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  • 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]':
  • NaNs 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 Objects or Arrays 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));