• # underscore.js
  • Underscore.js 1.8.3 http://underscorejs.org (c) 2009-2015 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];
  };

*

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);
  };

*

Trim out all falsy values from an array.

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

*

Internal implementation of a recursive flatten function.

  var flatten = function(input, shallow, 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);

* .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 = {
    '&': '&',
    '<': '<',
    '>': '>',
    '"': '"',
    "'": ''',
    '`': '`'
  };
  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('|') + '|${DATA}#39;, '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 offest.

      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;
  };

* *

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));