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Underscore.js 1.8.3

Underscore provides over 100 functions that support both your favorite workaday functional helpers

此脚本不应直接安装。它是供其他脚本使用的外部库，要使用该库请加入元指令 // @require https://update.cn-greasyfork.org/scripts/17419/109803/Underscorejs%20183.js 

  1. //     Underscore.js 1.8.3
  2. //     http://underscorejs.org
  3. //     (c) 2009-2016 Jeremy Ashkenas, DocumentCloud and Investigative Reporters & Editors
  4. //     Underscore may be freely distributed under the MIT license.
  5.  
  6. (function() {
  7.  
  8.   // Baseline setup
  9.   // --------------
  10.  
  11.   // Establish the root object, `window` (`self`) in the browser, `global`
  12.   // on the server, or `this` in some virtual machines. We use `self`
  13.   // instead of `window` for `WebWorker` support.
  14.   var root = typeof self == 'object' && self.self === self && self ||
  15.             typeof global == 'object' && global.global === global && global ||
  16.             this;
  17.  
  18.   // Save the previous value of the `_` variable.
  19.   var previousUnderscore = root._;
  20.  
  21.   // Save bytes in the minified (but not gzipped) version:
  22.   var ArrayProto = Array.prototype, ObjProto = Object.prototype;
  23.  
  24.   // Create quick reference variables for speed access to core prototypes.
  25.   var push = ArrayProto.push,
  26.       slice = ArrayProto.slice,
  27.       toString = ObjProto.toString,
  28.       hasOwnProperty = ObjProto.hasOwnProperty;
  29.  
  30.   // All **ECMAScript 5** native function implementations that we hope to use
  31.   // are declared here.
  32.   var nativeIsArray = Array.isArray,
  33.       nativeKeys = Object.keys,
  34.       nativeCreate = Object.create;
  35.  
  36.   // Naked function reference for surrogate-prototype-swapping.
  37.   var Ctor = function(){};
  38.  
  39.   // Create a safe reference to the Underscore object for use below.
  40.   var _ = function(obj) {
  41.     if (obj instanceof _) return obj;
  42.     if (!(this instanceof _)) return new _(obj);
  43.     this._wrapped = obj;
  44.   };
  45.  
  46.   // Export the Underscore object for **Node.js**, with
  47.   // backwards-compatibility for their old module API. If we're in
  48.   // the browser, add `_` as a global object.
  49.   // (`nodeType` is checked to ensure that `module`
  50.   // and `exports` are not HTML elements.)
  51.   if (typeof exports != 'undefined' && !exports.nodeType) {
  52.     if (typeof module != 'undefined' && !module.nodeType && module.exports) {
  53.       exports = module.exports = _;
  54.     }
  55.     exports._ = _;
  56.   } else {
  57.     root._ = _;
  58.   }
  59.  
  60.   // Current version.
  61.   _.VERSION = '1.8.3';
  62.  
  63.   // Internal function that returns an efficient (for current engines) version
  64.   // of the passed-in callback, to be repeatedly applied in other Underscore
  65.   // functions.
  66.   var optimizeCb = function(func, context, argCount) {
  67.     if (context === void 0) return func;
  68.     switch (argCount == null ? 3 : argCount) {
  69.       case 1: return function(value) {
  70.         return func.call(context, value);
  71.       };
  72.       // The 2-parameter case has been omitted only because no current consumers
  73.       // made use of it.
  74.       case 3: return function(value, index, collection) {
  75.         return func.call(context, value, index, collection);
  76.       };
  77.       case 4: return function(accumulator, value, index, collection) {
  78.         return func.call(context, accumulator, value, index, collection);
  79.       };
  80.     }
  81.     return function() {
  82.       return func.apply(context, arguments);
  83.     };
  84.   };
  85.  
  86.   // An internal function to generate callbacks that can be applied to each
  87.   // element in a collection, returning the desired result — either `identity`,
  88.   // an arbitrary callback, a property matcher, or a property accessor.
  89.   var cb = function(value, context, argCount) {
  90.     if (value == null) return _.identity;
  91.     if (_.isFunction(value)) return optimizeCb(value, context, argCount);
  92.     if (_.isObject(value)) return _.matcher(value);
  93.     return _.property(value);
  94.   };
  95.  
  96.   // An external wrapper for the internal callback generator.
  97.   _.iteratee = function(value, context) {
  98.     return cb(value, context, Infinity);
  99.   };
  100.  
  101.   // Similar to ES6's rest param (http://ariya.ofilabs.com/2013/03/es6-and-rest-parameter.html)
  102.   // This accumulates the arguments passed into an array, after a given index.
  103.   var restArgs = function(func, startIndex) {
  104.     startIndex = startIndex == null ? func.length - 1 : +startIndex;
  105.     return function() {
  106.       var length = Math.max(arguments.length - startIndex, 0);
  107.       var rest = Array(length);
  108.       for (var index = 0; index < length; index++) {
  109.         rest[index] = arguments[index + startIndex];
  110.       }
  111.       switch (startIndex) {
  112.         case 0: return func.call(this, rest);
  113.         case 1: return func.call(this, arguments[0], rest);
  114.         case 2: return func.call(this, arguments[0], arguments[1], rest);
  115.       }
  116.       var args = Array(startIndex + 1);
  117.       for (index = 0; index < startIndex; index++) {
  118.         args[index] = arguments[index];
  119.       }
  120.       args[startIndex] = rest;
  121.       return func.apply(this, args);
  122.     };
  123.   };
  124.  
  125.   // An internal function for creating a new object that inherits from another.
  126.   var baseCreate = function(prototype) {
  127.     if (!_.isObject(prototype)) return {};
  128.     if (nativeCreate) return nativeCreate(prototype);
  129.     Ctor.prototype = prototype;
  130.     var result = new Ctor;
  131.     Ctor.prototype = null;
  132.     return result;
  133.   };
  134.  
  135.   var property = function(key) {
  136.     return function(obj) {
  137.       return obj == null ? void 0 : obj[key];
  138.     };
  139.   };
  140.  
  141.   // Helper for collection methods to determine whether a collection
  142.   // should be iterated as an array or as an object.
  143.   // Related: http://people.mozilla.org/~jorendorff/es6-draft.html#sec-tolength
  144.   // Avoids a very nasty iOS 8 JIT bug on ARM-64. #2094
  145.   var MAX_ARRAY_INDEX = Math.pow(2, 53) - 1;
  146.   var getLength = property('length');
  147.   var isArrayLike = function(collection) {
  148.     var length = getLength(collection);
  149.     return typeof length == 'number' && length >= 0 && length <= MAX_ARRAY_INDEX;
  150.   };
  151.  
  152.   // Collection Functions
  153.   // --------------------
  154.  
  155.   // The cornerstone, an `each` implementation, aka `forEach`.
  156.   // Handles raw objects in addition to array-likes. Treats all
  157.   // sparse array-likes as if they were dense.
  158.   _.each = _.forEach = function(obj, iteratee, context) {
  159.     iteratee = optimizeCb(iteratee, context);
  160.     var i, length;
  161.     if (isArrayLike(obj)) {
  162.       for (i = 0, length = obj.length; i < length; i++) {
  163.         iteratee(obj[i], i, obj);
  164.       }
  165.     } else {
  166.       var keys = _.keys(obj);
  167.       for (i = 0, length = keys.length; i < length; i++) {
  168.         iteratee(obj[keys[i]], keys[i], obj);
  169.       }
  170.     }
  171.     return obj;
  172.   };
  173.  
  174.   // Return the results of applying the iteratee to each element.
  175.   _.map = _.collect = function(obj, iteratee, context) {
  176.     iteratee = cb(iteratee, context);
  177.     var keys = !isArrayLike(obj) && _.keys(obj),
  178.         length = (keys || obj).length,
  179.         results = Array(length);
  180.     for (var index = 0; index < length; index++) {
  181.       var currentKey = keys ? keys[index] : index;
  182.       results[index] = iteratee(obj[currentKey], currentKey, obj);
  183.     }
  184.     return results;
  185.   };
  186.  
  187.   // Create a reducing function iterating left or right.
  188.   var createReduce = function(dir) {
  189.     // Wrap code that reassigns argument variables in a separate function than
  190.     // the one that accesses `arguments.length` to avoid a perf hit. (#1991)
  191.     var reducer = function(obj, iteratee, memo, initial) {
  192.       var keys = !isArrayLike(obj) && _.keys(obj),
  193.           length = (keys || obj).length,
  194.           index = dir > 0 ? 0 : length - 1;
  195.       if (!initial) {
  196.         memo = obj[keys ? keys[index] : index];
  197.         index += dir;
  198.       }
  199.       for (; index >= 0 && index < length; index += dir) {
  200.         var currentKey = keys ? keys[index] : index;
  201.         memo = iteratee(memo, obj[currentKey], currentKey, obj);
  202.       }
  203.       return memo;
  204.     };
  205.  
  206.     return function(obj, iteratee, memo, context) {
  207.       var initial = arguments.length >= 3;
  208.       return reducer(obj, optimizeCb(iteratee, context, 4), memo, initial);
  209.     };
  210.   };
  211.  
  212.   // **Reduce** builds up a single result from a list of values, aka `inject`,
  213.   // or `foldl`.
  214.   _.reduce = _.foldl = _.inject = createReduce(1);
  215.  
  216.   // The right-associative version of reduce, also known as `foldr`.
  217.   _.reduceRight = _.foldr = createReduce(-1);
  218.  
  219.   // Return the first value which passes a truth test. Aliased as `detect`.
  220.   _.find = _.detect = function(obj, predicate, context) {
  221.     var key;
  222.     if (isArrayLike(obj)) {
  223.       key = _.findIndex(obj, predicate, context);
  224.     } else {
  225.       key = _.findKey(obj, predicate, context);
  226.     }
  227.     if (key !== void 0 && key !== -1) return obj[key];
  228.   };
  229.  
  230.   // Return all the elements that pass a truth test.
  231.   // Aliased as `select`.
  232.   _.filter = _.select = function(obj, predicate, context) {
  233.     var results = [];
  234.     predicate = cb(predicate, context);
  235.     _.each(obj, function(value, index, list) {
  236.       if (predicate(value, index, list)) results.push(value);
  237.     });
  238.     return results;
  239.   };
  240.  
  241.   // Return all the elements for which a truth test fails.
  242.   _.reject = function(obj, predicate, context) {
  243.     return _.filter(obj, _.negate(cb(predicate)), context);
  244.   };
  245.  
  246.   // Determine whether all of the elements match a truth test.
  247.   // Aliased as `all`.
  248.   _.every = _.all = function(obj, predicate, context) {
  249.     predicate = cb(predicate, context);
  250.     var keys = !isArrayLike(obj) && _.keys(obj),
  251.         length = (keys || obj).length;
  252.     for (var index = 0; index < length; index++) {
  253.       var currentKey = keys ? keys[index] : index;
  254.       if (!predicate(obj[currentKey], currentKey, obj)) return false;
  255.     }
  256.     return true;
  257.   };
  258.  
  259.   // Determine if at least one element in the object matches a truth test.
  260.   // Aliased as `any`.
  261.   _.some = _.any = function(obj, predicate, context) {
  262.     predicate = cb(predicate, context);
  263.     var keys = !isArrayLike(obj) && _.keys(obj),
  264.         length = (keys || obj).length;
  265.     for (var index = 0; index < length; index++) {
  266.       var currentKey = keys ? keys[index] : index;
  267.       if (predicate(obj[currentKey], currentKey, obj)) return true;
  268.     }
  269.     return false;
  270.   };
  271.  
  272.   // Determine if the array or object contains a given item (using `===`).
  273.   // Aliased as `includes` and `include`.
  274.   _.contains = _.includes = _.include = function(obj, item, fromIndex, guard) {
  275.     if (!isArrayLike(obj)) obj = _.values(obj);
  276.     if (typeof fromIndex != 'number' || guard) fromIndex = 0;
  277.     return _.indexOf(obj, item, fromIndex) >= 0;
  278.   };
  279.  
  280.   // Invoke a method (with arguments) on every item in a collection.
  281.   _.invoke = restArgs(function(obj, method, args) {
  282.     var isFunc = _.isFunction(method);
  283.     return _.map(obj, function(value) {
  284.       var func = isFunc ? method : value[method];
  285.       return func == null ? func : func.apply(value, args);
  286.     });
  287.   });
  288.  
  289.   // Convenience version of a common use case of `map`: fetching a property.
  290.   _.pluck = function(obj, key) {
  291.     return _.map(obj, _.property(key));
  292.   };
  293.  
  294.   // Convenience version of a common use case of `filter`: selecting only objects
  295.   // containing specific `key:value` pairs.
  296.   _.where = function(obj, attrs) {
  297.     return _.filter(obj, _.matcher(attrs));
  298.   };
  299.  
  300.   // Convenience version of a common use case of `find`: getting the first object
  301.   // containing specific `key:value` pairs.
  302.   _.findWhere = function(obj, attrs) {
  303.     return _.find(obj, _.matcher(attrs));
  304.   };
  305.  
  306.   // Return the maximum element (or element-based computation).
  307.   _.max = function(obj, iteratee, context) {
  308.     var result = -Infinity, lastComputed = -Infinity,
  309.         value, computed;
  310.     if (iteratee == null || (typeof iteratee == 'number' && typeof obj[0] != 'object') && obj != null) {
  311.       obj = isArrayLike(obj) ? obj : _.values(obj);
  312.       for (var i = 0, length = obj.length; i < length; i++) {
  313.         value = obj[i];
  314.         if (value != null && value > result) {
  315.           result = value;
  316.         }
  317.       }
  318.     } else {
  319.       iteratee = cb(iteratee, context);
  320.       _.each(obj, function(v, index, list) {
  321.         computed = iteratee(v, index, list);
  322.         if (computed > lastComputed || computed === -Infinity && result === -Infinity) {
  323.           result = v;
  324.           lastComputed = computed;
  325.         }
  326.       });
  327.     }
  328.     return result;
  329.   };
  330.  
  331.   // Return the minimum element (or element-based computation).
  332.   _.min = function(obj, iteratee, context) {
  333.     var result = Infinity, lastComputed = Infinity,
  334.         value, computed;
  335.     if (iteratee == null || (typeof iteratee == 'number' && typeof obj[0] != 'object') && obj != null) {
  336.       obj = isArrayLike(obj) ? obj : _.values(obj);
  337.       for (var i = 0, length = obj.length; i < length; i++) {
  338.         value = obj[i];
  339.         if (value != null && value < result) {
  340.           result = value;
  341.         }
  342.       }
  343.     } else {
  344.       iteratee = cb(iteratee, context);
  345.       _.each(obj, function(v, index, list) {
  346.         computed = iteratee(v, index, list);
  347.         if (computed < lastComputed || computed === Infinity && result === Infinity) {
  348.           result = v;
  349.           lastComputed = computed;
  350.         }
  351.       });
  352.     }
  353.     return result;
  354.   };
  355.  
  356.   // Shuffle a collection.
  357.   _.shuffle = function(obj) {
  358.     return _.sample(obj, Infinity);
  359.   };
  360.  
  361.   // Sample **n** random values from a collection using the modern version of the
  362.   // [Fisher-Yates shuffle](http://en.wikipedia.org/wiki/Fisher–Yates_shuffle).
  363.   // If **n** is not specified, returns a single random element.
  364.   // The internal `guard` argument allows it to work with `map`.
  365.   _.sample = function(obj, n, guard) {
  366.     if (n == null || guard) {
  367.       if (!isArrayLike(obj)) obj = _.values(obj);
  368.       return obj[_.random(obj.length - 1)];
  369.     }
  370.     var sample = isArrayLike(obj) ? _.clone(obj) : _.values(obj);
  371.     var length = getLength(sample);
  372.     n = Math.max(Math.min(n, length), 0);
  373.     var last = length - 1;
  374.     for (var index = 0; index < n; index++) {
  375.       var rand = _.random(index, last);
  376.       var temp = sample[index];
  377.       sample[index] = sample[rand];
  378.       sample[rand] = temp;
  379.     }
  380.     return sample.slice(0, n);
  381.   };
  382.  
  383.   // Sort the object's values by a criterion produced by an iteratee.
  384.   _.sortBy = function(obj, iteratee, context) {
  385.     var index = 0;
  386.     iteratee = cb(iteratee, context);
  387.     return _.pluck(_.map(obj, function(value, key, list) {
  388.       return {
  389.         value: value,
  390.         index: index++,
  391.         criteria: iteratee(value, key, list)
  392.       };
  393.     }).sort(function(left, right) {
  394.       var a = left.criteria;
  395.       var b = right.criteria;
  396.       if (a !== b) {
  397.         if (a > b || a === void 0) return 1;
  398.         if (a < b || b === void 0) return -1;
  399.       }
  400.       return left.index - right.index;
  401.     }), 'value');
  402.   };
  403.  
  404.   // An internal function used for aggregate "group by" operations.
  405.   var group = function(behavior, partition) {
  406.     return function(obj, iteratee, context) {
  407.       var result = partition ? [[], []] : {};
  408.       iteratee = cb(iteratee, context);
  409.       _.each(obj, function(value, index) {
  410.         var key = iteratee(value, index, obj);
  411.         behavior(result, value, key);
  412.       });
  413.       return result;
  414.     };
  415.   };
  416.  
  417.   // Groups the object's values by a criterion. Pass either a string attribute
  418.   // to group by, or a function that returns the criterion.
  419.   _.groupBy = group(function(result, value, key) {
  420.     if (_.has(result, key)) result[key].push(value); else result[key] = [value];
  421.   });
  422.  
  423.   // Indexes the object's values by a criterion, similar to `groupBy`, but for
  424.   // when you know that your index values will be unique.
  425.   _.indexBy = group(function(result, value, key) {
  426.     result[key] = value;
  427.   });
  428.  
  429.   // Counts instances of an object that group by a certain criterion. Pass
  430.   // either a string attribute to count by, or a function that returns the
  431.   // criterion.
  432.   _.countBy = group(function(result, value, key) {
  433.     if (_.has(result, key)) result[key]++; else result[key] = 1;
  434.   });
  435.  
  436.   var reStrSymbol = /[^\ud800-\udfff]|[\ud800-\udbff][\udc00-\udfff]|[\ud800-\udfff]/g;
  437.   // Safely create a real, live array from anything iterable.
  438.   _.toArray = function(obj) {
  439.     if (!obj) return [];
  440.     if (_.isArray(obj)) return slice.call(obj);
  441.     if (_.isString(obj)) {
  442.       // Keep surrogate pair characters together
  443.       return obj.match(reStrSymbol);
  444.     }
  445.     if (isArrayLike(obj)) return _.map(obj);
  446.     return _.values(obj);
  447.   };
  448.  
  449.   // Return the number of elements in an object.
  450.   _.size = function(obj) {
  451.     if (obj == null) return 0;
  452.     return isArrayLike(obj) ? obj.length : _.keys(obj).length;
  453.   };
  454.  
  455.   // Split a collection into two arrays: one whose elements all satisfy the given
  456.   // predicate, and one whose elements all do not satisfy the predicate.
  457.   _.partition = group(function(result, value, pass) {
  458.     result[pass ? 0 : 1].push(value);
  459.   }, true);
  460.  
  461.   // Array Functions
  462.   // ---------------
  463.  
  464.   // Get the first element of an array. Passing **n** will return the first N
  465.   // values in the array. Aliased as `head` and `take`. The **guard** check
  466.   // allows it to work with `_.map`.
  467.   _.first = _.head = _.take = function(array, n, guard) {
  468.     if (array == null) return void 0;
  469.     if (n == null || guard) return array[0];
  470.     return _.initial(array, array.length - n);
  471.   };
  472.  
  473.   // Returns everything but the last entry of the array. Especially useful on
  474.   // the arguments object. Passing **n** will return all the values in
  475.   // the array, excluding the last N.
  476.   _.initial = function(array, n, guard) {
  477.     return slice.call(array, 0, Math.max(0, array.length - (n == null || guard ? 1 : n)));
  478.   };
  479.  
  480.   // Get the last element of an array. Passing **n** will return the last N
  481.   // values in the array.
  482.   _.last = function(array, n, guard) {
  483.     if (array == null) return void 0;
  484.     if (n == null || guard) return array[array.length - 1];
  485.     return _.rest(array, Math.max(0, array.length - n));
  486.   };
  487.  
  488.   // Returns everything but the first entry of the array. Aliased as `tail` and `drop`.
  489.   // Especially useful on the arguments object. Passing an **n** will return
  490.   // the rest N values in the array.
  491.   _.rest = _.tail = _.drop = function(array, n, guard) {
  492.     return slice.call(array, n == null || guard ? 1 : n);
  493.   };
  494.  
  495.   // Trim out all falsy values from an array.
  496.   _.compact = function(array) {
  497.     return _.filter(array);
  498.   };
  499.  
  500.   // Internal implementation of a recursive `flatten` function.
  501.   var flatten = function(input, shallow, strict, output) {
  502.     output = output || [];
  503.     var idx = output.length;
  504.     for (var i = 0, length = getLength(input); i < length; i++) {
  505.       var value = input[i];
  506.       if (isArrayLike(value) && (_.isArray(value) || _.isArguments(value))) {
  507.         // Flatten current level of array or arguments object.
  508.         if (shallow) {
  509.           var j = 0, len = value.length;
  510.           while (j < len) output[idx++] = value[j++];
  511.         } else {
  512.           flatten(value, shallow, strict, output);
  513.           idx = output.length;
  514.         }
  515.       } else if (!strict) {
  516.         output[idx++] = value;
  517.       }
  518.     }
  519.     return output;
  520.   };
  521.  
  522.   // Flatten out an array, either recursively (by default), or just one level.
  523.   _.flatten = function(array, shallow) {
  524.     return flatten(array, shallow, false);
  525.   };
  526.  
  527.   // Return a version of the array that does not contain the specified value(s).
  528.   _.without = restArgs(function(array, otherArrays) {
  529.     return _.difference(array, otherArrays);
  530.   });
  531.  
  532.   // Produce a duplicate-free version of the array. If the array has already
  533.   // been sorted, you have the option of using a faster algorithm.
  534.   // Aliased as `unique`.
  535.   _.uniq = _.unique = function(array, isSorted, iteratee, context) {
  536.     if (!_.isBoolean(isSorted)) {
  537.       context = iteratee;
  538.       iteratee = isSorted;
  539.       isSorted = false;
  540.     }
  541.     if (iteratee != null) iteratee = cb(iteratee, context);
  542.     var result = [];
  543.     var seen = [];
  544.     for (var i = 0, length = getLength(array); i < length; i++) {
  545.       var value = array[i],
  546.           computed = iteratee ? iteratee(value, i, array) : value;
  547.       if (isSorted) {
  548.         if (!i || seen !== computed) result.push(value);
  549.         seen = computed;
  550.       } else if (iteratee) {
  551.         if (!_.contains(seen, computed)) {
  552.           seen.push(computed);
  553.           result.push(value);
  554.         }
  555.       } else if (!_.contains(result, value)) {
  556.         result.push(value);
  557.       }
  558.     }
  559.     return result;
  560.   };
  561.  
  562.   // Produce an array that contains the union: each distinct element from all of
  563.   // the passed-in arrays.
  564.   _.union = restArgs(function(arrays) {
  565.     return _.uniq(flatten(arrays, true, true));
  566.   });
  567.  
  568.   // Produce an array that contains every item shared between all the
  569.   // passed-in arrays.
  570.   _.intersection = function(array) {
  571.     var result = [];
  572.     var argsLength = arguments.length;
  573.     for (var i = 0, length = getLength(array); i < length; i++) {
  574.       var item = array[i];
  575.       if (_.contains(result, item)) continue;
  576.       var j;
  577.       for (j = 1; j < argsLength; j++) {
  578.         if (!_.contains(arguments[j], item)) break;
  579.       }
  580.       if (j === argsLength) result.push(item);
  581.     }
  582.     return result;
  583.   };
  584.  
  585.   // Take the difference between one array and a number of other arrays.
  586.   // Only the elements present in just the first array will remain.
  587.   _.difference = restArgs(function(array, rest) {
  588.     rest = flatten(rest, true, true);
  589.     return _.filter(array, function(value){
  590.       return !_.contains(rest, value);
  591.     });
  592.   });
  593.  
  594.   // Complement of _.zip. Unzip accepts an array of arrays and groups
  595.   // each array's elements on shared indices.
  596.   _.unzip = function(array) {
  597.     var length = array && _.max(array, getLength).length || 0;
  598.     var result = Array(length);
  599.  
  600.     for (var index = 0; index < length; index++) {
  601.       result[index] = _.pluck(array, index);
  602.     }
  603.     return result;
  604.   };
  605.  
  606.   // Zip together multiple lists into a single array -- elements that share
  607.   // an index go together.
  608.   _.zip = restArgs(_.unzip);
  609.  
  610.   // Converts lists into objects. Pass either a single array of `[key, value]`
  611.   // pairs, or two parallel arrays of the same length -- one of keys, and one of
  612.   // the corresponding values.
  613.   _.object = function(list, values) {
  614.     var result = {};
  615.     for (var i = 0, length = getLength(list); i < length; i++) {
  616.       if (values) {
  617.         result[list[i]] = values[i];
  618.       } else {
  619.         result[list[i][0]] = list[i][1];
  620.       }
  621.     }
  622.     return result;
  623.   };
  624.  
  625.   // Generator function to create the findIndex and findLastIndex functions.
  626.   var createPredicateIndexFinder = function(dir) {
  627.     return function(array, predicate, context) {
  628.       predicate = cb(predicate, context);
  629.       var length = getLength(array);
  630.       var index = dir > 0 ? 0 : length - 1;
  631.       for (; index >= 0 && index < length; index += dir) {
  632.         if (predicate(array[index], index, array)) return index;
  633.       }
  634.       return -1;
  635.     };
  636.   };
  637.  
  638.   // Returns the first index on an array-like that passes a predicate test.
  639.   _.findIndex = createPredicateIndexFinder(1);
  640.   _.findLastIndex = createPredicateIndexFinder(-1);
  641.  
  642.   // Use a comparator function to figure out the smallest index at which
  643.   // an object should be inserted so as to maintain order. Uses binary search.
  644.   _.sortedIndex = function(array, obj, iteratee, context) {
  645.     iteratee = cb(iteratee, context, 1);
  646.     var value = iteratee(obj);
  647.     var low = 0, high = getLength(array);
  648.     while (low < high) {
  649.       var mid = Math.floor((low + high) / 2);
  650.       if (iteratee(array[mid]) < value) low = mid + 1; else high = mid;
  651.     }
  652.     return low;
  653.   };
  654.  
  655.   // Generator function to create the indexOf and lastIndexOf functions.
  656.   var createIndexFinder = function(dir, predicateFind, sortedIndex) {
  657.     return function(array, item, idx) {
  658.       var i = 0, length = getLength(array);
  659.       if (typeof idx == 'number') {
  660.         if (dir > 0) {
  661.           i = idx >= 0 ? idx : Math.max(idx + length, i);
  662.         } else {
  663.           length = idx >= 0 ? Math.min(idx + 1, length) : idx + length + 1;
  664.         }
  665.       } else if (sortedIndex && idx && length) {
  666.         idx = sortedIndex(array, item);
  667.         return array[idx] === item ? idx : -1;
  668.       }
  669.       if (item !== item) {
  670.         idx = predicateFind(slice.call(array, i, length), _.isNaN);
  671.         return idx >= 0 ? idx + i : -1;
  672.       }
  673.       for (idx = dir > 0 ? i : length - 1; idx >= 0 && idx < length; idx += dir) {
  674.         if (array[idx] === item) return idx;
  675.       }
  676.       return -1;
  677.     };
  678.   };
  679.  
  680.   // Return the position of the first occurrence of an item in an array,
  681.   // or -1 if the item is not included in the array.
  682.   // If the array is large and already in sort order, pass `true`
  683.   // for **isSorted** to use binary search.
  684.   _.indexOf = createIndexFinder(1, _.findIndex, _.sortedIndex);
  685.   _.lastIndexOf = createIndexFinder(-1, _.findLastIndex);
  686.  
  687.   // Generate an integer Array containing an arithmetic progression. A port of
  688.   // the native Python `range()` function. See
  689.   // [the Python documentation](http://docs.python.org/library/functions.html#range).
  690.   _.range = function(start, stop, step) {
  691.     if (stop == null) {
  692.       stop = start || 0;
  693.       start = 0;
  694.     }
  695.     if (!step) {
  696.       step = stop < start ? -1 : 1;
  697.     }
  698.  
  699.     var length = Math.max(Math.ceil((stop - start) / step), 0);
  700.     var range = Array(length);
  701.  
  702.     for (var idx = 0; idx < length; idx++, start += step) {
  703.       range[idx] = start;
  704.     }
  705.  
  706.     return range;
  707.   };
  708.  
  709.   // Split an **array** into several arrays containing **count** or less elements
  710.   // of initial array.
  711.   _.chunk = function(array, count) {
  712.     if (count == null || count < 1) return [];
  713.  
  714.     var result = [];
  715.     var i = 0, length = array.length;
  716.     while (i < length) {
  717.       result.push(slice.call(array, i, i += count));
  718.     }
  719.     return result;
  720.   };
  721.  
  722.   // Function (ahem) Functions
  723.   // ------------------
  724.  
  725.   // Determines whether to execute a function as a constructor
  726.   // or a normal function with the provided arguments.
  727.   var executeBound = function(sourceFunc, boundFunc, context, callingContext, args) {
  728.     if (!(callingContext instanceof boundFunc)) return sourceFunc.apply(context, args);
  729.     var self = baseCreate(sourceFunc.prototype);
  730.     var result = sourceFunc.apply(self, args);
  731.     if (_.isObject(result)) return result;
  732.     return self;
  733.   };
  734.  
  735.   // Create a function bound to a given object (assigning `this`, and arguments,
  736.   // optionally). Delegates to **ECMAScript 5**'s native `Function.bind` if
  737.   // available.
  738.   _.bind = restArgs(function(func, context, args) {
  739.     if (!_.isFunction(func)) throw new TypeError('Bind must be called on a function');
  740.     var bound = restArgs(function(callArgs) {
  741.       return executeBound(func, bound, context, this, args.concat(callArgs));
  742.     });
  743.     return bound;
  744.   });
  745.  
  746.   // Partially apply a function by creating a version that has had some of its
  747.   // arguments pre-filled, without changing its dynamic `this` context. _ acts
  748.   // as a placeholder by default, allowing any combination of arguments to be
  749.   // pre-filled. Set `_.partial.placeholder` for a custom placeholder argument.
  750.   _.partial = restArgs(function(func, boundArgs) {
  751.     var placeholder = _.partial.placeholder;
  752.     var bound = function() {
  753.       var position = 0, length = boundArgs.length;
  754.       var args = Array(length);
  755.       for (var i = 0; i < length; i++) {
  756.         args[i] = boundArgs[i] === placeholder ? arguments[position++] : boundArgs[i];
  757.       }
  758.       while (position < arguments.length) args.push(arguments[position++]);
  759.       return executeBound(func, bound, this, this, args);
  760.     };
  761.     return bound;
  762.   });
  763.  
  764.   _.partial.placeholder = _;
  765.  
  766.   // Bind a number of an object's methods to that object. Remaining arguments
  767.   // are the method names to be bound. Useful for ensuring that all callbacks
  768.   // defined on an object belong to it.
  769.   _.bindAll = restArgs(function(obj, keys) {
  770.     keys = flatten(keys, false, false);
  771.     var index = keys.length;
  772.     if (index < 1) throw new Error('bindAll must be passed function names');
  773.     while (index--) {
  774.       var key = keys[index];
  775.       obj[key] = _.bind(obj[key], obj);
  776.     }
  777.   });
  778.  
  779.   // Memoize an expensive function by storing its results.
  780.   _.memoize = function(func, hasher) {
  781.     var memoize = function(key) {
  782.       var cache = memoize.cache;
  783.       var address = '' + (hasher ? hasher.apply(this, arguments) : key);
  784.       if (!_.has(cache, address)) cache[address] = func.apply(this, arguments);
  785.       return cache[address];
  786.     };
  787.     memoize.cache = {};
  788.     return memoize;
  789.   };
  790.  
  791.   // Delays a function for the given number of milliseconds, and then calls
  792.   // it with the arguments supplied.
  793.   _.delay = restArgs(function(func, wait, args) {
  794.     return setTimeout(function() {
  795.       return func.apply(null, args);
  796.     }, wait);
  797.   });
  798.  
  799.   // Defers a function, scheduling it to run after the current call stack has
  800.   // cleared.
  801.   _.defer = _.partial(_.delay, _, 1);
  802.  
  803.   // Returns a function, that, when invoked, will only be triggered at most once
  804.   // during a given window of time. Normally, the throttled function will run
  805.   // as much as it can, without ever going more than once per `wait` duration;
  806.   // but if you'd like to disable the execution on the leading edge, pass
  807.   // `{leading: false}`. To disable execution on the trailing edge, ditto.
  808.   _.throttle = function(func, wait, options) {
  809.     var timeout, context, args, result;
  810.     var previous = 0;
  811.     if (!options) options = {};
  812.  
  813.     var later = function() {
  814.       previous = options.leading === false ? 0 : _.now();
  815.       timeout = null;
  816.       result = func.apply(context, args);
  817.       if (!timeout) context = args = null;
  818.     };
  819.  
  820.     var throttled = function() {
  821.       var now = _.now();
  822.       if (!previous && options.leading === false) previous = now;
  823.       var remaining = wait - (now - previous);
  824.       context = this;
  825.       args = arguments;
  826.       if (remaining <= 0 || remaining > wait) {
  827.         if (timeout) {
  828.           clearTimeout(timeout);
  829.           timeout = null;
  830.         }
  831.         previous = now;
  832.         result = func.apply(context, args);
  833.         if (!timeout) context = args = null;
  834.       } else if (!timeout && options.trailing !== false) {
  835.         timeout = setTimeout(later, remaining);
  836.       }
  837.       return result;
  838.     };
  839.  
  840.     throttled.cancel = function() {
  841.       clearTimeout(timeout);
  842.       previous = 0;
  843.       timeout = context = args = null;
  844.     };
  845.  
  846.     return throttled;
  847.   };
  848.  
  849.   // Returns a function, that, as long as it continues to be invoked, will not
  850.   // be triggered. The function will be called after it stops being called for
  851.   // N milliseconds. If `immediate` is passed, trigger the function on the
  852.   // leading edge, instead of the trailing.
  853.   _.debounce = function(func, wait, immediate) {
  854.     var timeout, result;
  855.  
  856.     var later = function(context, args) {
  857.       timeout = null;
  858.       if (args) result = func.apply(context, args);
  859.     };
  860.  
  861.     var debounced = restArgs(function(args) {
  862.       var callNow = immediate && !timeout;
  863.       if (timeout) clearTimeout(timeout);
  864.       if (callNow) {
  865.         timeout = setTimeout(later, wait);
  866.         result = func.apply(this, args);
  867.       } else if (!immediate) {
  868.         timeout = _.delay(later, wait, this, args);
  869.       }
  870.  
  871.       return result;
  872.     });
  873.  
  874.     debounced.cancel = function() {
  875.       clearTimeout(timeout);
  876.       timeout = null;
  877.     };
  878.  
  879.     return debounced;
  880.   };
  881.  
  882.   // Returns the first function passed as an argument to the second,
  883.   // allowing you to adjust arguments, run code before and after, and
  884.   // conditionally execute the original function.
  885.   _.wrap = function(func, wrapper) {
  886.     return _.partial(wrapper, func);
  887.   };
  888.  
  889.   // Returns a negated version of the passed-in predicate.
  890.   _.negate = function(predicate) {
  891.     return function() {
  892.       return !predicate.apply(this, arguments);
  893.     };
  894.   };
  895.  
  896.   // Returns a function that is the composition of a list of functions, each
  897.   // consuming the return value of the function that follows.
  898.   _.compose = function() {
  899.     var args = arguments;
  900.     var start = args.length - 1;
  901.     return function() {
  902.       var i = start;
  903.       var result = args[start].apply(this, arguments);
  904.       while (i--) result = args[i].call(this, result);
  905.       return result;
  906.     };
  907.   };
  908.  
  909.   // Returns a function that will only be executed on and after the Nth call.
  910.   _.after = function(times, func) {
  911.     return function() {
  912.       if (--times < 1) {
  913.         return func.apply(this, arguments);
  914.       }
  915.     };
  916.   };
  917.  
  918.   // Returns a function that will only be executed up to (but not including) the Nth call.
  919.   _.before = function(times, func) {
  920.     var memo;
  921.     return function() {
  922.       if (--times > 0) {
  923.         memo = func.apply(this, arguments);
  924.       }
  925.       if (times <= 1) func = null;
  926.       return memo;
  927.     };
  928.   };
  929.  
  930.   // Returns a function that will be executed at most one time, no matter how
  931.   // often you call it. Useful for lazy initialization.
  932.   _.once = _.partial(_.before, 2);
  933.  
  934.   _.restArgs = restArgs;
  935.  
  936.   // Object Functions
  937.   // ----------------
  938.  
  939.   // Keys in IE < 9 that won't be iterated by `for key in ...` and thus missed.
  940.   var hasEnumBug = !{toString: null}.propertyIsEnumerable('toString');
  941.   var nonEnumerableProps = ['valueOf', 'isPrototypeOf', 'toString',
  942.                       'propertyIsEnumerable', 'hasOwnProperty', 'toLocaleString'];
  943.  
  944.   var collectNonEnumProps = function(obj, keys) {
  945.     var nonEnumIdx = nonEnumerableProps.length;
  946.     var constructor = obj.constructor;
  947.     var proto = _.isFunction(constructor) && constructor.prototype || ObjProto;
  948.  
  949.     // Constructor is a special case.
  950.     var prop = 'constructor';
  951.     if (_.has(obj, prop) && !_.contains(keys, prop)) keys.push(prop);
  952.  
  953.     while (nonEnumIdx--) {
  954.       prop = nonEnumerableProps[nonEnumIdx];
  955.       if (prop in obj && obj[prop] !== proto[prop] && !_.contains(keys, prop)) {
  956.         keys.push(prop);
  957.       }
  958.     }
  959.   };
  960.  
  961.   // Retrieve the names of an object's own properties.
  962.   // Delegates to **ECMAScript 5**'s native `Object.keys`.
  963.   _.keys = function(obj) {
  964.     if (!_.isObject(obj)) return [];
  965.     if (nativeKeys) return nativeKeys(obj);
  966.     var keys = [];
  967.     for (var key in obj) if (_.has(obj, key)) keys.push(key);
  968.     // Ahem, IE < 9.
  969.     if (hasEnumBug) collectNonEnumProps(obj, keys);
  970.     return keys;
  971.   };
  972.  
  973.   // Retrieve all the property names of an object.
  974.   _.allKeys = function(obj) {
  975.     if (!_.isObject(obj)) return [];
  976.     var keys = [];
  977.     for (var key in obj) keys.push(key);
  978.     // Ahem, IE < 9.
  979.     if (hasEnumBug) collectNonEnumProps(obj, keys);
  980.     return keys;
  981.   };
  982.  
  983.   // Retrieve the values of an object's properties.
  984.   _.values = function(obj) {
  985.     var keys = _.keys(obj);
  986.     var length = keys.length;
  987.     var values = Array(length);
  988.     for (var i = 0; i < length; i++) {
  989.       values[i] = obj[keys[i]];
  990.     }
  991.     return values;
  992.   };
  993.  
  994.   // Returns the results of applying the iteratee to each element of the object.
  995.   // In contrast to _.map it returns an object.
  996.   _.mapObject = function(obj, iteratee, context) {
  997.     iteratee = cb(iteratee, context);
  998.     var keys = _.keys(obj),
  999.         length = keys.length,
  1000.         results = {};
  1001.     for (var index = 0; index < length; index++) {
  1002.       var currentKey = keys[index];
  1003.       results[currentKey] = iteratee(obj[currentKey], currentKey, obj);
  1004.     }
  1005.     return results;
  1006.   };
  1007.  
  1008.   // Convert an object into a list of `[key, value]` pairs.
  1009.   _.pairs = function(obj) {
  1010.     var keys = _.keys(obj);
  1011.     var length = keys.length;
  1012.     var pairs = Array(length);
  1013.     for (var i = 0; i < length; i++) {
  1014.       pairs[i] = [keys[i], obj[keys[i]]];
  1015.     }
  1016.     return pairs;
  1017.   };
  1018.  
  1019.   // Invert the keys and values of an object. The values must be serializable.
  1020.   _.invert = function(obj) {
  1021.     var result = {};
  1022.     var keys = _.keys(obj);
  1023.     for (var i = 0, length = keys.length; i < length; i++) {
  1024.       result[obj[keys[i]]] = keys[i];
  1025.     }
  1026.     return result;
  1027.   };
  1028.  
  1029.   // Return a sorted list of the function names available on the object.
  1030.   // Aliased as `methods`.
  1031.   _.functions = _.methods = function(obj) {
  1032.     var names = [];
  1033.     for (var key in obj) {
  1034.       if (_.isFunction(obj[key])) names.push(key);
  1035.     }
  1036.     return names.sort();
  1037.   };
  1038.  
  1039.   // An internal function for creating assigner functions.
  1040.   var createAssigner = function(keysFunc, defaults) {
  1041.     return function(obj) {
  1042.       var length = arguments.length;
  1043.       if (defaults) obj = Object(obj);
  1044.       if (length < 2 || obj == null) return obj;
  1045.       for (var index = 1; index < length; index++) {
  1046.         var source = arguments[index],
  1047.             keys = keysFunc(source),
  1048.             l = keys.length;
  1049.         for (var i = 0; i < l; i++) {
  1050.           var key = keys[i];
  1051.           if (!defaults || obj[key] === void 0) obj[key] = source[key];
  1052.         }
  1053.       }
  1054.       return obj;
  1055.     };
  1056.   };
  1057.  
  1058.   // Extend a given object with all the properties in passed-in object(s).
  1059.   _.extend = createAssigner(_.allKeys);
  1060.  
  1061.   // Assigns a given object with all the own properties in the passed-in object(s).
  1062.   // (https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Object/assign)
  1063.   _.extendOwn = _.assign = createAssigner(_.keys);
  1064.  
  1065.   // Returns the first key on an object that passes a predicate test.
  1066.   _.findKey = function(obj, predicate, context) {
  1067.     predicate = cb(predicate, context);
  1068.     var keys = _.keys(obj), key;
  1069.     for (var i = 0, length = keys.length; i < length; i++) {
  1070.       key = keys[i];
  1071.       if (predicate(obj[key], key, obj)) return key;
  1072.     }
  1073.   };
  1074.  
  1075.   // Internal pick helper function to determine if `obj` has key `key`.
  1076.   var keyInObj = function(value, key, obj) {
  1077.     return key in obj;
  1078.   };
  1079.  
  1080.   // Return a copy of the object only containing the whitelisted properties.
  1081.   _.pick = restArgs(function(obj, keys) {
  1082.     var result = {}, iteratee = keys[0];
  1083.     if (obj == null) return result;
  1084.     if (_.isFunction(iteratee)) {
  1085.       if (keys.length > 1) iteratee = optimizeCb(iteratee, keys[1]);
  1086.       keys = _.allKeys(obj);
  1087.     } else {
  1088.       iteratee = keyInObj;
  1089.       keys = flatten(keys, false, false);
  1090.       obj = Object(obj);
  1091.     }
  1092.     for (var i = 0, length = keys.length; i < length; i++) {
  1093.       var key = keys[i];
  1094.       var value = obj[key];
  1095.       if (iteratee(value, key, obj)) result[key] = value;
  1096.     }
  1097.     return result;
  1098.   });
  1099.  
  1100.    // Return a copy of the object without the blacklisted properties.
  1101.   _.omit = restArgs(function(obj, keys) {
  1102.     var iteratee = keys[0], context;
  1103.     if (_.isFunction(iteratee)) {
  1104.       iteratee = _.negate(iteratee);
  1105.       if (keys.length > 1) context = keys[1];
  1106.     } else {
  1107.       keys = _.map(flatten(keys, false, false), String);
  1108.       iteratee = function(value, key) {
  1109.         return !_.contains(keys, key);
  1110.       };
  1111.     }
  1112.     return _.pick(obj, iteratee, context);
  1113.   });
  1114.  
  1115.   // Fill in a given object with default properties.
  1116.   _.defaults = createAssigner(_.allKeys, true);
  1117.  
  1118.   // Creates an object that inherits from the given prototype object.
  1119.   // If additional properties are provided then they will be added to the
  1120.   // created object.
  1121.   _.create = function(prototype, props) {
  1122.     var result = baseCreate(prototype);
  1123.     if (props) _.extendOwn(result, props);
  1124.     return result;
  1125.   };
  1126.  
  1127.   // Create a (shallow-cloned) duplicate of an object.
  1128.   _.clone = function(obj) {
  1129.     if (!_.isObject(obj)) return obj;
  1130.     return _.isArray(obj) ? obj.slice() : _.extend({}, obj);
  1131.   };
  1132.  
  1133.   // Invokes interceptor with the obj, and then returns obj.
  1134.   // The primary purpose of this method is to "tap into" a method chain, in
  1135.   // order to perform operations on intermediate results within the chain.
  1136.   _.tap = function(obj, interceptor) {
  1137.     interceptor(obj);
  1138.     return obj;
  1139.   };
  1140.  
  1141.   // Returns whether an object has a given set of `key:value` pairs.
  1142.   _.isMatch = function(object, attrs) {
  1143.     var keys = _.keys(attrs), length = keys.length;
  1144.     if (object == null) return !length;
  1145.     var obj = Object(object);
  1146.     for (var i = 0; i < length; i++) {
  1147.       var key = keys[i];
  1148.       if (attrs[key] !== obj[key] || !(key in obj)) return false;
  1149.     }
  1150.     return true;
  1151.   };
  1152.  
  1153.  
  1154.   // Internal recursive comparison function for `isEqual`.
  1155.   var eq, deepEq;
  1156.   eq = function(a, b, aStack, bStack) {
  1157.     // Identical objects are equal. `0 === -0`, but they aren't identical.
  1158.     // See the [Harmony `egal` proposal](http://wiki.ecmascript.org/doku.php?id=harmony:egal).
  1159.     if (a === b) return a !== 0 || 1 / a === 1 / b;
  1160.     // A strict comparison is necessary because `null == undefined`.
  1161.     if (a == null || b == null) return a === b;
  1162.     // `NaN`s are equivalent, but non-reflexive.
  1163.     if (a !== a) return b !== b;
  1164.     // Exhaust primitive checks
  1165.     var type = typeof a;
  1166.     if (type !== 'function' && type !== 'object' && typeof b != 'object') return false;
  1167.     return deepEq(a, b, aStack, bStack);
  1168.   };
  1169.  
  1170.   // Internal recursive comparison function for `isEqual`.
  1171.   deepEq = function(a, b, aStack, bStack) {
  1172.     // Unwrap any wrapped objects.
  1173.     if (a instanceof _) a = a._wrapped;
  1174.     if (b instanceof _) b = b._wrapped;
  1175.     // Compare `[[Class]]` names.
  1176.     var className = toString.call(a);
  1177.     if (className !== toString.call(b)) return false;
  1178.     switch (className) {
  1179.       // Strings, numbers, regular expressions, dates, and booleans are compared by value.
  1180.       case '[object RegExp]':
  1181.       // RegExps are coerced to strings for comparison (Note: '' + /a/i === '/a/i')
  1182.       case '[object String]':
  1183.         // Primitives and their corresponding object wrappers are equivalent; thus, `"5"` is
  1184.         // equivalent to `new String("5")`.
  1185.         return '' + a === '' + b;
  1186.       case '[object Number]':
  1187.         // `NaN`s are equivalent, but non-reflexive.
  1188.         // Object(NaN) is equivalent to NaN.
  1189.         if (+a !== +a) return +b !== +b;
  1190.         // An `egal` comparison is performed for other numeric values.
  1191.         return +a === 0 ? 1 / +a === 1 / b : +a === +b;
  1192.       case '[object Date]':
  1193.       case '[object Boolean]':
  1194.         // Coerce dates and booleans to numeric primitive values. Dates are compared by their
  1195.         // millisecond representations. Note that invalid dates with millisecond representations
  1196.         // of `NaN` are not equivalent.
  1197.         return +a === +b;
  1198.     }
  1199.  
  1200.     var areArrays = className === '[object Array]';
  1201.     if (!areArrays) {
  1202.       if (typeof a != 'object' || typeof b != 'object') return false;
  1203.  
  1204.       // Objects with different constructors are not equivalent, but `Object`s or `Array`s
  1205.       // from different frames are.
  1206.       var aCtor = a.constructor, bCtor = b.constructor;
  1207.       if (aCtor !== bCtor && !(_.isFunction(aCtor) && aCtor instanceof aCtor &&
  1208.                                _.isFunction(bCtor) && bCtor instanceof bCtor)
  1209.                           && ('constructor' in a && 'constructor' in b)) {
  1210.         return false;
  1211.       }
  1212.     }
  1213.     // Assume equality for cyclic structures. The algorithm for detecting cyclic
  1214.     // structures is adapted from ES 5.1 section 15.12.3, abstract operation `JO`.
  1215.  
  1216.     // Initializing stack of traversed objects.
  1217.     // It's done here since we only need them for objects and arrays comparison.
  1218.     aStack = aStack || [];
  1219.     bStack = bStack || [];
  1220.     var length = aStack.length;
  1221.     while (length--) {
  1222.       // Linear search. Performance is inversely proportional to the number of
  1223.       // unique nested structures.
  1224.       if (aStack[length] === a) return bStack[length] === b;
  1225.     }
  1226.  
  1227.     // Add the first object to the stack of traversed objects.
  1228.     aStack.push(a);
  1229.     bStack.push(b);
  1230.  
  1231.     // Recursively compare objects and arrays.
  1232.     if (areArrays) {
  1233.       // Compare array lengths to determine if a deep comparison is necessary.
  1234.       length = a.length;
  1235.       if (length !== b.length) return false;
  1236.       // Deep compare the contents, ignoring non-numeric properties.
  1237.       while (length--) {
  1238.         if (!eq(a[length], b[length], aStack, bStack)) return false;
  1239.       }
  1240.     } else {
  1241.       // Deep compare objects.
  1242.       var keys = _.keys(a), key;
  1243.       length = keys.length;
  1244.       // Ensure that both objects contain the same number of properties before comparing deep equality.
  1245.       if (_.keys(b).length !== length) return false;
  1246.       while (length--) {
  1247.         // Deep compare each member
  1248.         key = keys[length];
  1249.         if (!(_.has(b, key) && eq(a[key], b[key], aStack, bStack))) return false;
  1250.       }
  1251.     }
  1252.     // Remove the first object from the stack of traversed objects.
  1253.     aStack.pop();
  1254.     bStack.pop();
  1255.     return true;
  1256.   };
  1257.  
  1258.   // Perform a deep comparison to check if two objects are equal.
  1259.   _.isEqual = function(a, b) {
  1260.     return eq(a, b);
  1261.   };
  1262.  
  1263.   // Is a given array, string, or object empty?
  1264.   // An "empty" object has no enumerable own-properties.
  1265.   _.isEmpty = function(obj) {
  1266.     if (obj == null) return true;
  1267.     if (isArrayLike(obj) && (_.isArray(obj) || _.isString(obj) || _.isArguments(obj))) return obj.length === 0;
  1268.     return _.keys(obj).length === 0;
  1269.   };
  1270.  
  1271.   // Is a given value a DOM element?
  1272.   _.isElement = function(obj) {
  1273.     return !!(obj && obj.nodeType === 1);
  1274.   };
  1275.  
  1276.   // Is a given value an array?
  1277.   // Delegates to ECMA5's native Array.isArray
  1278.   _.isArray = nativeIsArray || function(obj) {
  1279.     return toString.call(obj) === '[object Array]';
  1280.   };
  1281.  
  1282.   // Is a given variable an object?
  1283.   _.isObject = function(obj) {
  1284.     var type = typeof obj;
  1285.     return type === 'function' || type === 'object' && !!obj;
  1286.   };
  1287.  
  1288.   // Add some isType methods: isArguments, isFunction, isString, isNumber, isDate, isRegExp, isError.
  1289.   _.each(['Arguments', 'Function', 'String', 'Number', 'Date', 'RegExp', 'Error', 'Symbol'], function(name) {
  1290.     _['is' + name] = function(obj) {
  1291.       return toString.call(obj) === '[object ' + name + ']';
  1292.     };
  1293.   });
  1294.  
  1295.   // Define a fallback version of the method in browsers (ahem, IE < 9), where
  1296.   // there isn't any inspectable "Arguments" type.
  1297.   if (!_.isArguments(arguments)) {
  1298.     _.isArguments = function(obj) {
  1299.       return _.has(obj, 'callee');
  1300.     };
  1301.   }
  1302.  
  1303.   // Optimize `isFunction` if appropriate. Work around some typeof bugs in old v8,
  1304.   // IE 11 (#1621), Safari 8 (#1929), and PhantomJS (#2236).
  1305.   var nodelist = root.document && root.document.childNodes;
  1306.   if (typeof /./ != 'function' && typeof Int8Array != 'object' && typeof nodelist != 'function') {
  1307.     _.isFunction = function(obj) {
  1308.       return typeof obj == 'function' || false;
  1309.     };
  1310.   }
  1311.  
  1312.   // Is a given object a finite number?
  1313.   _.isFinite = function(obj) {
  1314.     return !_.isSymbol(obj) && isFinite(obj) && !isNaN(parseFloat(obj));
  1315.   };
  1316.  
  1317.   // Is the given value `NaN`?
  1318.   _.isNaN = function(obj) {
  1319.     return _.isNumber(obj) && isNaN(obj);
  1320.   };
  1321.  
  1322.   // Is a given value a boolean?
  1323.   _.isBoolean = function(obj) {
  1324.     return obj === true || obj === false || toString.call(obj) === '[object Boolean]';
  1325.   };
  1326.  
  1327.   // Is a given value equal to null?
  1328.   _.isNull = function(obj) {
  1329.     return obj === null;
  1330.   };
  1331.  
  1332.   // Is a given variable undefined?
  1333.   _.isUndefined = function(obj) {
  1334.     return obj === void 0;
  1335.   };
  1336.  
  1337.   // Shortcut function for checking if an object has a given property directly
  1338.   // on itself (in other words, not on a prototype).
  1339.   _.has = function(obj, key) {
  1340.     return obj != null && hasOwnProperty.call(obj, key);
  1341.   };
  1342.  
  1343.   // Utility Functions
  1344.   // -----------------
  1345.  
  1346.   // Run Underscore.js in *noConflict* mode, returning the `_` variable to its
  1347.   // previous owner. Returns a reference to the Underscore object.
  1348.   _.noConflict = function() {
  1349.     root._ = previousUnderscore;
  1350.     return this;
  1351.   };
  1352.  
  1353.   // Keep the identity function around for default iteratees.
  1354.   _.identity = function(value) {
  1355.     return value;
  1356.   };
  1357.  
  1358.   // Predicate-generating functions. Often useful outside of Underscore.
  1359.   _.constant = function(value) {
  1360.     return function() {
  1361.       return value;
  1362.     };
  1363.   };
  1364.  
  1365.   _.noop = function(){};
  1366.  
  1367.   _.property = property;
  1368.  
  1369.   // Generates a function for a given object that returns a given property.
  1370.   _.propertyOf = function(obj) {
  1371.     return obj == null ? function(){} : function(key) {
  1372.       return obj[key];
  1373.     };
  1374.   };
  1375.  
  1376.   // Returns a predicate for checking whether an object has a given set of
  1377.   // `key:value` pairs.
  1378.   _.matcher = _.matches = function(attrs) {
  1379.     attrs = _.extendOwn({}, attrs);
  1380.     return function(obj) {
  1381.       return _.isMatch(obj, attrs);
  1382.     };
  1383.   };
  1384.  
  1385.   // Run a function **n** times.
  1386.   _.times = function(n, iteratee, context) {
  1387.     var accum = Array(Math.max(0, n));
  1388.     iteratee = optimizeCb(iteratee, context, 1);
  1389.     for (var i = 0; i < n; i++) accum[i] = iteratee(i);
  1390.     return accum;
  1391.   };
  1392.  
  1393.   // Return a random integer between min and max (inclusive).
  1394.   _.random = function(min, max) {
  1395.     if (max == null) {
  1396.       max = min;
  1397.       min = 0;
  1398.     }
  1399.     return min + Math.floor(Math.random() * (max - min + 1));
  1400.   };
  1401.  
  1402.   // A (possibly faster) way to get the current timestamp as an integer.
  1403.   _.now = Date.now || function() {
  1404.     return new Date().getTime();
  1405.   };
  1406.  
  1407.    // List of HTML entities for escaping.
  1408.   var escapeMap = {
  1409.     '&': '&amp;',
  1410.     '<': '&lt;',
  1411.     '>': '&gt;',
  1412.     '"': '&quot;',
  1413.     "'": '&#x27;',
  1414.     '`': '&#x60;'
  1415.   };
  1416.   var unescapeMap = _.invert(escapeMap);
  1417.  
  1418.   // Functions for escaping and unescaping strings to/from HTML interpolation.
  1419.   var createEscaper = function(map) {
  1420.     var escaper = function(match) {
  1421.       return map[match];
  1422.     };
  1423.     // Regexes for identifying a key that needs to be escaped.
  1424.     var source = '(?:' + _.keys(map).join('|') + ')';
  1425.     var testRegexp = RegExp(source);
  1426.     var replaceRegexp = RegExp(source, 'g');
  1427.     return function(string) {
  1428.       string = string == null ? '' : '' + string;
  1429.       return testRegexp.test(string) ? string.replace(replaceRegexp, escaper) : string;
  1430.     };
  1431.   };
  1432.   _.escape = createEscaper(escapeMap);
  1433.   _.unescape = createEscaper(unescapeMap);
  1434.  
  1435.   // If the value of the named `property` is a function then invoke it with the
  1436.   // `object` as context; otherwise, return it.
  1437.   _.result = function(object, prop, fallback) {
  1438.     var value = object == null ? void 0 : object[prop];
  1439.     if (value === void 0) {
  1440.       value = fallback;
  1441.     }
  1442.     return _.isFunction(value) ? value.call(object) : value;
  1443.   };
  1444.  
  1445.   // Generate a unique integer id (unique within the entire client session).
  1446.   // Useful for temporary DOM ids.
  1447.   var idCounter = 0;
  1448.   _.uniqueId = function(prefix) {
  1449.     var id = ++idCounter + '';
  1450.     return prefix ? prefix + id : id;
  1451.   };
  1452.  
  1453.   // By default, Underscore uses ERB-style template delimiters, change the
  1454.   // following template settings to use alternative delimiters.
  1455.   _.templateSettings = {
  1456.     evaluate: /<%([\s\S]+?)%>/g,
  1457.     interpolate: /<%=([\s\S]+?)%>/g,
  1458.     escape: /<%-([\s\S]+?)%>/g
  1459.   };
  1460.  
  1461.   // When customizing `templateSettings`, if you don't want to define an
  1462.   // interpolation, evaluation or escaping regex, we need one that is
  1463.   // guaranteed not to match.
  1464.   var noMatch = /(.)^/;
  1465.  
  1466.   // Certain characters need to be escaped so that they can be put into a
  1467.   // string literal.
  1468.   var escapes = {
  1469.     "'": "'",
  1470.     '\\': '\\',
  1471.     '\r': 'r',
  1472.     '\n': 'n',
  1473.     '\u2028': 'u2028',
  1474.     '\u2029': 'u2029'
  1475.   };
  1476.  
  1477.   var escapeRegExp = /\\|'|\r|\n|\u2028|\u2029/g;
  1478.  
  1479.   var escapeChar = function(match) {
  1480.     return '\\' + escapes[match];
  1481.   };
  1482.  
  1483.   // JavaScript micro-templating, similar to John Resig's implementation.
  1484.   // Underscore templating handles arbitrary delimiters, preserves whitespace,
  1485.   // and correctly escapes quotes within interpolated code.
  1486.   // NB: `oldSettings` only exists for backwards compatibility.
  1487.   _.template = function(text, settings, oldSettings) {
  1488.     if (!settings && oldSettings) settings = oldSettings;
  1489.     settings = _.defaults({}, settings, _.templateSettings);
  1490.  
  1491.     // Combine delimiters into one regular expression via alternation.
  1492.     var matcher = RegExp([
  1493.       (settings.escape || noMatch).source,
  1494.       (settings.interpolate || noMatch).source,
  1495.       (settings.evaluate || noMatch).source
  1496.     ].join('|') + '|$', 'g');
  1497.  
  1498.     // Compile the template source, escaping string literals appropriately.
  1499.     var index = 0;
  1500.     var source = "__p+='";
  1501.     text.replace(matcher, function(match, escape, interpolate, evaluate, offset) {
  1502.       source += text.slice(index, offset).replace(escapeRegExp, escapeChar);
  1503.       index = offset + match.length;
  1504.  
  1505.       if (escape) {
  1506.         source += "'+\n((__t=(" + escape + "))==null?'':_.escape(__t))+\n'";
  1507.       } else if (interpolate) {
  1508.         source += "'+\n((__t=(" + interpolate + "))==null?'':__t)+\n'";
  1509.       } else if (evaluate) {
  1510.         source += "';\n" + evaluate + "\n__p+='";
  1511.       }
  1512.  
  1513.       // Adobe VMs need the match returned to produce the correct offset.
  1514.       return match;
  1515.     });
  1516.     source += "';\n";
  1517.  
  1518.     // If a variable is not specified, place data values in local scope.
  1519.     if (!settings.variable) source = 'with(obj||{}){\n' + source + '}\n';
  1520.  
  1521.     source = "var __t,__p='',__j=Array.prototype.join," +
  1522.       "print=function(){__p+=__j.call(arguments,'');};\n" +
  1523.       source + 'return __p;\n';
  1524.  
  1525.     var render;
  1526.     try {
  1527.       render = new Function(settings.variable || 'obj', '_', source);
  1528.     } catch (e) {
  1529.       e.source = source;
  1530.       throw e;
  1531.     }
  1532.  
  1533.     var template = function(data) {
  1534.       return render.call(this, data, _);
  1535.     };
  1536.  
  1537.     // Provide the compiled source as a convenience for precompilation.
  1538.     var argument = settings.variable || 'obj';
  1539.     template.source = 'function(' + argument + '){\n' + source + '}';
  1540.  
  1541.     return template;
  1542.   };
  1543.  
  1544.   // Add a "chain" function. Start chaining a wrapped Underscore object.
  1545.   _.chain = function(obj) {
  1546.     var instance = _(obj);
  1547.     instance._chain = true;
  1548.     return instance;
  1549.   };
  1550.  
  1551.   // OOP
  1552.   // ---------------
  1553.   // If Underscore is called as a function, it returns a wrapped object that
  1554.   // can be used OO-style. This wrapper holds altered versions of all the
  1555.   // underscore functions. Wrapped objects may be chained.
  1556.  
  1557.   // Helper function to continue chaining intermediate results.
  1558.   var chainResult = function(instance, obj) {
  1559.     return instance._chain ? _(obj).chain() : obj;
  1560.   };
  1561.  
  1562.   // Add your own custom functions to the Underscore object.
  1563.   _.mixin = function(obj) {
  1564.     _.each(_.functions(obj), function(name) {
  1565.       var func = _[name] = obj[name];
  1566.       _.prototype[name] = function() {
  1567.         var args = [this._wrapped];
  1568.         push.apply(args, arguments);
  1569.         return chainResult(this, func.apply(_, args));
  1570.       };
  1571.     });
  1572.   };
  1573.  
  1574.   // Add all of the Underscore functions to the wrapper object.
  1575.   _.mixin(_);
  1576.  
  1577.   // Add all mutator Array functions to the wrapper.
  1578.   _.each(['pop', 'push', 'reverse', 'shift', 'sort', 'splice', 'unshift'], function(name) {
  1579.     var method = ArrayProto[name];
  1580.     _.prototype[name] = function() {
  1581.       var obj = this._wrapped;
  1582.       method.apply(obj, arguments);
  1583.       if ((name === 'shift' || name === 'splice') && obj.length === 0) delete obj[0];
  1584.       return chainResult(this, obj);
  1585.     };
  1586.   });
  1587.  
  1588.   // Add all accessor Array functions to the wrapper.
  1589.   _.each(['concat', 'join', 'slice'], function(name) {
  1590.     var method = ArrayProto[name];
  1591.     _.prototype[name] = function() {
  1592.       return chainResult(this, method.apply(this._wrapped, arguments));
  1593.     };
  1594.   });
  1595.  
  1596.   // Extracts the result from a wrapped and chained object.
  1597.   _.prototype.value = function() {
  1598.     return this._wrapped;
  1599.   };
  1600.  
  1601.   // Provide unwrapping proxy for some methods used in engine operations
  1602.   // such as arithmetic and JSON stringification.
  1603.   _.prototype.valueOf = _.prototype.toJSON = _.prototype.value;
  1604.  
  1605.   _.prototype.toString = function() {
  1606.     return '' + this._wrapped;
  1607.   };
  1608.  
  1609.   // AMD registration happens at the end for compatibility with AMD loaders
  1610.   // that may not enforce next-turn semantics on modules. Even though general
  1611.   // practice for AMD registration is to be anonymous, underscore registers
  1612.   // as a named module because, like jQuery, it is a base library that is
  1613.   // popular enough to be bundled in a third party lib, but not be part of
  1614.   // an AMD load request. Those cases could generate an error when an
  1615.   // anonymous define() is called outside of a loader request.
  1616.   if (typeof define == 'function' && define.amd) {
  1617.     define('underscore', [], function() {
  1618.       return _;
  1619.     });
  1620.   }
  1621. }());