Wrapper to organize all internal variables and functions inside

Wrapper for prototypes and methods underscoreJS depends on

Wrapper to include all internal functions for creating underscore public API

External wrapper for our callback generator cb. Users may customize _.iteratee if they want additional predicate/iteratee shorthand styles. This abstraction hides the internal-only argCount argument.

OFFICIAL: https://underscorejs.org/#each // 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.

run callback on each element of an array, arrayLike, or object, return obj

OFFICIAL: https://underscorejs.org/#map Return the results of applying the iteratee to each element.

run callback on each element of array, arrayLike or other object, return an array with each result in it

what's behind _.reduce = createReduce(1)?

0. createReduce(1)
1. created reducer function to do real reduction work
2. passed 1 to set reducer to work from left to right
3. returned a wrapper function of reducer
4. this wrapper function
5. ask users for 4 arguments (obj, iteratee, memo, context)
6. further prepares initial or memo, and iteratee for running reducer internally

_.find(obj, predicate, context) => the first element that matches predicate

.groupBy(obj,iteratee,context): behavior for `.groupByis the following 1. ifresultdoes not havekey`, result[ key ] = [ value ]; 2. if it does, result[ key ].push(value)

// use1: _.groupBy(array, callback) group elements by callback(el,idx,array) result
_.groupBy([1, 2, 3, 4, 5, 6], function(num){ return num % 2; }) 
// => {"0": [2, 4, 6], "1": [1, 3, 5]}

// use2: _.groupBy(array, 'length') => group elements by _.property('length')(...) result
var list = ['one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine', 'ten'];
var grouped = _.groupBy(list, 'length');
arrayEq(grouped['3'], ['one', 'two', 'six', 'ten']);
arrayEq(grouped['4'], ['four', 'five', 'nine']);
arrayEq(grouped['5'], ['three', 'seven', 'eight']);

// use3 _.groupBy(array) => group elements by _.identity(el) result
var array = [1, 2, 1, 2, 3];
var grouped = _.groupBy(array);
eq(grouped['1'].length, 2);
eq(grouped['3'].length, 1);

// use4 _.groupBy(arrayOfArrays, number) => group element(a sub-array) by _.property(number)(el-subarray)
var matrix = [
  [1, 2],
  [1, 3],
  [2, 3]
];
_.isEqual(_.groupBy(matrix, 0), {1: [[1, 2], [1, 3]], 2: [[2, 3]]});
_.isEqual(_.groupBy(matrix, 1), {2: [[1, 2]], 3: [[1, 3], [2, 3]]});

// use5 _.groupBy(arrayOfObjects, arrayOfKeys) => group element(an object) by _.property(arrayOfKeys)(el-object)
var liz = {name: 'Liz', stats: {power: 10}};
var chelsea = {name: 'Chelsea', stats: {power: 10}};
var jordan = {name: 'Jordan', stats: {power: 6}};
var collection = [liz, chelsea, jordan];
var expected = {
  10: [liz, chelsea],
  6: [jordan]
};
_.isEqual(_.groupBy(collection, ['stats', 'power']), expected);

// use6 _.groupBy(arrayOfObjects, key) => group element(an object) by _.property(key)(el-object)
var foos = [{foo: [1, 2]}, {foo: [1, 2]}]
_.isEqual(_.groupBy(foos, 'foo'), {"1,2": foos});

tests // debugger; // _.union([1,2],[3,4]);

OFFICIAL Retrieve the names of an object's own properties. Delegates to ECMAScript 5's native Object.keys.

_.keys(obj) => [] or [key1, key2, ...] of direct properties not properties on prototype

_.isArray(obj): check whether an array or not use Array.isArray or toString.call(obj)

_.isObject(obj): check whether an obj is of type 'function' or 'object' obj: must not be undefined, null, NaN

_.property(path)(obj) => get a value from shallow object or nested object

tests // debugger; _.matcher({num: 1})({num:1}) => true

func(args) => restArguments(func, startIndex)(arg1, arg2, ...) => no array format is needed func([el1, el2, el3, ... ]) => restArguments(func, 0)(el1, el2, el3, ...) func(arg1, [el1, el2, ...]) => restArguments(func, 1)(arg1, el1, el2, ...) func(arg1, arg2, [el1, el2, ...]) => restArguments(func, 2)(arg1, arg2, el1, el2, ...) .union(arrays) => restArgument(.union, 0)([1,2], [3,4], ...) .without(array, otherArrays) => restArgument(.without, 1)([1,2,3,4], 2,4,...) var log = .bind(console.log, console); .delay(func, wait, args) => restArgument(_.delay, 2)(log, 1000, 'logged later', 'and more', 'and less')

baseCreate({num: 1}) => {}; whose {}.proto === {num:1}

shallowProperty(key)(obj) => obj[key] or undefined

has(obj, path) ===> obj.hasOwnProperty(path) considering undefined and null too

deepGet(obj, path) => a value at many level deep of a nested array, or arrayLike or other object

OFFICIAL // 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

isArrayLike(collection): check array, or arguments, or NodeList

createReduce(dir)(obj, iteratee, memo, context)

reducer(obj, iteratee, memo, initial) => memo

group(behavior, partition) => [[...],[...]] or {...} 1. create the returned container [[ ],[ ]], or { }, result 2. callbackTransformer based on value of iteratee 3. invoke callback on each element and get result key 4. run behavior which sets rules for how result use key to store value 5. return result

createPredicateIndexFinder(1)(array, predicate, context) => _.findIndex(...)