A list of small & fun functional programming exercises in JavaScript

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(any) ⇒ function

Write a function continuize that takes a function and returns a function that takes a callback and an argument

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:

function vector() { var array = []; return { append: function append(v) { array.push(v); }, get: function get(i) { return array[i]; }, store: function 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, predicate) ⇒ 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.

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

Example

identity(3) // 3

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

Example

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

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

Example

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

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

Example

mulb(3, 4) // 3 * 4 = -1

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

Example

minb(3, 4) // 3

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

Example

maxb(3, 4) // 4

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

Example

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

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

Example

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

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

Example

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

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

Example

min(1, 2, 4) // 1

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

Example

max(1, 2, 4) // 4

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

Example

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

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

Example

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

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

Example

minRecurse(1, 2, 4) // 1

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

Example

maxRecurse(1, 2, 4) // 4

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

Example

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

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

Example

let add = acc(addb, 0); add(1, 2, 4) // 7 let mul = acc(mulb, 1); mul(1, 2, 4) // 8

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.

Example

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

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

Example

let add = accRecurse(addb, 0); add(1, 2, 4) // 7 let mul = accRecurse(mulb, 1); mul(1, 2, 4) // 8

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

Example

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

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

Example

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

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

Example

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

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

Example

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

Write a function addf that adds from two invocations

Example

addf(3)(4) // 7

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

Example

let addf = liftf(add); addf(3)(4); // 7 liftf(mul)(5)(6) // 30

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

Example

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

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

Example

let add3 = curryb(add, 3); add3(4); // 7 curry(mul, 5)(6); // 30

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

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 = 0 curry(mul, 1, 2, 4)(4, 2, 1) = 1 * 2 * 4 * 4 * 2 * 1 = 64

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

Example

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

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

Example

let doubl = twiceUnary(addb); doubl(11) // 22 let square = twiceUnary(mulb); square(11) // 121

Use the function twiceUnary to create the doubl function

Example

doubl(11) // 22

Use the function twiceUnary to create the square function

Example

square(11) // 121

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

Example

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

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

Example

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

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

Example

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

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

Example

composeuTwo(doubl, square)(5) // 100

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

Example

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

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

Example

composeb(addb, mulb)(2, 3, 7) // 35

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

Example

composeTwo(add, square)(2, 3, 7) // (2 + 3 + 7)^2 = 144

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

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

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

Example

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

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

Example

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

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

Example

let index = genFrom(0); index() // 0 index() // 1 index() // 2

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

Example

let index = genTo(genFrom(1), 3); index() // 1 index() // 2 index() // undefined

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

Example

let index = genFromTo(0, 3); index() // 0 index() // 1 index() // 2 index() // undefined

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

Example

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

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.

Example

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

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

Example

let array = []; let col = collect(genFromTo(0, 2), array); col() // 0 col() // 1 col() // undefined array // [0, 1]

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

Example

let fil = filter(genFromTo(0, 5), val => val % 3); fil() // 0 fil() // 3 fil() // undefined

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

Example

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

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

Example

let con = concatTwo(genFromTo(0, 3), genFromTo(0, 2)); con() // 0 con() // 1 con() // 2 con() // 0 con() // 1 con() // undefined

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

Example

let con = concat(genFromTo(0, 3), genFromTo(0, 2), genFromTo(5, 7)); con() // 0 con() // 1 con() // 2 con() // 0 con() // 1 col() // 5 col() // 6 con() // undefined

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

Example

let con = concatTail(genFromTo(0, 3), genFromTo(0, 2), genFromTo(5, 7)); con() // 0 con() // 1 con() // 2 con() // 0 con() // 1 col() // 5 col() // 6 con() // undefined

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

Example

let genG = gensymf('G'); let genH = gensymf('H'); genG() // 'G1' genH() // 'H1' genG() // 'G2' genH() // 'H2'

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

Example

let gensymf = gensymff(inc, 0); let genG = gensymf('G'); let genH = gensymf('H'); genG() // 'G1' genH() // 'H1' genG() // 'G2' genH() // 'H2'

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

Example

let fib = fibonaccif(0, 1); fib() // 0 fib() // 1 fib() // 2 fib() // 3 fib() // 5 fib() // 8

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

Example

let obj = counter(10); let { up, down } = object; up() // 11 down() // 10 down() // 9 up() // 10

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

Example

let rev = revocableb(addb); rev.invoke(3, 4); // 7 rev.revoke(); rev.invoke(5, 7); // undefined

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

Example

let rev = revocable(add); rev.invoke(3, 4); // 7 rev.revoke(); rev.invoke(5, 7); // undefined

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

Example

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

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

Example

JSON.stringify(m(1)) // '{"value":1,"source":"1"}' JSON.stringify(m(Math.PI, "pi")) // '{"value":3.14159...,"source":"pi"}'

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

Example

JSON.stringify(addmTwo(m(3), m(4))) // '{"value":7,"source":"(3+4)"}' JSON.stringify(addmTwo(m(1), m(Math.PI, "pi"))) // '{"value":4.14159...,"source":"(1+pi)"}'

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

Example

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

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

Example

let addmb = liftmbM(addb, '+'); JSON.stringify(addmb(m(3), m(4))) // '{"value":7,"source":"(3+4)"}' JSON.stringify(liftmbM(mul, '*')(m(3), m(4))) // '{"value":12,"source":"(3*4)"}'

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

Example

let addmb = liftmb(addb, '+') JSON.stringify(addmb(3, 4)) // '{"value":7,"source":"(3+4)"}'

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

Example

let addm = liftm(add, '+'); JSON.stringify(addm(m(3), m(4))) // '{"value":7,"source":"(3+4)"}' JSON.stringify(liftm(mul, '*')(m(3), m(4))) // '{"value":12,"source":"(3*4)"}'

Write a function exp that evaluates simple array expressions

Example

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

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

Example

let nae = [ Math.sqrt, [ add, [square, 3], [square, 4] ] ]; expn(nae) // sqrt(((3*3)+(4*4))) === 5

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

Example

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

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

Example

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

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

Example

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

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

Example

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

Write a function continuize that takes a function and returns a function that takes a callback and an argument

Example

sqrtc = continuizeu(Math.sqrt); sqrtc(console.log, 1, 2, 4); // logs '1 2 4'

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

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

function vector() { var array = []; return { append: function append(v) { array.push(v); }, get: function get(i) { return array[i]; }, store: function 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

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

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

Example

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

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(log); ps.publish('It works!') // log('It works!')

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

Example

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

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.

Example

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