Reduce()

Question 1

What is the reduce() method used for in JavaScript?

Answer 1

The reduce() method executes a reducer function on each element of the array, resulting in a single output value.

Question 2

How do you sum an array of numbers using reduce()?

Answer 2

numbers.reduce((acc, n) => acc + n, 0)

Question 3

How do you multiply all numbers in an array using reduce()?

Answer 3

numbers.reduce((acc, n) => acc * n, 1)

Question 4

How can you find the maximum value in an array using reduce()?

Answer 4

numbers.reduce((max, n) => n > max ? n : max, numbers[0])

Question 5

How can you find the minimum value in an array using reduce()?

Answer 5

numbers.reduce((min, n) => n < min ? n : min, numbers[0])

Question 6

How do you concatenate an array of strings using reduce()?

Answer 6

strings.reduce((acc, s) => acc + s, ‘’)

Question 7

How can you count elements in an array using reduce()?

Answer 7

arr.reduce(count => count + 1, 0)

Question 8

How do you convert an array of numbers to the sum of squares using reduce()?

Answer 8

numbers.reduce((acc, n) => acc + n * n, 0)

Question 9

How can you create a comma-separated string from an array using reduce()?

Answer 9

arr.reduce((acc, x) => acc ? acc + ‘,’ + x : x, ‘’)

Question 10

How do you reverse an array using reduce()?

Answer 10

arr.reduce((acc, x) => [x, …acc], [])

Question 11

How can you remove duplicates from an array using reduce()?

Answer 11

arr.reduce((acc, x) => acc.includes(x) ? acc : […acc, x], [])

Question 12

How do you count the frequency of elements in an array using reduce()?

Answer 12

arr.reduce((acc, x) => { acc[x] = (acc[x] || 0) + 1; return acc; }, {})

Question 13

How can you group objects by a property using reduce()?

Answer 13

arr.reduce((acc, obj) => { (acc[obj.key] ||= []).push(obj); return acc; }, {})

Question 14

How do you flatten a 2D array using reduce()?

Answer 14

arr.reduce((acc, x) => acc.concat(x), [])

Question 15

How can you find the average of numbers in an array using reduce()?

Answer 15

arr.reduce((acc, n, _, { length }) => acc + n/length, 0)

Question 16

How do you partition an array into even and odd numbers using reduce()?

Answer 16

arr.reduce((acc, n) => { acc[n%2?’odd’:’even’].push(n); return acc; }, {even:[], odd:[]})

Question 17

How can you find unique characters in a string using reduce()?

Answer 17

str.split(‘’).reduce((acc, c) => acc.includes(c) ? acc : acc + c, ‘’)

Question 18

How do you merge an array of objects into one using reduce()?

Answer 18

arr.reduce((acc, obj) => ({…acc, …obj}), {})

Question 19

How can you count vowels in a string using reduce()?

Answer 19

str.split(‘’).reduce((acc, c) => ‘aeiou’.includes(c)?acc+1:acc, 0)

Question 20

How do you find the longest word in an array using reduce()?

Answer 20

arr.reduce((longest, w) => w.length > longest.length ? w : longest, ‘’)

Question 21

How can you check if all elements are positive using reduce()?

Answer 21

arr.reduce((acc, n) => acc && n > 0, true)

Question 22

How do you convert an array of key-value pairs to an object using reduce()?

Answer 22

arr.reduce((obj,[k,v]) => (obj[k]=v,obj),{})

Question 23

How can you implement map() using reduce()?

Answer 23

arr.reduce((acc, x) => […acc, x*2], [])

Question 24

How can you implement filter() using reduce()?

Answer 24

arr.reduce((acc, x) => x>0?[…acc,x]:acc, [])

Question 25

How do you implement some() using reduce()?

Answer 25

arr.reduce((acc, x) => acc || x>0, false)

Question 26

How can you implement every() using reduce()?

Answer 26

arr.reduce((acc, x) => acc && x>0, true)

Question 27

How do you find the second largest number in an array using reduce()?

Answer 27

arr.reduce((acc, n) => {if(n>acc[0])return[n,acc[0]];if(n>acc[1])acc[1]=n;return acc;},[-Infinity,-Infinity])[1]

Question 28

How can you create a histogram of word lengths using reduce()?

Answer 28

words.reduce((acc,w)=>(acc[w.length]=(acc[w.length]||0)+1,acc),{})

Question 29

How do you compute running totals using reduce()?

Answer 29

arr.reduce((acc,x,i)=>(acc.push((acc[i-1]||0)+x),acc),[])

Question 30

How can you zip two arrays using reduce()?

Answer 30

[...Array(arr1.length)].reduce((acc,_,i)=>acc.concat([[arr1[i],arr2[i]]]),[])

Question 31

How do you shuffle an array using reduce()?

Answer 31

arr.reduce((acc,x,i)=>{const j=Math.floor(Math.random()*(i+1));[acc[i],acc[j]]=[acc[j],x];return acc;},[...arr])

Question 32

How can you implement compose() function using reduceRight?

Answer 32

funcs.reduceRight((acc,f)=>(...args)=>f(acc(...args)),x=>x)

Question 33

How do you deep flatten nested arrays using reduce()?

Answer 33

const flatten = arr => arr.reduce((acc,x)=>acc.concat(Array.isArray(x)?flatten(x):x),[])

Question 34

How can you calculate the factorial of n using reduce()?

Answer 34

[...Array(n).keys()].map(i=>i+1).reduce((acc,x)=>acc*x,1)

Question 35

How do you parse a query string into an object using reduce()?

Answer 35

str.split('&').reduce((acc,p)=>{let[k,v]=p.split('=');acc[k]=v;return acc;},{})

Question 36

How can you find min and max values in a single pass using reduce()?

Answer 36

arr.reduce((acc,n)=>({min:Math.min(acc.min,n),max:Math.max(acc.max,n)}),{min:Infinity,max:-Infinity})

Question 37

How do you chain promises sequentially using reduce()?

Answer 37

tasks.reduce((p,task)=>p.then(task),Promise.resolve())

Question 38

How can you create a frequency map for top K frequent elements using reduce()?

Answer 38

arr.reduce((acc,x)=>{acc[x]=(acc[x]||0)+1;return acc;},{})

Question 39

How do you convert a sentence into camelCase using reduce()?

Answer 39

sentence.split(' ').reduce((acc,word,i)=>acc+(i?word[0].toUpperCase()+word.slice(1):word.toLowerCase()),'')

Question 40

How can you create a trie structure from an array of words using reduce()?

Answer 40

words.reduce((trie,word)=>{let node=trie;word.split('').forEach(c=>node=node[c]=node[c]||{});node.end=true;return trie;}, {})

Question 41

What is a conceptual use of reduce() for custom debounce accumulator?

Answer 41

(Conceptual: combining inputs with reduce before emitting)