## Array

#### all

Returns `true` if the provided predicate function returns `true` for all elements in a collection, `false` otherwise.

Use `Array.every()` to test if all elements in the collection return `true` based on `fn`. Omit the second argument, `fn`, to use `Boolean` as a default.

    const all = (arr, fn = Boolean) => arr.every(fn);

examples

    all([4, 2, 3], x => x > 1); // true
    all([1, 2, 3]); // true

#### allEqual

Check if all elements in an array are equal.

Use `Array.every()` to check if all the elements of the array are the same as the first one.

    const allEqual = arr => arr.every(val => val === arr[0]);

examples

    allEqual([1, 2, 3, 4, 5, 6]); // false
    allEqual([1, 1, 1, 1]); // true

#### any

Returns `true` if the provided predicate function returns `true` for at least one element in a collection, `false` otherwise.

Use `Array.some()` to test if any elements in the collection return `true` based on `fn`. Omit the second argument, `fn`, to use `Boolean` as a default.

    const any = (arr, fn = Boolean) => arr.some(fn);

examples

    any([0, 1, 2, 0], x => x >= 2); // true
    any([0, 0, 1, 0]); // true

#### arrayToCSV

Converts a 2D array to a comma-separated values (CSV) string.

Use `Array.map()` and `Array.join(delimiter)` to combine individual 1D arrays (rows) into strings. Use `Array.join('\n')` to combine all rows into a CSV string, separating each row with a newline. Omit the second argument, `delimiter`, to use a default delimiter of `,`.

    const arrayToCSV = (arr, delimiter = ',') =>
      arr.map(v => v.map(x => `"${x}"`).join(delimiter)).join('\n');

examples

    arrayToCSV([['a', 'b'], ['c', 'd']]); // '"a","b"\n"c","d"'
    arrayToCSV([['a', 'b'], ['c', 'd']], ';'); // '"a";"b"\n"c";"d"'

#### bifurcate

Splits values into two groups. If an element in `filter` is truthy, the corresponding element in the collection belongs to the first group; otherwise, it belongs to the second group.

Use `Array.reduce()` and `Array.push()` to add elements to groups, based on `filter`.

    const bifurcate = (arr, filter) =>
      arr.reduce((acc, val, i) => (acc[filter[i] ? 0 : 1].push(val), acc), [[], []]);

examples

    bifurcate(['beep', 'boop', 'foo', 'bar'], [true, true, false, true]); // [ ['beep', 'boop', 'bar'], ['foo'] ]

#### bifurcateBy

Splits values into two groups according to a predicate function, which specifies which group an element in the input collection belongs to. If the predicate function returns a truthy value, the collection element belongs to the first group; otherwise, it belongs to the second group.

Use `Array.reduce()` and `Array.push()` to add elements to groups, based on the value returned by `fn` for each element.

    const bifurcateBy = (arr, fn) =>
      arr.reduce((acc, val, i) => (acc[fn(val, i) ? 0 : 1].push(val), acc), [[], []]);

examples

    bifurcateBy(['beep', 'boop', 'foo', 'bar'], x => x[0] === 'b'); // [ ['beep', 'boop', 'bar'], ['foo'] ]

#### chunk

Chunks an array into smaller arrays of a specified size.

Use `Array.from()` to create a new array, that fits the number of chunks that will be produced. Use `Array.slice()` to map each element of the new array to a chunk the length of `size`. If the original array can't be split evenly, the final chunk will contain the remaining elements.

    const chunk = (arr, size) =>
      Array.from({ length: Math.ceil(arr.length / size) }, (v, i) =>
        arr.slice(i * size, i * size + size)
      );

examples

    chunk([1, 2, 3, 4, 5], 2); // [[1,2],[3,4],[5]]

#### compact

Removes falsey values from an array.

Use `Array.filter()` to filter out falsey values (`false`, `null`, `0`, `""`, `undefined`, and `NaN`).

    const compact = arr => arr.filter(Boolean);

examples

    compact([0, 1, false, 2, '', 3, 'a', 'e' * 23, NaN, 's', 34]); // [ 1, 2, 3, 'a', 's', 34 ]

#### countBy

Groups the elements of an array based on the given function and returns the count of elements in each group.

Use `Array.map()` to map the values of an array to a function or property name. Use `Array.reduce()` to create an object, where the keys are produced from the mapped results.

    const countBy = (arr, fn) =>
      arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val, i) => {
        acc[val] = (acc[val] || 0) + 1;
        return acc;
      }, {});

examples

    countBy([6.1, 4.2, 6.3], Math.floor); // {4: 1, 6: 2}
    countBy(['one', 'two', 'three'], 'length'); // {3: 2, 5: 1}

#### countOccurrences

Counts the occurrences of a value in an array.

Use `Array.reduce()` to increment a counter each time you encounter the specific value inside the array.

    const countOccurrences = (arr, val) => arr.reduce((a, v) => (v === val ? a + 1 : a), 0);

examples

    countOccurrences([1, 1, 2, 1, 2, 3], 1); // 3

#### deepFlatten

Deep flattens an array.

Use recursion. Use `Array.concat()` with an empty array (`[]`) and the spread operator (`...`) to flatten an array. Recursively flatten each element that is an array.

    const deepFlatten = arr => [].concat(...arr.map(v => (Array.isArray(v) ? deepFlatten(v) : v)));

examples

    deepFlatten([1, [2], [[3], 4], 5]); // [1,2,3,4,5]

#### difference

Returns the difference between two arrays.

Create a `Set` from `b`, then use `Array.filter()` on `a` to only keep values not contained in `b`.

    const difference = (a, b) => {
      const s = new Set(b);
      return a.filter(x => !s.has(x));
    };

examples

    difference([1, 2, 3], [1, 2, 4]); // [3]

#### differenceBy

Returns the difference between two arrays, after applying the provided function to each array element of both.

Create a `Set` by applying `fn` to each element in `b`, then use `Array.filter()` in combination with `fn` on `a` to only keep values not contained in the previously created set.

    const differenceBy = (a, b, fn) => {
      const s = new Set(b.map(v => fn(v)));
      return a.filter(x => !s.has(fn(x)));
    };

examples

    differenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [1.2]
    differenceBy([{ x: 2 }, { x: 1 }], [{ x: 1 }], v => v.x); // [ { x: 2 } ]

#### differenceWith

Filters out all values from an array for which the comparator function does not return `true`.

Use `Array.filter()` and `Array.findIndex()` to find the appropriate values.

    const differenceWith = (arr, val, comp) => arr.filter(a => val.findIndex(b => comp(a, b)) === -1);

examples

    differenceWith([1, 1.2, 1.5, 3, 0], [1.9, 3, 0], (a, b) => Math.round(a) === Math.round(b)); // [1, 1.2]

#### drop

Returns a new array with `n` elements removed from the left.

Use `Array.slice()` to slice the remove the specified number of elements from the left.

    const drop = (arr, n = 1) => arr.slice(n);

examples

    drop([1, 2, 3]); // [2,3]
    drop([1, 2, 3], 2); // [3]
    drop([1, 2, 3], 42); // []

#### dropRight

Returns a new array with `n` elements removed from the right.

Use `Array.slice()` to slice the remove the specified number of elements from the right.

    const dropRight = (arr, n = 1) => arr.slice(0, -n);

examples

    dropRight([1, 2, 3]); // [1,2]
    dropRight([1, 2, 3], 2); // [1]
    dropRight([1, 2, 3], 42); // []

#### dropRightWhile

Removes elements from the end of an array until the passed function returns `true`. Returns the remaining elements in the array.

Loop through the array, using `Array.slice()` to drop the last element of the array until the returned value from the function is `true`. Returns the remaining elements.

    const dropRightWhile = (arr, func) => {
      while (arr.length > 0 && !func(arr[arr.length - 1])) arr = arr.slice(0, -1);
      return arr;
    };

examples

    dropRightWhile([1, 2, 3, 4], n => n < 3); // [1, 2]

#### dropWhile

Removes elements in an array until the passed function returns `true`. Returns the remaining elements in the array.

Loop through the array, using `Array.slice()` to drop the first element of the array until the returned value from the function is `true`. Returns the remaining elements.

    const dropWhile = (arr, func) => {
      while (arr.length > 0 && !func(arr[0])) arr = arr.slice(1);
      return arr;
    };

examples

    dropWhile([1, 2, 3, 4], n => n >= 3); // [3,4]

#### everyNth

Returns every nth element in an array.

Use `Array.filter()` to create a new array that contains every nth element of a given array.

    const everyNth = (arr, nth) => arr.filter((e, i) => i % nth === nth - 1);

examples

    everyNth([1, 2, 3, 4, 5, 6], 2); // [ 2, 4, 6 ]

#### filterNonUnique

Filters out the non-unique values in an array.

Use `Array.filter()` for an array containing only the unique values.

    const filterNonUnique = arr => arr.filter(i => arr.indexOf(i) === arr.lastIndexOf(i));

examples

    filterNonUnique([1, 2, 2, 3, 4, 4, 5]); // [1, 3, 5]

#### filterNonUniqueBy

Filters out the non-unique values in an array, based on a provided comparator function.

Use `Array.filter()` and `Array.every()` for an array containing only the unique values, based on the comparator function, `fn`. The comparator function takes four arguments: the values of the two elements being compared and their indexes.

    const filterNonUniqueBy = (arr, fn) =>
      arr.filter((v, i) => arr.every((x, j) => (i === j) === fn(v, x, i, j)));

examples

    filterNonUniqueBy(
      [
        { id: 0, value: 'a' },
        { id: 1, value: 'b' },
        { id: 2, value: 'c' },
        { id: 1, value: 'd' },
        { id: 0, value: 'e' }
      ],
      (a, b) => a.id == b.id
    ); // [ { id: 2, value: 'c' } ]

#### findLast

Returns the last element for which the provided function returns a truthy value.

Use `Array.filter()` to remove elements for which `fn` returns falsey values, `Array.pop()` to get the last one.

    const findLast = (arr, fn) => arr.filter(fn).pop();

examples

    findLast([1, 2, 3, 4], n => n % 2 === 1); // 3

#### findLastIndex

Returns the index of the last element for which the provided function returns a truthy value.

Use `Array.map()` to map each element to an array with its index and value. Use `Array.filter()` to remove elements for which `fn` returns falsey values, `Array.pop()` to get the last one.

    const findLastIndex = (arr, fn) =>
      arr
        .map((val, i) => [i, val])
        .filter(([i, val]) => fn(val, i, arr))
        .pop()[0];

examples

    findLastIndex([1, 2, 3, 4], n => n % 2 === 1); // 2 (index of the value 3)

#### flatten

Flattens an array up to the specified depth.

Use recursion, decrementing `depth` by 1 for each level of depth. Use `Array.reduce()` and `Array.concat()` to merge elements or arrays. Base case, for `depth` equal to `1` stops recursion. Omit the second argument, `depth` to flatten only to a depth of `1` (single flatten).

    const flatten = (arr, depth = 1) =>
      arr.reduce((a, v) => a.concat(depth > 1 && Array.isArray(v) ? flatten(v, depth - 1) : v), []);

examples

    flatten([1, [2], 3, 4]); // [1, 2, 3, 4]
    flatten([1, [2, [3, [4, 5], 6], 7], 8], 2); // [1, 2, 3, [4, 5], 6, 7, 8]

#### forEachRight

Executes a provided function once for each array element, starting from the array's last element.

Use `Array.slice(0)` to clone the given array, `Array.reverse()` to reverse it and `Array.forEach()` to iterate over the reversed array.

    const forEachRight = (arr, callback) =>
      arr
        .slice(0)
        .reverse()
        .forEach(callback);

examples

    forEachRight([1, 2, 3, 4], val => console.log(val)); // '4', '3', '2', '1'

#### groupBy

Groups the elements of an array based on the given function.

Use `Array.map()` to map the values of an array to a function or property name. Use `Array.reduce()` to create an object, where the keys are produced from the mapped results.

    const groupBy = (arr, fn) =>
      arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val, i) => {
        acc[val] = (acc[val] || []).concat(arr[i]);
        return acc;
      }, {});

examples

    groupBy([6.1, 4.2, 6.3], Math.floor); // {4: [4.2], 6: [6.1, 6.3]}
    groupBy(['one', 'two', 'three'], 'length'); // {3: ['one', 'two'], 5: ['three']}

#### head

Returns the head of a list.

Use `arr[0]` to return the first element of the passed array.

    const head = arr => arr[0];

examples

    head([1, 2, 3]); // 1

#### indexOfAll

Returns all indices of `val` in an array. If `val` never occurs, returns `[]`.

Use `Array.reduce()` to loop over elements and store indices for matching elements. Return the array of indices.

    const indexOfAll = (arr, val) => arr.reduce((acc, el, i) => (el === val ? [...acc, i] : acc), []);

examples

    indexOfAll([1, 2, 3, 1, 2, 3], 1); // [0,3]
    indexOfAll([1, 2, 3], 4); // []

#### initial

Returns all the elements of an array except the last one.

Use `arr.slice(0,-1)` to return all but the last element of the array.

    const initial = arr => arr.slice(0, -1);

examples

    initial([1, 2, 3]); // [1,2]

#### initialize2DArray

Initializes a 2D array of given width and height and value.

Use `Array.map()` to generate h rows where each is a new array of size w initialize with value. If the value is not provided, default to `null`.

    const initialize2DArray = (w, h, val = null) =>
      Array.from({ length: h }).map(() => Array.from({ length: w }).fill(val));

examples

    initialize2DArray(2, 2, 0); // [[0,0], [0,0]]

#### initializeArrayWithRange

Initializes an array containing the numbers in the specified range where `start` and `end` are inclusive with their common difference `step`.

Use `Array.from()` to create an array of the desired length, `(end - start + 1)/step`, and a map function to fill it with the desired values in the given range. You can omit `start` to use a default value of `0`. You can omit `step` to use a default value of `1`.

    const initializeArrayWithRange = (end, start = 0, step = 1) =>
      Array.from({ length: Math.ceil((end - start + 1) / step) }, (v, i) => i * step + start);

examples

    initializeArrayWithRange(5); // [0,1,2,3,4,5]
    initializeArrayWithRange(7, 3); // [3,4,5,6,7]
    initializeArrayWithRange(9, 0, 2); // [0,2,4,6,8]

#### initializeArrayWithRangeRight

Initializes an array containing the numbers in the specified range (in reverse) where `start` and `end` are inclusive with their common difference `step`.

Use `Array.from(Math.ceil((end+1-start)/step))` to create an array of the desired length(the amounts of elements is equal to `(end-start)/step` or `(end+1-start)/step` for inclusive end), `Array.map()` to fill with the desired values in a range. You can omit `start` to use a default value of `0`. You can omit `step` to use a default value of `1`.

    const initializeArrayWithRangeRight = (end, start = 0, step = 1) =>
      Array.from({ length: Math.ceil((end + 1 - start) / step) }).map(
        (v, i, arr) => (arr.length - i - 1) * step + start
      );

examples

    initializeArrayWithRangeRight(5); // [5,4,3,2,1,0]
    initializeArrayWithRangeRight(7, 3); // [7,6,5,4,3]
    initializeArrayWithRangeRight(9, 0, 2); // [8,6,4,2,0]

#### initializeArrayWithValues

Initializes and fills an array with the specified values.

Use `Array(n)` to create an array of the desired length, `fill(v)` to fill it with the desired values. You can omit `val` to use a default value of `0`.

    const initializeArrayWithValues = (n, val = 0) => Array(n).fill(val);

examples

    initializeArrayWithValues(5, 2); // [2, 2, 2, 2, 2]

#### initializeNDArray

Create a n-dimensional array with given value.

Use recursion. Use `Array.map()` to generate rows where each is a new array initialized using `initializeNDArray`.

    const initializeNDArray = (val, ...args) =>
      args.length === 0
        ? val
        : Array.from({ length: args[0] }).map(() => initializeNDArray(val, ...args.slice(1)));

examples

    initializeNDArray(1, 3); // [1,1,1]
    initializeNDArray(5, 2, 2, 2); // [[[5,5],[5,5]],[[5,5],[5,5]]]

#### intersection

Returns a list of elements that exist in both arrays.

Create a `Set` from `b`, then use `Array.filter()` on `a` to only keep values contained in `b`.

    const intersection = (a, b) => {
      const s = new Set(b);
      return a.filter(x => s.has(x));
    };

examples

    intersection([1, 2, 3], [4, 3, 2]); // [2, 3]

#### intersectionBy

Returns a list of elements that exist in both arrays, after applying the provided function to each array element of both.

Create a `Set` by applying `fn` to all elements in `b`, then use `Array.filter()` on `a` to only keep elements, which produce values contained in `b` when `fn` is applied to them.

    const intersectionBy = (a, b, fn) => {
      const s = new Set(b.map(fn));
      return a.filter(x => s.has(fn(x)));
    };

examples

    intersectionBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [2.1]

#### intersectionWith

Returns a list of elements that exist in both arrays, using a provided comparator function.

Use `Array.filter()` and `Array.findIndex()` in combination with the provided comparator to determine intersecting values.

    const intersectionWith = (a, b, comp) => a.filter(x => b.findIndex(y => comp(x, y)) !== -1);

examples

    intersectionWith([1, 1.2, 1.5, 3, 0], [1.9, 3, 0, 3.9], (a, b) => Math.round(a) === Math.round(b)); // [1.5, 3, 0]

#### isSorted

Returns `1` if the array is sorted in ascending order, `-1` if it is sorted in descending order or `0` if it is not sorted.

Calculate the ordering `direction` for the first two elements. Use `Object.entries()` to loop over array objects and compare them in pairs. Return `0` if the `direction` changes or the `direction` if the last element is reached.

    const isSorted = arr => {
      let direction = -(arr[0] - arr[1]);
      for (let [i, val] of arr.entries()) {
        direction = !direction ? -(arr[i - 1] - arr[i]) : direction;
        if (i === arr.length - 1) return !direction ? 0 : direction;
        else if ((val - arr[i + 1]) * direction > 0) return 0;
      }
    };

examples

    isSorted([0, 1, 2, 2]); // 1
    isSorted([4, 3, 2]); // -1
    isSorted([4, 3, 5]); // 0

#### join

Joins all elements of an array into a string and returns this string. Uses a separator and an end separator.

Use `Array.reduce()` to combine elements into a string. Omit the second argument, `separator`, to use a default separator of `','`. Omit the third argument, `end`, to use the same value as `separator` by default.

    const join = (arr, separator = ',', end = separator) =>
      arr.reduce(
        (acc, val, i) =>
          i === arr.length - 2
            ? acc + val + end
            : i === arr.length - 1
              ? acc + val
              : acc + val + separator,
        ''
      );

examples

    join(['pen', 'pineapple', 'apple', 'pen'], ',', '&'); // "pen,pineapple,apple&pen"
    join(['pen', 'pineapple', 'apple', 'pen'], ','); // "pen,pineapple,apple,pen"
    join(['pen', 'pineapple', 'apple', 'pen']); // "pen,pineapple,apple,pen"

#### JSONtoCSV

Converts an array of objects to a comma-separated values (CSV) string that contains only the `columns` specified.

Use `Array.join(demiliter)` to combine all the names in `columns` to create the first row. Use `Array.map()` and `Array.reduce()` to create a row for each object, substituting non-existent values with empty strings and only mapping values in `columns`. Use `Array.join('\n')` to combine all rows into a string. Omit the third argument, `delimiter`, to use a default delimiter of `,`.

    const JSONtoCSV = (arr, columns, delimiter = ',') =>
      [
        columns.join(delimiter),
        ...arr.map(obj =>
          columns.reduce(
            (acc, key) => `${acc}${!acc.length ? '' : delimiter}"${!obj[key] ? '' : obj[key]}"`,
            ''
          )
        )
      ].join('\n');

examples

    JSONtoCSV([{ a: 1, b: 2 }, { a: 3, b: 4, c: 5 }, { a: 6 }, { b: 7 }], ['a', 'b']); // 'a,b\n"1","2"\n"3","4"\n"6",""\n"","7"'
    JSONtoCSV([{ a: 1, b: 2 }, { a: 3, b: 4, c: 5 }, { a: 6 }, { b: 7 }], ['a', 'b'], ';'); // 'a;b\n"1";"2"\n"3";"4"\n"6";""\n"";"7"'

#### last

Returns the last element in an array.

Use `arr.length - 1` to compute the index of the last element of the given array and returning it.

    const last = arr => arr[arr.length - 1];

examples

    last([1, 2, 3]); // 3

#### longestItem

Takes any number of iterable objects or objects with a `length` property and returns the longest one. If multiple objects have the same length, the first one will be returned. Returns `undefined` if no arguments are provided.

Use `Array.reduce()`, comparing the `length` of objects to find the longest one.

    const longestItem = (val, ...vals) =>
      [val, ...vals].reduce((a, x) => (x.length > a.length ? x : a));

examples

    longestItem('this', 'is', 'a', 'testcase'); // 'testcase'
    longestItem(...['a', 'ab', 'abc']); // 'abc'
    longestItem(...['a', 'ab', 'abc'], 'abcd'); // 'abcd'
    longestItem([1, 2, 3], [1, 2], [1, 2, 3, 4, 5]); // [1, 2, 3, 4, 5]
    longestItem([1, 2, 3], 'foobar'); // 'foobar'

#### mapObject

Maps the values of an array to an object using a function, where the key-value pairs consist of the original value as the key and the mapped value.

Use an anonymous inner function scope to declare an undefined memory space, using closures to store a return value. Use a new `Array` to store the array with a map of the function over its data set and a comma operator to return a second step, without needing to move from one context to another (due to closures and order of operations).

    const mapObject = (arr, fn) =>
      (a => (
        (a = [arr, arr.map(fn)]), a[0].reduce((acc, val, ind) => ((acc[val] = a[1][ind]), acc), {})
      ))();

examples

    const squareIt = arr => mapObject(arr, a => a * a);
    squareIt([1, 2, 3]); // { 1: 1, 2: 4, 3: 9 }

#### maxN

Returns the `n` maximum elements from the provided array. If `n` is greater than or equal to the provided array's length, then return the original array (sorted in descending order).

Use `Array.sort()` combined with the spread operator (`...`) to create a shallow clone of the array and sort it in descending order. Use `Array.slice()` to get the specified number of elements. Omit the second argument, `n`, to get a one-element array.

    const maxN = (arr, n = 1) => [...arr].sort((a, b) => b - a).slice(0, n);

examples

    maxN([1, 2, 3]); // [3]
    maxN([1, 2, 3], 2); // [3,2]

#### minN

Returns the `n` minimum elements from the provided array. If `n` is greater than or equal to the provided array's length, then return the original array (sorted in ascending order).

Use `Array.sort()` combined with the spread operator (`...`) to create a shallow clone of the array and sort it in ascending order. Use `Array.slice()` to get the specified number of elements. Omit the second argument, `n`, to get a one-element array.

    const minN = (arr, n = 1) => [...arr].sort((a, b) => a - b).slice(0, n);

examples

    minN([1, 2, 3]); // [1]
    minN([1, 2, 3], 2); // [1,2]

#### none

Returns `true` if the provided predicate function returns `false` for all elements in a collection, `false` otherwise.

Use `Array.some()` to test if any elements in the collection return `true` based on `fn`. Omit the second argument, `fn`, to use `Boolean` as a default.

    const none = (arr, fn = Boolean) => !arr.some(fn);

examples

    none([0, 1, 3, 0], x => x == 2); // true
    none([0, 0, 0]); // true

#### nthElement

Returns the nth element of an array.

Use `Array.slice()` to get an array containing the nth element at the first place. If the index is out of bounds, return `undefined`. Omit the second argument, `n`, to get the first element of the array.

    const nthElement = (arr, n = 0) => (n === -1 ? arr.slice(n) : arr.slice(n, n + 1))[0];

examples

    nthElement(['a', 'b', 'c'], 1); // 'b'
    nthElement(['a', 'b', 'b'], -3); // 'a'

#### offset

Moves the specified amount of elements to the end of the array.

Use `Array.slice()` twice to get the elements after the specified index and the elements before that. Use the spread operator(`...`) to combine the two into one array. If `offset` is negative, the elements will be moved from end to start.

    const offset = (arr, offset) => [...arr.slice(offset), ...arr.slice(0, offset)];

examples

    offset([1, 2, 3, 4, 5], 2); // [3, 4, 5, 1, 2]
    offset([1, 2, 3, 4, 5], -2); // [4, 5, 1, 2, 3]

#### partition

Groups the elements into two arrays, depending on the provided function's truthiness for each element.

Use `Array.reduce()` to create an array of two arrays. Use `Array.push()` to add elements for which `fn` returns `true` to the first array and elements for which `fn` returns `false` to the second one.

    const partition = (arr, fn) =>
      arr.reduce(
        (acc, val, i, arr) => {
          acc[fn(val, i, arr) ? 0 : 1].push(val);
          return acc;
        },
        [[], []]
      );

examples

    const users = [{ user: 'barney', age: 36, active: false }, { user: 'fred', age: 40, active: true }];
    partition(users, o => o.active); // [[{ 'user': 'fred',    'age': 40, 'active': true }],[{ 'user': 'barney',  'age': 36, 'active': false }]]

#### permutations

⚠️ WARNING: This function's execution time increases exponentially with each array element. Anything more than 8 to 10 entries will cause your browser to hang as it tries to solve all the different combinations.

Generates all permutations of an array's elements (contains duplicates).

Use recursion. For each element in the given array, create all the partial permutations for the rest of its elements. Use `Array.map()` to combine the element with each partial permutation, then `Array.reduce()` to combine all permutations in one array. Base cases are for array `length` equal to `2` or `1`.

    const permutations = arr => {
      if (arr.length <= 2) return arr.length === 2 ? [arr, [arr[1], arr[0]]] : arr;
      return arr.reduce(
        (acc, item, i) =>
          acc.concat(
            permutations([...arr.slice(0, i), ...arr.slice(i + 1)]).map(val => [item, ...val])
          ),
        []
      );
    };

examples

    permutations([1, 33, 5]); // [ [ 1, 33, 5 ], [ 1, 5, 33 ], [ 33, 1, 5 ], [ 33, 5, 1 ], [ 5, 1, 33 ], [ 5, 33, 1 ] ]

#### pull

Mutates the original array to filter out the values specified.

Use `Array.filter()` and `Array.includes()` to pull out the values that are not needed. Use `Array.length = 0` to mutate the passed in an array by resetting it's length to zero and `Array.push()` to re-populate it with only the pulled values.

(For a snippet that does not mutate the original array see `without`)

    const pull = (arr, ...args) => {
      let argState = Array.isArray(args[0]) ? args[0] : args;
      let pulled = arr.filter((v, i) => !argState.includes(v));
      arr.length = 0;
      pulled.forEach(v => arr.push(v));
    };

examples

    let myArray = ['a', 'b', 'c', 'a', 'b', 'c'];
    pull(myArray, 'a', 'c'); // myArray = [ 'b', 'b' ]

#### pullAtIndex

Mutates the original array to filter out the values at the specified indexes.

Use `Array.filter()` and `Array.includes()` to pull out the values that are not needed. Use `Array.length = 0` to mutate the passed in an array by resetting it's length to zero and `Array.push()` to re-populate it with only the pulled values. Use `Array.push()` to keep track of pulled values

    const pullAtIndex = (arr, pullArr) => {
      let removed = [];
      let pulled = arr
        .map((v, i) => (pullArr.includes(i) ? removed.push(v) : v))
        .filter((v, i) => !pullArr.includes(i));
      arr.length = 0;
      pulled.forEach(v => arr.push(v));
      return removed;
    };

examples

    let myArray = ['a', 'b', 'c', 'd'];
    let pulled = pullAtIndex(myArray, [1, 3]); // myArray = [ 'a', 'c' ] , pulled = [ 'b', 'd' ]

#### pullAtValue

Mutates the original array to filter out the values specified. Returns the removed elements.

Use `Array.filter()` and `Array.includes()` to pull out the values that are not needed. Use `Array.length = 0` to mutate the passed in an array by resetting it's length to zero and `Array.push()` to re-populate it with only the pulled values. Use `Array.push()` to keep track of pulled values

    const pullAtValue = (arr, pullArr) => {
      let removed = [],
        pushToRemove = arr.forEach((v, i) => (pullArr.includes(v) ? removed.push(v) : v)),
        mutateTo = arr.filter((v, i) => !pullArr.includes(v));
      arr.length = 0;
      mutateTo.forEach(v => arr.push(v));
      return removed;
    };

examples

    let myArray = ['a', 'b', 'c', 'd'];
    let pulled = pullAtValue(myArray, ['b', 'd']); // myArray = [ 'a', 'c' ] , pulled = [ 'b', 'd' ]

#### pullBy

Mutates the original array to filter out the values specified, based on a given iterator function.

Check if the last argument provided in a function. Use `Array.map()` to apply the iterator function `fn` to all array elements. Use `Array.filter()` and `Array.includes()` to pull out the values that are not needed. Use `Array.length = 0` to mutate the passed in an array by resetting it's length to zero and `Array.push()` to re-populate it with only the pulled values.

    const pullBy = (arr, ...args) => {
      const length = args.length;
      let fn = length > 1 ? args[length - 1] : undefined;
      fn = typeof fn == 'function' ? (args.pop(), fn) : undefined;
      let argState = (Array.isArray(args[0]) ? args[0] : args).map(val => fn(val));
      let pulled = arr.filter((v, i) => !argState.includes(fn(v)));
      arr.length = 0;
      pulled.forEach(v => arr.push(v));
    };

examples

    var myArray = [{ x: 1 }, { x: 2 }, { x: 3 }, { x: 1 }];
    pullBy(myArray, [{ x: 1 }, { x: 3 }], o => o.x); // myArray = [{ x: 2 }]

#### reducedFilter

Filter an array of objects based on a condition while also filtering out unspecified keys.

Use `Array.filter()` to filter the array based on the predicate `fn` so that it returns the objects for which the condition returned a truthy value. On the filtered array, use `Array.map()` to return the new object using `Array.reduce()` to filter out the keys which were not supplied as the `keys` argument.

    const reducedFilter = (data, keys, fn) =>
      data.filter(fn).map(el =>
        keys.reduce((acc, key) => {
          acc[key] = el[key];
          return acc;
        }, {})
      );

examples

    const data = [
      {
        id: 1,
        name: 'john',
        age: 24
      },
      {
        id: 2,
        name: 'mike',
        age: 50
      }
    ];

    reducedFilter(data, ['id', 'name'], item => item.age > 24); // [{ id: 2, name: 'mike'}]

#### reduceSuccessive

Applies a function against an accumulator and each element in the array (from left to right), returning an array of successively reduced values.

Use `Array.reduce()` to apply the given function to the given array, storing each new result.

    const reduceSuccessive = (arr, fn, acc) =>
      arr.reduce((res, val, i, arr) => (res.push(fn(res.slice(-1)[0], val, i, arr)), res), [acc]);

examples

    reduceSuccessive([1, 2, 3, 4, 5, 6], (acc, val) => acc + val, 0); // [0, 1, 3, 6, 10, 15, 21]

#### reduceWhich

Returns the minimum/maximum value of an array, after applying the provided function to set comparing rule.

Use `Array.reduce()` in combination with the `comparator` function to get the appropriate element in the array. You can omit the second parameter, `comparator`, to use the default one that returns the minimum element in the array.

    const reduceWhich = (arr, comparator = (a, b) => a - b) =>
      arr.reduce((a, b) => (comparator(a, b) >= 0 ? b : a));

examples

    reduceWhich([1, 3, 2]); // 1
    reduceWhich([1, 3, 2], (a, b) => b - a); // 3
    reduceWhich(
      [{ name: 'Tom', age: 12 }, { name: 'Jack', age: 18 }, { name: 'Lucy', age: 9 }],
      (a, b) => a.age - b.age
    ); // {name: "Lucy", age: 9}

#### reject

Takes a predicate and array, like `Array.filter()`, but only keeps `x` if `pred(x) === false`.

    const reject = (pred, array) => array.filter((...args) => !pred(...args));

examples

    reject(x => x % 2 === 0, [1, 2, 3, 4, 5]); // [1, 3, 5]
    reject(word => word.length > 4, ['Apple', 'Pear', 'Kiwi', 'Banana']); // ['Pear', 'Kiwi']

#### remove

Removes elements from an array for which the given function returns `false`.

Use `Array.filter()` to find array elements that return truthy values and `Array.reduce()` to remove elements using `Array.splice()`. The `func` is invoked with three arguments (`value, index, array`).

    const remove = (arr, func) =>
      Array.isArray(arr)
        ? arr.filter(func).reduce((acc, val) => {
            arr.splice(arr.indexOf(val), 1);
            return acc.concat(val);
          }, [])
        : [];

examples

    remove([1, 2, 3, 4], n => n % 2 === 0); // [2, 4]

#### sample

Returns a random element from an array.

Use `Math.random()` to generate a random number, multiply it by `length` and round it of to the nearest whole number using `Math.floor()`. This method also works with strings.

    const sample = arr => arr[Math.floor(Math.random() * arr.length)];

examples

    sample([3, 7, 9, 11]); // 9

#### sampleSize

Gets `n` random elements at unique keys from `array` up to the size of `array`.

Shuffle the array using the Fisher-Yates algorithm. Use `Array.slice()` to get the first `n` elements. Omit the second argument, `n` to get only one element at random from the array.

    const sampleSize = ([...arr], n = 1) => {
      let m = arr.length;
      while (m) {
        const i = Math.floor(Math.random() * m--);
        [arr[m], arr[i]] = [arr[i], arr[m]];
      }
      return arr.slice(0, n);
    };

examples

    sampleSize([1, 2, 3], 2); // [3,1]
    sampleSize([1, 2, 3], 4); // [2,3,1]

#### shank

Has the same functionality as `Array.prototype.splice()`, but returning a new array instead of mutating the original array.

Use `Array.slice()` and `Array.concat()` to get a new array with the new contents after removing existing elements and/or adding new elements. Omit the second argument, `index`, to start at `0`. Omit the third argument, `delCount`, to remove `0` elements. Omit the fourth argument, `elements`, in order to not add any new elements.

    const shank = (arr, index = 0, delCount = 0, ...elements) =>
      arr
        .slice(0, index)
        .concat(elements)
        .concat(arr.slice(index + delCount));

examples

    const names = ['alpha', 'bravo', 'charlie'];
    const namesAndDelta = shank(names, 1, 0, 'delta'); // [ 'alpha', 'delta', 'bravo', 'charlie' ]
    const namesNoBravo = shank(names, 1, 1); // [ 'alpha', 'charlie' ]
    console.log(names); // ['alpha', 'bravo', 'charlie']

#### shuffle

Randomizes the order of the values of an array, returning a new array.

Uses the Fisher-Yates algorithm to reorder the elements of the array.

    const shuffle = ([...arr]) => {
      let m = arr.length;
      while (m) {
        const i = Math.floor(Math.random() * m--);
        [arr[m], arr[i]] = [arr[i], arr[m]];
      }
      return arr;
    };

examples

    const foo = [1, 2, 3];
    shuffle(foo); // [2, 3, 1], foo = [1, 2, 3]

#### similarity

Returns an array of elements that appear in both arrays.

Use `Array.filter()` to remove values that are not part of `values`, determined using `Array.includes()`.

    const similarity = (arr, values) => arr.filter(v => values.includes(v));

examples

    similarity([1, 2, 3], [1, 2, 4]); // [1, 2]

#### sortedIndex

Returns the lowest index at which value should be inserted into array in order to maintain its sort order.

Check if the array is sorted in descending order (loosely). Use `Array.findIndex()` to find the appropriate index where the element should be inserted.

    const sortedIndex = (arr, n) => {
      const isDescending = arr[0] > arr[arr.length - 1];
      const index = arr.findIndex(el => (isDescending ? n >= el : n <= el));
      return index === -1 ? arr.length : index;
    };

examples

    sortedIndex([5, 3, 2, 1], 4); // 1
    sortedIndex([30, 50], 40); // 1

#### sortedIndexBy

Returns the lowest index at which value should be inserted into array in order to maintain its sort order, based on a provided iterator function.

Check if the array is sorted in descending order (loosely). Use `Array.findIndex()` to find the appropriate index where the element should be inserted, based on the iterator function `fn`.

    const sortedIndexBy = (arr, n, fn) => {
      const isDescending = fn(arr[0]) > fn(arr[arr.length - 1]);
      const val = fn(n);
      const index = arr.findIndex(el => (isDescending ? val >= fn(el) : val <= fn(el)));
      return index === -1 ? arr.length : index;
    };

examples

    sortedIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, o => o.x); // 0

#### sortedLastIndex

Returns the highest index at which value should be inserted into array in order to maintain its sort order.

Check if the array is sorted in descending order (loosely). Use `Array.reverse()` and `Array.findIndex()` to find the appropriate last index where the element should be inserted.

    const sortedLastIndex = (arr, n) => {
      const isDescending = arr[0] > arr[arr.length - 1];
      const index = arr.reverse().findIndex(el => (isDescending ? n <= el : n >= el));
      return index === -1 ? 0 : arr.length - index;
    };

examples

    sortedLastIndex([10, 20, 30, 30, 40], 30); // 4

#### sortedLastIndexBy

Returns the highest index at which value should be inserted into array in order to maintain its sort order, based on a provided iterator function.

Check if the array is sorted in descending order (loosely). Use `Array.map()` to apply the iterator function to all elements of the array. Use `Array.reverse()` and `Array.findIndex()` to find the appropriate last index where the element should be inserted, based on the provided iterator function.

    const sortedLastIndexBy = (arr, n, fn) => {
      const isDescending = fn(arr[0]) > fn(arr[arr.length - 1]);
      const val = fn(n);
      const index = arr
        .map(fn)
        .reverse()
        .findIndex(el => (isDescending ? val <= el : val >= el));
      return index === -1 ? 0 : arr.length - index;
    };

examples

    sortedLastIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, o => o.x); // 1

#### stableSort

Performs stable sorting of an array, preserving the initial indexes of items when their values are the same. Does not mutate the original array, but returns a new array instead.

Use `Array.map()` to pair each element of the input array with its corresponding index. Use `Array.sort()` and a `compare` function to sort the list, preserving their initial order if the items compared are equal. Use `Array.map()` to convert back to the initial array items.

    const stableSort = (arr, compare) =>
      arr
        .map((item, index) => ({ item, index }))
        .sort((a, b) => compare(a.item, b.item) || a.index - b.index)
        .map(({ item }) => item);

examples

    const arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
    const stable = stableSort(arr, () => 0); // [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

#### symmetricDifference

Returns the symmetric difference between two arrays, without filtering out duplicate values.

Create a `Set` from each array, then use `Array.filter()` on each of them to only keep values not contained in the other.

    const symmetricDifference = (a, b) => {
      const sA = new Set(a),
        sB = new Set(b);
      return [...a.filter(x => !sB.has(x)), ...b.filter(x => !sA.has(x))];
    };

examples

    symmetricDifference([1, 2, 3], [1, 2, 4]); // [3, 4]
    symmetricDifference([1, 2, 2], [1, 3, 1]); // [2, 2, 3]

#### symmetricDifferenceBy

Returns the symmetric difference between two arrays, after applying the provided function to each array element of both.

Create a `Set` by applying `fn` to each array's elements, then use `Array.filter()` on each of them to only keep values not contained in the other.

    const symmetricDifferenceBy = (a, b, fn) => {
      const sA = new Set(a.map(v => fn(v))),
        sB = new Set(b.map(v => fn(v)));
      return [...a.filter(x => !sB.has(fn(x))), ...b.filter(x => !sA.has(fn(x)))];
    };

examples

    symmetricDifferenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [ 1.2, 3.4 ]

#### symmetricDifferenceWith

Returns the symmetric difference between two arrays, using a provided function as a comparator.

Use `Array.filter()` and `Array.findIndex()` to find the appropriate values.

    const symmetricDifferenceWith = (arr, val, comp) => [
      ...arr.filter(a => val.findIndex(b => comp(a, b)) === -1),
      ...val.filter(a => arr.findIndex(b => comp(a, b)) === -1)
    ];

examples

    symmetricDifferenceWith(
      [1, 1.2, 1.5, 3, 0],
      [1.9, 3, 0, 3.9],
      (a, b) => Math.round(a) === Math.round(b)
    ); // [1, 1.2, 3.9]

#### tail

Returns all elements in an array except for the first one.

Return `Array.slice(1)` if the array's `length` is more than `1`, otherwise, return the whole array.

    const tail = arr => (arr.length > 1 ? arr.slice(1) : arr);

examples

    tail([1, 2, 3]); // [2,3]
    tail([1]); // [1]

#### take

Returns an array with n elements removed from the beginning.

Use `Array.slice()` to create a slice of the array with `n` elements taken from the beginning.

    const take = (arr, n = 1) => arr.slice(0, n);

examples

    take([1, 2, 3], 5); // [1, 2, 3]
    take([1, 2, 3], 0); // []

#### takeRight

Returns an array with n elements removed from the end.

Use `Array.slice()` to create a slice of the array with `n` elements taken from the end.

    const takeRight = (arr, n = 1) => arr.slice(arr.length - n, arr.length);

examples

    takeRight([1, 2, 3], 2); // [ 2, 3 ]
    takeRight([1, 2, 3]); // [3]

#### takeRightWhile

Removes elements from the end of an array until the passed function returns `true`. Returns the removed elements.

Loop through the array, using a `for...of` loop over `Array.keys()` until the returned value from the function is `true`. Return the removed elements, using `Array.reverse()` and `Array.slice()`.

    const takeRightWhile = (arr, func) => {
      for (let i of arr.reverse().keys())
        if (func(arr[i])) return arr.reverse().slice(arr.length - i, arr.length);
      return arr;
    };

examples

    takeRightWhile([1, 2, 3, 4], n => n < 3); // [3, 4]

#### takeWhile

Removes elements in an array until the passed function returns `true`. Returns the removed elements.

Loop through the array, using a `for...of` loop over `Array.entries()` until the returned value from the function is `true`. Return the removed elements, using `Array.slice()`.

    const takeWhile = (arr, func) => {
      for (const [i, val] of arr.entries()) if (func(val)) return arr.slice(0, i);
      return arr;
    };

examples

    takeWhile([1, 2, 3, 4], n => n >= 3); // [1, 2]

#### toHash

Reduces a given Array-like into a value hash (keyed data store).

Given an Iterable or Array-like structure, call `Array.prototype.reduce.call()` on the provided object to step over it and return an Object, keyed by the reference value.

    const toHash = (object, key) =>
      Array.prototype.reduce.call(
        object,
        (acc, data, index) => ((acc[!key ? index : data[key]] = data), acc),
        {}
      );

examples

    toHash([4, 3, 2, 1]); // { 0: 4, 1: 3, 2: 2, 1: 1 }
    toHash([{ a: 'label' }], 'a'); // { label: { a: 'label' } }
    // A more in depth example:
    let users = [{ id: 1, first: 'Jon' }, { id: 2, first: 'Joe' }, { id: 3, first: 'Moe' }];
    let managers = [{ manager: 1, employees: [2, 3] }];
    // We use function here because we want a bindable reference, but a closure referencing the hash would work, too.
    managers.forEach(
      manager =>
        (manager.employees = manager.employees.map(function(id) {
          return this[id];
        }, toHash(users, 'id')))
    );
    managers; // [ { manager:1, employees: [ { id: 2, first: "Joe" }, { id: 3, first: "Moe" } ] } ]

#### union

Returns every element that exists in any of the two arrays once.

Create a `Set` with all values of `a` and `b` and convert to an array.

    const union = (a, b) => Array.from(new Set([...a, ...b]));

examples

    union([1, 2, 3], [4, 3, 2]); // [1,2,3,4]

#### unionBy

Returns every element that exists in any of the two arrays once, after applying the provided function to each array element of both.

Create a `Set` by applying all `fn` to all values of `a`. Create a `Set` from `a` and all elements in `b` whose value, after applying `fn` does not match a value in the previously created set. Return the last set converted to an array.

    const unionBy = (a, b, fn) => {
      const s = new Set(a.map(v => fn(v)));
      return Array.from(new Set([...a, ...b.filter(x => !s.has(fn(x)))]));
    };

examples

    unionBy([2.1], [1.2, 2.3], Math.floor); // [2.1, 1.2]

#### unionWith

Returns every element that exists in any of the two arrays once, using a provided comparator function.

Create a `Set` with all values of `a` and values in `b` for which the comparator finds no matches in `a`, using `Array.findIndex()`.

    const unionWith = (a, b, comp) =>
      Array.from(new Set([...a, ...b.filter(x => a.findIndex(y => comp(x, y)) === -1)]));

examples

    unionWith([1, 1.2, 1.5, 3, 0], [1.9, 3, 0, 3.9], (a, b) => Math.round(a) === Math.round(b)); // [1, 1.2, 1.5, 3, 0, 3.9]

#### uniqueElements

Returns all unique values of an array.

Use ES6 `Set` and the `...rest` operator to discard all duplicated values.

    const uniqueElements = arr => [...new Set(arr)];

examples

    uniqueElements([1, 2, 2, 3, 4, 4, 5]); // [1, 2, 3, 4, 5]

#### uniqueElementsBy

Returns all unique values of an array, based on a provided comparator function.

Use `Array.reduce()` and `Array.some()` for an array containing only the first unique occurence of each value, based on the comparator function, `fn`. The comparator function takes two arguments: the values of the two elements being compared.

    const uniqueElementsBy = (arr, fn) =>
      arr.reduce((acc, v) => {
        if (!acc.some(x => fn(v, x))) acc.push(v);
        return acc;
      }, []);

examples

    uniqueElementsBy(
      [
        { id: 0, value: 'a' },
        { id: 1, value: 'b' },
        { id: 2, value: 'c' },
        { id: 1, value: 'd' },
        { id: 0, value: 'e' }
      ],
      (a, b) => a.id == b.id
    ); // [ { id: 0, value: 'a' }, { id: 1, value: 'b' }, { id: 2, value: 'c' } ]

#### uniqueElementsByRight

Returns all unique values of an array, based on a provided comparator function.

Use `Array.reduce()` and `Array.some()` for an array containing only the last unique occurence of each value, based on the comparator function, `fn`. The comparator function takes two arguments: the values of the two elements being compared.

    const uniqueElementsByRight = (arr, fn) =>
      arr.reduceRight((acc, v) => {
        if (!acc.some(x => fn(v, x))) acc.push(v);
        return acc;
      }, []);

examples

    uniqueElementsByRight(
      [
        { id: 0, value: 'a' },
        { id: 1, value: 'b' },
        { id: 2, value: 'c' },
        { id: 1, value: 'd' },
        { id: 0, value: 'e' }
      ],
      (a, b) => a.id == b.id
    ); // [ { id: 0, value: 'e' }, { id: 1, value: 'd' }, { id: 2, value: 'c' } ]

#### uniqueSymmetricDifference

Returns the unique symmetric difference between two arrays, not containing duplicate values from either array.

Use `Array.filter()` and `Array.includes()` on each array to remove values contained in the other, then create a `Set` from the results, removing duplicate values.

    const uniqueSymmetricDifference = (a, b) => [
      ...new Set([...a.filter(v => !b.includes(v)), ...b.filter(v => !a.includes(v))])
    ];

examples

    uniqueSymmetricDifference([1, 2, 3], [1, 2, 4]); // [3, 4]
    uniqueSymmetricDifference([1, 2, 2], [1, 3, 1]); // [2, 3]

#### unzip

Creates an array of arrays, ungrouping the elements in an array produced by zip.

Use `Math.max.apply()` to get the longest subarray in the array, `Array.map()` to make each element an array. Use `Array.reduce()` and `Array.forEach()` to map grouped values to individual arrays.

    const unzip = arr =>
      arr.reduce(
        (acc, val) => (val.forEach((v, i) => acc[i].push(v)), acc),
        Array.from({
          length: Math.max(...arr.map(x => x.length))
        }).map(x => [])
      );

examples

    unzip([['a', 1, true], ['b', 2, false]]); //[['a', 'b'], [1, 2], [true, false]]
    unzip([['a', 1, true], ['b', 2]]); //[['a', 'b'], [1, 2], [true]]

#### unzipWith

Creates an array of elements, ungrouping the elements in an array produced by zip and applying the provided function.

Use `Math.max.apply()` to get the longest subarray in the array, `Array.map()` to make each element an array. Use `Array.reduce()` and `Array.forEach()` to map grouped values to individual arrays. Use `Array.map()` and the spread operator (`...`) to apply `fn` to each individual group of elements.

    const unzipWith = (arr, fn) =>
      arr
        .reduce(
          (acc, val) => (val.forEach((v, i) => acc[i].push(v)), acc),
          Array.from({
            length: Math.max(...arr.map(x => x.length))
          }).map(x => [])
        )
        .map(val => fn(...val));

examples

    unzipWith([[1, 10, 100], [2, 20, 200]], (...args) => args.reduce((acc, v) => acc + v, 0)); // [3, 30, 300]

#### without

Filters out the elements of an array, that have one of the specified values.

Use `Array.filter()` to create an array excluding(using `!Array.includes()`) all given values.

(For a snippet that mutates the original array see `pull`)

    const without = (arr, ...args) => arr.filter(v => !args.includes(v));

examples

    without([2, 1, 2, 3], 1, 2); // [3]

#### xProd

Creates a new array out of the two supplied by creating each possible pair from the arrays.

Use `Array.reduce()`, `Array.map()` and `Array.concat()` to produce every possible pair from the elements of the two arrays and save them in an array.

    const xProd = (a, b) => a.reduce((acc, x) => acc.concat(b.map(y => [x, y])), []);

examples

    xProd([1, 2], ['a', 'b']); // [[1, 'a'], [1, 'b'], [2, 'a'], [2, 'b']]

#### zip

Creates an array of elements, grouped based on the position in the original arrays.

Use `Math.max.apply()` to get the longest array in the arguments. Creates an array with that length as return value and use `Array.from()` with a map-function to create an array of grouped elements. If lengths of the argument-arrays vary, `undefined` is used where no value could be found.

    const zip = (...arrays) => {
      const maxLength = Math.max(...arrays.map(x => x.length));
      return Array.from({ length: maxLength }).map((_, i) => {
        return Array.from({ length: arrays.length }, (_, k) => arrays[k][i]);
      });
    };

examples

    zip(['a', 'b'], [1, 2], [true, false]); // [['a', 1, true], ['b', 2, false]]
    zip(['a'], [1, 2], [true, false]); // [['a', 1, true], [undefined, 2, false]]

#### zipObject

Given an array of valid property identifiers and an array of values, return an object associating the properties to the values.

Since an object can have undefined values but not undefined property pointers, the array of properties is used to decide the structure of the resulting object using `Array.reduce()`.

    const zipObject = (props, values) =>
      props.reduce((obj, prop, index) => ((obj[prop] = values[index]), obj), {});

examples

    zipObject(['a', 'b', 'c'], [1, 2]); // {a: 1, b: 2, c: undefined}
    zipObject(['a', 'b'], [1, 2, 3]); // {a: 1, b: 2}

#### zipWith

Creates an array of elements, grouped based on the position in the original arrays and using function as the last value to specify how grouped values should be combined.

Check if the last argument provided is a function. Use `Math.max()` to get the longest array in the arguments. Creates an array with that length as return value and use `Array.from()` with a map-function to create an array of grouped elements. If lengths of the argument-arrays vary, `undefined` is used where no value could be found. The function is invoked with the elements of each group `(...group)`.

    const zipWith = (...array) => {
      const fn = typeof array[array.length - 1] === 'function' ? array.pop() : undefined;
      return Array.from(
        { length: Math.max(...array.map(a => a.length)) },
        (_, i) => (fn ? fn(...array.map(a => a[i])) : array.map(a => a[i]))
      );
    };

examples

    zipWith([1, 2], [10, 20], [100, 200], (a, b, c) => a + b + c); // [111,222]
    zipWith(
      [1, 2, 3],
      [10, 20],
      [100, 200],
      (a, b, c) => (a != null ? a : 'a') + (b != null ? b : 'b') + (c != null ? c : 'c')
    ); // [111, 222, '3bc']