js-function-fun

A list of small & fun functional programming exercises in JavaScript.

Contributing

Please see CONTRIBUTING.

Testing

To test the functions:

1. Run npm install to install the dependencies (need node.js for npm). If you don't have node please visit the Node JS website to download. It is recommended to download the LTS version.
2. Change filename in test/tests.js to the name of your solution file (optional).
3. Make sure your solution file is in the Solutions folder.
4. Make sure your function names match the ones listed below as you're coding them.
5. At the bottom of your solution file, copy and paste the following code:

module.exports = {
    identity,
    addb,
    subb,
    mulb,
    minb,
    maxb,
    add,
    sub,
    mul,
    min,
    max,
    addRecurse,
    mulRecurse,
    minRecurse,
    maxRecurse,
    not,
    acc,
    accPartial,
    accRecurse,
    fill,
    fillRecurse,
    set,
    identityf,
    addf,
    liftf,
    pure,
    curryb,
    curry,
    inc,
    twiceUnary,
    doubl,
    square,
    twice,
    reverseb,
    reverse,
    composeuTwo,
    composeu,
    composeb,
    composeTwo,
    compose,
    limitb,
    limit,
    genFrom,
    genTo,
    genFromTo,
    elementGen,
    element,
    collect,
    filter,
    filterTail,
    concatTwo,
    concat,
    concatTail,
    gensymf,
    gensymff,
    fibonaccif,
    counter,
    revocableb,
    revocable,
    extract,
    m,
    addmTwo,
    addm,
    liftmbM,
    liftmb,
    liftm,
    exp,
    expn,
    addg,
    liftg,
    arrayg,
    continuizeu,
    continuize,
    vector,
    exploitVector,
    vectorSafe,
    pubsub,
    mapRecurse,
    filterRecurse,
};

6. You can comment out any function names in the module.exports that you haven't written yet, but a lot of the tests depend on previous functions to run properly so it's safer to write the functions in order.
7. Finally, npm run test to run the tests.

Functions

identity(x) ⇒ any

Write a function identity that takes an argument and returns that argument

addb(a, b) ⇒ number

Write a binary function addb that takes two numbers and returns their sum

subb(a, b) ⇒ number

Write a binary function subb that takes two numbers and returns their difference

mulb(a, b) ⇒ number

Write a binary function mulb that takes two numbers and returns their product

minb(a, b) ⇒ number

Write a binary function minb that takes two numbers and returns the smaller one

maxb(a, b) ⇒ number

Write a binary function maxb that takes two numbers and returns the larger one

add(...nums) ⇒ number

Write a function add that is generalized for any amount of arguments

sub(...nums) ⇒ number

Write a function sub that is generalized for any amount of arguments

mul(...nums) ⇒ number

Write a function mul that is generalized for any amount of arguments

min(...nums) ⇒ number

Write a function min that is generalized for any amount of arguments

max(...nums) ⇒ number

Write a function max that is generalized for any amount of arguments

addRecurse(...nums) ⇒ number

Write a function addRecurse that is the generalized add function but uses recursion

mulRecurse(...nums) ⇒ number

Write a function mulRecurse that is the generalized mul function but uses recursion

minRecurse(...nums) ⇒ number

Write a function minRecurse that is the generalized min function but uses recursion

maxRecurse(...nums) ⇒ number

Write a function maxRecurse that is the generalized max function but uses recursion

not(func) ⇒ function

Write a function not that takes a function and returns the negation of its result

acc(func, initial) ⇒ function

Write a function acc that takes a function and an initial value and returns a function that runs the initial function on each argument, accumulating the result

accPartial(func, start, end) ⇒ function

Write a function accPartial that takes in a function, a start index, and an end index, and returns a function that accumulates a subset of its arguments by applying the given function to all elements between start and end.

accRecurse(func, initial) ⇒ function

Write a function accRecurse that does what acc does but uses recursion

fill(num) ⇒ array

Write a function fill that takes a number and returns an array with that many numbers equal to the given number

fillRecurse(num) ⇒ array

Write a function fillRecurse that does what fill does but uses recursion

set(...args) ⇒ array

Write a function set that is given a list of arguments and returns an array with all duplicates removed

identityf(x) ⇒ function

Write a function identityf that takes an argument and returns a function that returns that argument

addf(a) ⇒ function

Write a function addf that adds from two invocations

liftf(binary) ⇒ function

Write a function liftf that takes a binary function, and makes it callable with two invocations

pure(x, y) ⇒ array

Write a pure function pure that is a wrapper arround the impure function impure

function impure(x) {
  y++;
  z = x * y;
}

var y = 5, z;

impure(20);
z; // 120

impure(25);
z; // 175

curryb(binary, a) ⇒ function

Write a function curryb that takes a binary function and an argument, and returns a function that can take a second argument

curry(func, ...outer) ⇒ function

Write a function curry that is generalized for any amount of arguments

inc(x) ⇒ number

Without writting any new functions, show multiple ways to create the inc function

twiceUnary(binary) ⇒ function

Write a function twiceUnary that takes a binary function and returns a unary function that passes its argument to the binary function twice

doubl(x) ⇒ number

Use the function twiceUnary to create the doubl function

square(x) ⇒ number

Use the function twiceUnary to create the square function

twice(x) ⇒ any

Write a function twice that is generalized for any amount of arguments

reverseb(binary) ⇒ function

Write a function reverseb that reverses the arguments of a binary function

reverse(func) ⇒ function

Write a function reverse that is generalized for any amount of arguments

composeuTwo(unary1, unary2) ⇒ function

Write a function composeuTwo that takes two unary functions and returns a unary function that calls them both

composeu(...funcs) ⇒ any

Write a function composeu that is generalized for any amount of arguments

composeb(binary1, binary2) ⇒ function

Write a function composeb that takes two binary functions and returns a function that calls them both

composeTwo(func1, func2) ⇒ function

Write a function composeTwo that takes two functions and returns a function that calls them both

compose(...funcs) ⇒ function

Write a function compose that takes any amount of functions and returns a function that takes any amount of arguments and gives them to the first function, then that result to the second function and so on

limitb(binary, lmt) ⇒ function

Write a function limitb that allows a binary function to be called a limited number of times

limit(func, lmt) ⇒ function

Write a function limit that is generalized for any amount of arguments

genFrom(x) ⇒ function

Write a function genFrom that produces a generator that will produces a series of values

genTo(gen, lmt) ⇒ function

Write a function genTo that takes a generator and an end limit, and returns a generator that will produce numbers up to that limit

genFromTo(start, end) ⇒ function

Write a function genFromTo that produces a generator that will produce values in a range

elementGen(array, gen) ⇒ function

Write a function elementGen that takes an array and a generator and returns a generator that will produce elements from the array

element(array, gen) ⇒ function

Write a function element that is a modified elementGen function so that the generator argument is optional. If a generator is not provided, then each of the elements of the array will be produced.

collect(gen, array) ⇒ function

Write a function collect that takes a generator and an array and produces a function that will collect the results in the array

filter(gen, predicate) ⇒ function

Write a function filter that takes a generator and a predicate and produces a generator that produces only the values approved by the predicate

filterTail(gen, predicate) ⇒ function

Write a function filterTail that uses tail-recursion to perform the filtering

concatTwo(gen1, gen2) ⇒ function

Write a function concatTwo that takes two generators and produces a generator that combines the sequences

concat(...gens) ⇒ function

Write a function concat that is generalized for any amount of arguments

concatTail(...gens) ⇒ function

Write a function concatTail that uses tail-recursion to perform the concating

gensymf(symbol) ⇒ function

Write a function gensymf that makes a function that generates unique symbols

gensymff(unary, seed) ⇒ function

Write a function gensymff that takes a unary function and a seed and returns a gensymf

fibonaccif(first, second) ⇒ function

Write a function fibonaccif that returns a generator that will return the next fibonacci number

counter(i) ⇒ object

Write a function counter that returns an object containing two functions that implement an up/down counter, hiding the counter

revocableb(binary) ⇒ object

Write a function revocableb that takes a binary function, and returns an object containing an invoke function that can invoke a function and a revoke function that disables the invoke function

revocable(func) ⇒ object

Write a function revocable that is generalized for any amount of arguments

extract(array, prop) ⇒ array

Write a function extract that takes an array of objects and an object property name and converts each object in the array by extracting that property

m(value, source) ⇒ object

Write a function m that takes a value and an optional source string and returns them in an object

addmTwo(m1, m2) ⇒ object

Write a function addmTwo that adds two m objects and returns an m object

addm(...ms) ⇒ object

Write a function addm that is generalized for any amount of arguments

liftmbM(binary, op) ⇒ object

Write a function liftmbM that takes a binary function and a string and returns a function that acts on m objects

liftmb(binary, op) ⇒ object

Write a function liftmb that is a modified function liftmbM that can accept arguments that are either numbers or m objects

liftm(func, op) ⇒ object

Write a function liftm that is generalized for any amount of arguments

exp(value) ⇒ any

Write a function exp that evaluates simple array expressions

expn(value) ⇒ any

Write a function expn that is a modified exp that can evaluate nested array expressions

addg(value) ⇒ number | undefined

Write a function addg that adds from many invocations, until it sees an empty invocation

liftg(binary) ⇒ function

Write a function liftg that will take a binary function and apply it to many invocations

arrayg(value) ⇒ array

Write a function arrayg that will build an array from many invocations

continuizeu(unary) ⇒ function

Write a function continuizeu that takes a unary function and returns a function that takes a callback and an argument

continuize(func) ⇒ function

Write a function continuize that takes a function and returns a function that takes a callback and arguments

vector()

Make an array wrapper object with methods get, store, and append, such that an attacker cannot get access to the private array

exploitVector()

Let's assume your vector implementation looks like something like this:

let vector = () => {
  let array = []
  return {
    append: (v) => array.push(v),
    get: (i) => array[i],
    store: (i, v) => array[i] = v
  }
}

Can you spot any security concerns with this approach? Mainly, can we get access to the array outside of vector? Note: the issue has nothing to do with prototypes and we can assume that global prototypes cannot be altered. Hint: Think about using this in a method invocation. Can we override a method of vector?

vectorSafe()

How would you rewrite vector to deal with the issue from above?

pubsub()

Make a function pubsub that makes a publish/subscribe object. It will reliably deliver all publications to all subscribers in the right order.

mapRecurse(array, callback) ⇒ array

Make a function mapRecurse that performs a transformation for each element of a given array, recursively

filterRecurse(array, predicate) ⇒ array

Make a function filterRecurse that takes in an array and a predicate function and returns a new array by filtering out all items using the predicate, recursively.

identity(x) ⇒ any

Write a function identity that takes an argument and returns that argument

Param	Type
x	any

Example

identity(3) // 3

addb(a, b) ⇒ number

Write a binary function addb that takes two numbers and returns their sum

Param	Type
a	number
b	number

Example

addb(3, 4) // 3 + 4 = 7

subb(a, b) ⇒ number

Write a binary function subb that takes two numbers and returns their difference

Param	Type
a	number
b	number

Example

subb(3, 4) // 3 - 4 = -1

mulb(a, b) ⇒ number

Write a binary function mulb that takes two numbers and returns their product

Param	Type
a	number
b	number

Example

mulb(3, 4) // 3 * 4 = 12

minb(a, b) ⇒ number

Write a binary function minb that takes two numbers and returns the smaller one

Param	Type
a	number
b	number

Example

minb(3, 4) // 3

maxb(a, b) ⇒ number

Write a binary function maxb that takes two numbers and returns the larger one

Param	Type
a	number
b	number

Example

maxb(3, 4) // 4

add(...nums) ⇒ number

Write a function add that is generalized for any amount of arguments

Param	Type
...nums	number

Example

add(1, 2, 4) // 1 + 2 + 4 = 7

sub(...nums) ⇒ number

Write a function sub that is generalized for any amount of arguments

Param	Type
...nums	number

Example

sub(1, 2, 4) // 1 - 2 - 4 = -5

mul(...nums) ⇒ number

Write a function mul that is generalized for any amount of arguments

Param	Type
...nums	number

Example

mul(1, 2, 4) // 1 * 2 * 4 = 8

min(...nums) ⇒ number

Write a function min that is generalized for any amount of arguments

Param	Type
...nums	number

Example

min(1, 2, 4) // 1

max(...nums) ⇒ number

Write a function max that is generalized for any amount of arguments

Param	Type
...nums	number

Example

max(1, 2, 4) // 4

addRecurse(...nums) ⇒ number

Write a function addRecurse that is the generalized add function but uses recursion

Param	Type
...nums	number

Example

addRecurse(1, 2, 4) // 1 + 2 + 4 = 7

mulRecurse(...nums) ⇒ number

Write a function mulRecurse that is the generalized mul function but uses recursion

Param	Type
...nums	number

Example

mulRecurse(1, 2, 4) // 1 * 2 * 4 = 8

minRecurse(...nums) ⇒ number

Write a function minRecurse that is the generalized min function but uses recursion

Param	Type
...nums	number

Example

minRecurse(1, 2, 4) // 1

maxRecurse(...nums) ⇒ number

Write a function maxRecurse that is the generalized max function but uses recursion

Param	Type
...nums	number

Example

maxRecurse(1, 2, 4) // 4

not(func) ⇒ function

Write a function not that takes a function and returns the negation of its result

Param	Type
func	function

Example

const isOdd = (x) => x % 2 === 1
const isEven = not(isOdd)
isEven(1) // false
isEven(2) // true

acc(func, initial) ⇒ function

Write a function acc that takes a function and an initial value and returns a function that runs the initial function on each argument, accumulating the result

Param	Type
func	function
initial	any

Example

let add = acc(addb, 0)
add(1, 2, 4) // 7

let mul = acc(mulb, 1)
mul(1, 2, 4) // 8

accPartial(func, start, end) ⇒ function

Write a function accPartial that takes in a function, a start index, and an end index, and returns a function that accumulates a subset of its arguments by applying the given function to all elements between start and end.

Param	Type
func	function
start	number
end	number

Example

const addSecondToThird = accPartial(add, 1, 3)
addSecondToThird(1, 2, 4, 8) // [ 1, 6, 8 ]

accRecurse(func, initial) ⇒ function

Write a function accRecurse that does what acc does but uses recursion

Param	Type
func	function
initial	number

Example

let add = accRecurse(addb, 0)
add(1, 2, 4) // 7

let mul = accRecurse(mulb, 1)
mul(1, 2, 4) // 8

fill(num) ⇒ array

Write a function fill that takes a number and returns an array with that many numbers equal to the given number

Param	Type
num	number

Example

fill(3) // [ 3, 3, 3 ]

fillRecurse(num) ⇒ array

Write a function fillRecurse that does what fill does but uses recursion

Param	Type
num	number

Example

fillRecurse(3) // [ 3, 3, 3 ]

set(...args) ⇒ array

Write a function set that is given a list of arguments and returns an array with all duplicates removed

Param	Type
...args	any

Example

let oneAndTwo = set(1, 1, 1, 2, 2, 2) // [ 1, 2 ]

identityf(x) ⇒ function

Write a function identityf that takes an argument and returns a function that returns that argument

Param	Type
x	any

Example

let three = identityf(3)
three() // 3

addf(a) ⇒ function

Write a function addf that adds from two invocations

Param	Type
a	number

Example

addf(3)(4) // 7

liftf(binary) ⇒ function

Write a function liftf that takes a binary function, and makes it callable with two invocations

Param	Type
binary	function

Example

let addf = liftf(addb)
addf(3)(4) // 7

liftf(mulb)(5)(6) // 30

pure(x, y) ⇒ array

Write a pure function pure that is a wrapper arround the impure function impure

function impure(x) {
  y++;
  z = x * y;
}

var y = 5, z;

impure(20);
z; // 120

impure(25);
z; // 175

Returns: array - an array containing y and z

Param	Type
x	number
y	number

Example

pure(20, 5) // [ 6, 120 ]
pure(25, 6) // [ 7, 175 ]

curryb(binary, a) ⇒ function

Write a function curryb that takes a binary function and an argument, and returns a function that can take a second argument

Param	Type
binary	function
a	any

Example

let add3 = curryb(addb, 3)
add3(4) // 7

curryb(mulb, 5)(6) // 30

curry(func, ...outer) ⇒ function

Write a function curry that is generalized for any amount of arguments

Param	Type
func	function
...outer	any

Example

curry(add, 1, 2, 4)(4, 2, 1) = 1 + 2 + 4 + 4 + 2 + 1 = 14
curry(sub, 1, 2, 4)(4, 2, 1) = 1 - 2 - 4 - 4 - 2 - 1 = -12
curry(mul, 1, 2, 4)(4, 2, 1) = 1 * 2 * 4 * 4 * 2 * 1 = 64

inc(x) ⇒ number

Without writting any new functions, show multiple ways to create the inc function

Param	Type
x	number

Example

inc(5) // 6
inc(inc(5)) // 7

twiceUnary(binary) ⇒ function

Write a function twiceUnary that takes a binary function and returns a unary function that passes its argument to the binary function twice

Param	Type
binary	function

Example

let doubl = twiceUnary(addb)
doubl(11) // 22

let square = twiceUnary(mulb)
square(11) // 121

doubl(x) ⇒ number

Use the function twiceUnary to create the doubl function

Param	Type
x	number

Example

doubl(11) // 22

square(x) ⇒ number

Use the function twiceUnary to create the square function

Param	Type
x	number

Example

square(11) // 121

twice(x) ⇒ any

Write a function twice that is generalized for any amount of arguments

Param	Type
x	function

Example

let doubleSum = twice(add)
doubleSum(1, 2, 4) // 1 + 2 + 4 + 1 + 2 + 4 = 14

reverseb(binary) ⇒ function

Write a function reverseb that reverses the arguments of a binary function

Param	Type
binary	function

Example

let bus = reverseb(subb)
bus(3, 2) // -1

reverse(func) ⇒ function

Write a function reverse that is generalized for any amount of arguments

Param	Type
func	function

Example

reverse(sub)(1, 2, 4) // 4 - 2 - 1 = 1

composeuTwo(unary1, unary2) ⇒ function

Write a function composeuTwo that takes two unary functions and returns a unary function that calls them both

Param	Type
unary1	function
unary2	function

Example

composeuTwo(doubl, square)(5) // (5 * 2)^2 = 100

composeu(...funcs) ⇒ any

Write a function composeu that is generalized for any amount of arguments

Param	Type
...funcs	function

Example

composeu(doubl, square, identity, curry(add, 1, 2))(5) // (5 * 2)^2 + 1 + 2 = 103

composeb(binary1, binary2) ⇒ function

Write a function composeb that takes two binary functions and returns a function that calls them both

Param	Type
binary1	function
binary2	function

Example

composeb(addb, mulb)(2, 3, 7) // (2 + 3) * 7 = 35

composeTwo(func1, func2) ⇒ function

Write a function composeTwo that takes two functions and returns a function that calls them both

Param	Type
func1	function
func2	function

Example

composeTwo(add, square)(2, 3, 7, 5) // (2 + 3 + 7 + 5)^2 = 289

compose(...funcs) ⇒ function

Write a function compose that takes any amount of functions and returns a function that takes any amount of arguments and gives them to the first function, then that result to the second function and so on

Param	Type
...funcs	function

Example

const f = compose(add, doubl, fill, max)
f(0, 1, 2)
// add(0, 1, 2) -> 3
// doubl(3) -> 6
// fill(6) -> [ 6, 6, 6, 6, 6, 6 ]
// max(6, 6, 6, 6, 6, 6) -> 6

limitb(binary, lmt) ⇒ function

Write a function limitb that allows a binary function to be called a limited number of times

Param	Type
binary	function
lmt	number

Example

let addLmtb = limitb(addb, 1)
addLmtb(3, 4) // 7
addLmtb(3, 5) // undefined

limit(func, lmt) ⇒ function

Write a function limit that is generalized for any amount of arguments

Param	Type
func	function
lmt	number

Example

let addLmt = limit(add, 1)
addLmt(1, 2, 4) // 7
addLmt(3, 5, 9, 2) // undefined

genFrom(x) ⇒ function

Write a function genFrom that produces a generator that will produces a series of values. Follows the iterator protocol for the returned format.

Param	Type
x	number

Example

let index = genFrom(0)

index.next().value // 0
index.next().value // 1
index.next().value // 2

genTo(gen, lmt) ⇒ function

Write a function genTo that takes a generator and an end limit, and returns a generator that will produce numbers up to that limit

Param	Type
gen	function
lmt	number

Example

let index = genTo(genFrom(1), 3)

index.next().value // 1
index.next().value // 2
index.next().value // undefined

genFromTo(start, end) ⇒ function

Write a function genFromTo that produces a generator that will produce values in a range

Param	Type
start	number
end	number

Example

let index = genFromTo(0, 3)
index.next().value // 0
index.next().value // 1
index.next().value // 2
index.next().value // undefined

elementGen(array, gen) ⇒ function

Write a function elementGen that takes an array and a generator and returns a generator that will produce elements from the array

Param	Type
array	array
gen	function

Example

let ele = elementGen(['a', 'b', 'c', 'd'], genFromTo(1, 3))

ele.next().value // 'b'
ele.next().value // 'c'
ele.next().value // undefined

element(array, gen) ⇒ function

Write a function element that is a modified elementGen function so that the generator argument is optional. If a generator is not provided, then each of the elements of the array will be produced.

Param	Type
array	array
gen	function

Example

let ele = element(['a', 'b', 'c', 'd'])

ele.next().value // 'a'
ele.next().value // 'b'
ele.next().value // 'c'
ele.next().value // 'd'
ele.next().value // undefined

collect(gen, array) ⇒ function

Write a function collect that takes a generator and an array and produces a function that will collect the results in the array

Param	Type
gen	function
array	array

Example

let array = []
let col = collect(genFromTo(0, 2), array)

col.next().value // 0
col.next().value // 1
col.next().value // undefined
array // [0, 1]

filter(gen, predicate) ⇒ function

Write a function filter that takes a generator and a predicate and produces a generator that produces only the values approved by the predicate

Param	Type
gen	function
predicate	function

Example

let third = (val) => val % 3 === 0
let fil = filter(genFromTo(0, 5), third)

fil.next().value // 0
fil.next().value // 3
fil.next().value // undefined

filterTail(gen, predicate) ⇒ function

Write a function filterTail that uses tail-recursion to perform the filtering

Param	Type
gen	function
predicate	function

Example

let third = (val) => val % 3 === 0
let fil = filterTail(genFromTo(0, 5), third)

fil.next().value // 0
fil.next().value // 3
fil.next().value // undefined

concatTwo(gen1, gen2) ⇒ function

Write a function concatTwo that takes two generators and produces a generator that combines the sequences

Param	Type
gen1	function
gen2	function

Example

let con = concatTwo(genFromTo(0, 3), genFromTo(0, 2))
con.next().value // 0
con.next().value // 1
con.next().value // 2
con.next().value // 0
con.next().value // 1
con.next().value // undefined

concat(...gens) ⇒ function

Write a function concat that is generalized for any amount of arguments

Param	Type
...gens	function

Example

let con = concat(genFromTo(0, 3), genFromTo(0, 2), genFromTo(5, 7))
con.next().value // 0
con.next().value // 1
con.next().value // 2
con.next().value // 0
con.next().value // 1
con.next().value // 5
con.next().value // 6
con.next().value // undefined

concatTail(...gens) ⇒ function

Write a function concatTail that uses tail-recursion to perform the concating

Param	Type
...gens	function

Example

let con = concatTail(genFromTo(0, 3), genFromTo(0, 2), genFromTo(5, 7))
con.next().value // 0
con.next().value // 1
con.next().value // 2
con.next().value // 0
con.next().value // 1
con.next().value // 5
con.next().value // 6
con.next().value // undefined

gensymf(symbol) ⇒ function

Write a function gensymf that makes a function that generates unique symbols

Param	Type
symbol	string

Example

let genG = gensymf('G')
let genH = gensymf('H')

genG.next().value // 'G1'
genH.next().value // 'H1'
genG.next().value // 'G2'
genH.next().value // 'H2'

gensymff(unary, seed) ⇒ function

Write a function gensymff that takes a unary function and a seed and returns a gensymf

Param	Type
unary	function
seed	number

Example

let gensymf = gensymff(inc, 0)
let genG = gensymf('G')
let genH = gensymf('H')

genG.next().value // 'G1'
genH.next().value // 'H1'
genG.next().value // 'G2'
genH.next().value // 'H2'

fibonaccif(first, second) ⇒ function

Write a function fibonaccif that returns a generator that will return the next fibonacci number

Param	Type
first	number
second	number

Example

let fib = fibonaccif(0, 1)
fib.next().value // 0
fib.next().value // 1
fib.next().value // 1
fib.next().value // 2
fib.next().value // 3
fib.next().value // 5
fib.next().value // 8

counter(i) ⇒ object

Write a function counter that returns an object containing two functions that implement an up/down counter, hiding the counter

Param	Type
i	number

Example

let obj = counter(10)
let { up, down } = obj

up() // 11
down() // 10
down() // 9
up() // 10

revocableb(binary) ⇒ object

Write a function revocableb that takes a binary function, and returns an object containing an invoke function that can invoke a function and a revoke function that disables the invoke function

Param	Type
binary	function

Example

let rev = revocableb(addb)

rev.invoke(3, 4) // 7
rev.revoke()
rev.invoke(5, 7) // undefined

revocable(func) ⇒ object

Write a function revocable that is generalized for any amount of arguments

Param	Type
func	function

Example

let rev = revocable(add)

rev.invoke(3, 4) // 7
rev.revoke()
rev.invoke(5, 7) // undefined

extract(array, prop) ⇒ array

Write a function extract that takes an array of objects and an object property name and converts each object in the array by extracting that property

Param	Type
array	array
prop	string

Example

let people = [{ name: 'john' }, { name: 'bob' }]
let names = extract(people, 'name') // ['john', 'bob']

m(value, source) ⇒ object

Write a function m that takes a value and an optional source string and returns them in an object

Param	Type
value	any
source	any

Example

m(1) // {value:1, source:"1"}

m(Math.PI, 'pi') // {value:3.14159..., source:"pi"}

addmTwo(m1, m2) ⇒ object

Write a function addmTwo that adds two m objects and returns an m object

Param	Type
m1	function
m2	function

Example

addmTwo(m(3), m(4)) // {value:7, source:"(3+4)"}

addmTwo(m(1, m(Math.PI, 'pi'))) // {value:4.14159..., source:"(1+pi)"}

addm(...ms) ⇒ object

Write a function addm that is generalized for any amount of arguments

Param	Type
...ms	function

Example

addm(m(1), m(2), m(4)) // {value:7, source:"(1+2+4)"}

liftmbM(binary, op) ⇒ object

Write a function liftmbM that takes a binary function and a string and returns a function that acts on m objects

Param	Type
binary	function
op	string

Example

let addmb = liftmbM(addb, '+')

addmb(m(3), m(4)) // {value:7, source:"(3+4)"}

liftmbM(mul, '*')(m(3), m(4)) // {value:12, source:"(3*4)"}

liftmb(binary, op) ⇒ object

Write a function liftmb that is a modified function liftmbM that can accept arguments that are either numbers or m objects

Param	Type
binary	function
op	string

Example

let addmb = liftmb(addb, '+')

addmb(3, 4) // {value:7, source:"(3+4)"}

liftm(func, op) ⇒ object

Write a function liftm that is generalized for any amount of arguments

Param	Type
func	function
op	string

Example

let addm = liftm(add, '+')

addm(m(3), m(4)) // {value:7, source:"(3+4)"}

liftm(mul, '*')(m(3), m(4)) // {value:12, source:"(3*4)"}

exp(value) ⇒ any

Write a function exp that evaluates simple array expressions

Param	Type
value	any

Example

let sae = [mul, 1, 2, 4]
exp(sae) // 1 * 2 * 4 = 8
exp(42) // 42

expn(value) ⇒ any

Write a function expn that is a modified exp that can evaluate nested array expressions

Param	Type
value	any

Example

let nae = [Math.sqrt, [add, [square, 3], [square, 4]]]

expn(nae) // sqrt(((3*3)+(4*4))) === 5

addg(value) ⇒ number | undefined

Write a function addg that adds from many invocations, until it sees an empty invocation

Param	Type
value	number

Example

addg() // undefined
addg(2)() // 2
addg(2)(7)() // 9
addg(3)(0)(4)() // 7
addg(1)(2)(4)(8)() // 15

liftg(binary) ⇒ function

Write a function liftg that will take a binary function and apply it to many invocations

Param	Type
binary	function

Example

liftg(mulb)() // undefined
liftg(mulb)(3)() // 3
liftg(mulb)(3)(0)(4)() // 0
liftg(mulb)(1)(2)(4)(8)() // 64

arrayg(value) ⇒ array

Write a function arrayg that will build an array from many invocations

Param	Type
value	any

Example

arrayg() // []
arrayg(3)() // [3]
arrayg(3)(4)(5)() // [3, 4, 5]

continuizeu(unary) ⇒ function

Write a function continuizeu that takes a unary function and returns a function that takes a callback and an argument

Param	Type
unary	function

Example

let sqrtc = continuizeu(Math.sqrt)
sqrtc(console.log, 81) // logs '9'

continuize(func) ⇒ function

Write a function continuize that takes a function and returns a function that takes a callback and arguments

Param	Type
func	function

Example

let mullc = continuize(mul)
mullc(console.log, 81, 4, 2) // logs '648'

vector()

Make an array wrapper object with methods get, store, and append, such that an attacker cannot get access to the private array

Example

let v = vector()
v.append(7)
v.store(1, 8)
v.get(0) // 7
v.get(1) // 8

exploitVector()

Let's assume your vector implementation looks like something like this:

let vector = () => {
  let array = []
  return {
    append: (v) => array.push(v),
    get: (i) => array[i],
    store: (i, v) => array[i] = v
  }
}

Can you spot any security concerns with this approach? Mainly, can we get access to the array outside of vector? Note*: the issue has nothing to do with prototypes and we can assume that global prototypes cannot be altered. Hint*: Think about using this in a method invocation. Can we override a method of vector?

Example

let v = vector()
v.append(1)
v.append(2)
let internalData = exploitVector(v) // [1, 2]

vectorSafe()

How would you rewrite vector to deal with the issue from above?

Example

let v = vectorSafe()
v.append(1)
v.append(2)
let internalData = exploitVector(v) // undefined

pubsub()

Make a function pubsub that makes a publish/subscribe object. It will reliably deliver all publications to all subscribers in the right order.

Example

let ps = pubsub()
ps.subscribe(console.log)
ps.publish('It works!') // logs 'It works!'

mapRecurse(array, callback) ⇒ array

Make a function mapRecurse that performs a transformation for each element of a given array, recursively

Param	Type
array	array
callback	function

Example

mapRecurse([1, 2, 3, 4], (x) => x * 2) // [ 2, 4, 6, 8 ]

filterRecurse(array, predicate) ⇒ array

Make a function filterRecurse that takes in an array and a predicate function and returns a new array by filtering out all items using the predicate, recursively.

Param	Type
array	array
predicate	function

Example

filterRecurse([1, 2, 3, 4], (x) => x % 2 === 0) // [ 2, 4 ]