  Add some isType methods: isArguments, isFunction, isString, isNumber, isDate, isRegExp, isError.

        _.each(['Arguments', 'Function', 'String', 'Number', 'Date', 'RegExp', 'Error'], function(name) {
          _['is' + name] = function(obj) {
            return toString.call(obj) === '[object ' + name + ']';
          };
        });

- ¶

  Define a fallback version of the method in browsers (ahem, IE < 9), where there isn’t any inspectable “Arguments” type.

        if (!_.isArguments(arguments)) {
          _.isArguments = function(obj) {
            return _.has(obj, 'callee');
          };
        }

- ¶

  Optimize `isFunction` if appropriate. Work around some typeof bugs in old v8, IE 11 (#1621), and in Safari 8 (#1929).

        if (typeof /./ != 'function' && typeof Int8Array != 'object') {
          _.isFunction = function(obj) {
            return typeof obj == 'function' || false;
          };
        }

- ¶

  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 obj != null && 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 iteratees.

        _.identity = function(value) {
          return value;
        };

- ¶

  Predicate-generating functions. Often useful outside of Underscore.

        _.constant = function(value) {
          return function() {
            return value;
          };
        };

        _.noop = function(){};

        _.property = property;

- ¶

  Generates a function for a given object that returns a given property.

        _.propertyOf = function(obj) {
          return obj == null ? function(){} : function(key) {
            return obj[key];
          };
        };

- ¶

  Returns a predicate for checking whether an object has a given set of `key:value` pairs.

        _.matcher = _.matches = function(attrs) {
          attrs = _.extendOwn({}, attrs);
          return function(obj) {
            return _.isMatch(obj, attrs);
          };
        };

- ¶

  Run a function n times.

        _.times = function(n, iteratee, context) {
          var accum = Array(Math.max(0, n));
          iteratee = optimizeCb(iteratee, context, 1);
          for (var i = 0; i < n; i++) accum[i] = iteratee(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));
        };

- ¶

  A (possibly faster) way to get the current timestamp as an integer.

        _.now = Date.now || function() {
          return new Date().getTime();
        };

- ¶

  List of HTML entities for escaping.

        var escapeMap = {
          '&': '&amp;',
          '<': '&lt;',
          '>': '&gt;',
          '"': '&quot;',
          "'": '&#x27;',
          '`': '&#x60;'
        };
        var unescapeMap = _.invert(escapeMap);

- ¶

  Functions for escaping and unescaping strings to/from HTML interpolation.

        var createEscaper = function(map) {
          var escaper = function(match) {
            return map[match];
          };

- ¶

  Regexes for identifying a key that needs to be escaped

          var source = '(?:' + _.keys(map).join('|') + ')';
          var testRegexp = RegExp(source);
          var replaceRegexp = RegExp(source, 'g');
          return function(string) {
            string = string == null ? '' : '' + string;
            return testRegexp.test(string) ? string.replace(replaceRegexp, escaper) : string;
          };
        };
        _.escape = createEscaper(escapeMap);
        _.unescape = createEscaper(unescapeMap);

- ¶

  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, fallback) {
          var value = object == null ? void 0 : object[property];
          if (value === void 0) {
            value = fallback;
          }
          return _.isFunction(value) ? value.call(object) : value;
        };

- ¶

  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',
          '\u2028': 'u2028',
          '\u2029': 'u2029'
        };

        var escaper = /\\|'|\r|\n|\u2028|\u2029/g;

        var escapeChar = function(match) {
          return '\\' + escapes[match];
        };

- ¶

  JavaScript micro-templating, similar to John Resig’s implementation. Underscore templating handles arbitrary delimiters, preserves whitespace, and correctly escapes quotes within interpolated code. NB: `oldSettings` only exists for backwards compatibility.

        _.template = function(text, settings, oldSettings) {
          if (!settings && oldSettings) settings = oldSettings;
          settings = _.defaults({}, settings, _.templateSettings);

- ¶

  Combine delimiters into one regular expression via alternation.

          var matcher = 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, escapeChar);
            index = offset + match.length;

            if (escape) {
              source += "'+\n((__t=(" + escape + "))==null?'':_.escape(__t))+\n'";
            } else if (interpolate) {
              source += "'+\n((__t=(" + interpolate + "))==null?'':__t)+\n'";
            } else if (evaluate) {
              source += "';\n" + evaluate + "\n__p+='";
            }

- ¶

  Adobe VMs need the match returned to produce the correct offset.

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

          var template = function(data) {
            return render.call(this, data, _);
          };

- ¶

  Provide the compiled source as a convenience for precompilation.

          var argument = settings.variable || 'obj';
          template.source = 'function(' + argument + '){\n' + source + '}';

          return template;
        };

- ¶

  Add a “chain” function. Start chaining a wrapped Underscore object.

        _.chain = function(obj) {
          var instance = _(obj);
          instance._chain = true;
          return instance;
        };

- ¶

  ## 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(instance, obj) {
          return instance._chain ? _(obj).chain() : obj;
        };

- ¶

  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(this, func.apply(_, args));
            };
          });
        };

- ¶

  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(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(this, method.apply(this._wrapped, arguments));
          };
        });

- ¶

  Extracts the result from a wrapped and chained object.

        _.prototype.value = function() {
          return this._wrapped;
        };

- ¶

  Provide unwrapping proxy for some methods used in engine operations such as arithmetic and JSON stringification.

        _.prototype.valueOf = _.prototype.toJSON = _.prototype.value;

        _.prototype.toString = function() {
          return '' + this._wrapped;
        };

- ¶

  AMD registration happens at the end for compatibility with AMD loaders that may not enforce next-turn semantics on modules. Even though general practice for AMD registration is to be anonymous, underscore registers as a named module because, like jQuery, it is a base library that is popular enough to be bundled in a third party lib, but not be part of an AMD load request. Those cases could generate an error when an anonymous define() is called outside of a loader request.

        if (typeof define === 'function' && define.amd) {
          define('underscore', [], function() {
            return _;
          });
        }
      }.call(this));