JS Hot Question

Recently while preparing for a javascript interview I revised foundational questions of javascript and shared a few goods with you.

1. Check if an object is an array.

let arr = [1,2,3,4,5]

// METHOD 1
console.log(toString.call(arr) === "[object Array]"); // true
console.log(toString.apply(arr) === "[object Array]"); // true
console.log(toString.apply(arr) === "[object Array]"); // true
console.log(toString.bind(arr)() === "[object Array]"); // true


// METHOD 2
console.log(Array.isArray(arr)); // true


// METHOD 3
console.log(Object.prototype.toString.call(arr) === "[object Array]");
// true

2. Clone an array.

// Changing clone array would not affect original array
let arr = [10, 20, 30, 40, 50]

// METHOD 1
let clone_arr = arr.slice()
clone_arr[1] = 1
console.log(`arr: ${arr}  | clone_array: ${clone_arr}`); 
// arr: 10,20,30,40,50  | clone_array: 10,1,30,40,50


// METHOD 2
let clone_arr1 = Array.from(arr)
clone_arr1[1] = 1
console.log(`arr: ${arr}  | clone_array1: ${clone_arr1}`); 
// arr: 10,20,30,40,50  | clone_array1: 10,1,30,40,50 


// METHOD 3
let clone_arr2 = [... arr]
clone_arr2[1] = 1
console.log(`arr: ${arr}  | clone_array2: ${clone_arr1}`); 
// arr: 10,20,30,40,50  | clone_array2: 10,1,30,40,50


// OTHERS::
let clone_arr3 = arr.map(i => i)
let clone_arr4 = arr.filter(i => i)
let clone_arr5 = arr.filter(() => true)

3. Return first 'n' elements of array

const firsts = (arr, k=1) => {
    console.log(k);
    if(k>=0){
        return arr.slice(0,k);
    }
    return []
}

console.log(firsts([7, 9, 0, -2])); // [7]
console.log(firsts([],3)); // []
console.log(firsts([7, 9, 0, -2],3)); // [7, 9, 0]
console.log(firsts([7, 9, 0, -2],6)); // [ 7, 9, 0, -2 ]
console.log(firsts([7, 9, 0, -2],-3)); // []

4. Return the last 'n' elements of an array

const lasts = (arr, k=1) => {
    console.log(k);
    if(k>=0){
        return arr.slice(arr.length-k);
    }
    return []
}

console.log(lasts([7, 9, 0, -2])); // [2]
console.log(lasts([7, 9, 0, -2],3)); // [9, 0, 02]
console.log(lasts([7, 9, 0, -2],6)); // [ 0, -2 ]
console.log(lasts([7, 9, 0, -2],-3)); // []

5. Add hyphens between two evens

const addHyphenBetweenTwoEvens = (nums) => {
    nums = Array.from(nums.toString());
    let result = [nums[0]]
    for(let x=1; x<nums.length; x++)
    {
        if((nums[x-1]%2 === 0)&&(nums[x]%2 === 0))
        {
            result.push('-', nums[x]);
        }
        else
        {
            result.push(nums[x]);
        }
    }
    return result.join("");
}

console.log(addHyphenBetweenTwoEvens("025468")); //0-254-6-8

6. program to find the most frequent item of an array

const mostFrequent = (arr) => {
    let m = new Map();
    for (let x of arr){
        if(m.has(x)){
            m.set(x, m.get(x)+1)
        }
        else{
            m.set(x, 1)
        }
    }
    console.log(Math.max(... m.values()));
    let mx = Math.max(... m.values())
    const mxVal = [...m].find(([k, v]) => v == mx );
    return `${mxVal[0]} ( ${mxVal[1]} times )`
}

console.log(mostFrequent([3, 'a', 'a', 'a', 2, 3, 'a', 3, 'a', 2, 4, 9, 3]));

// TC = SC = O(n), where n is the length of the input array.

7. Swap the case of each character

const caseSwapping = (string) => {
    let revStr = ""
    for(let x=0; x<string.length; x++){
        if(string[x] == string[x].toUpperCase()){
            revStr += string[x].toLowerCase()
        }
        else{
            revStr += string[x].toUpperCase()
        }
    }
    return revStr
}

console.log(caseSwapping("The Quick Brown Fox"));

// TC = SC = O(n)

8. Program to find the sum of squares of a numeric vector

const sumOfSquares = (arr) => {
    let sums = arr.map(x => x**2)
                  .reduce((a,b) => a+b, 0)

    return sums
}

console.log(sumOfSquares([0,1,2,3,4])) // 30

9. Sum of each index value of two or more arrays

let array1 = [1,0,2,3,4];
let array2 = [3,5,6,7,8,13];

let mx = Math.max(array1.length, array2.length)
let res = []
for(let x=0; x<mx; x++){
    if(array1[x] && array2[x]){

        res.push( array1[x] + array2[x])
    }
    else if(array1[x]){
        res.push(array1[x])
    }
    else if(array2[x]){
        res.push(array2[x])
    }
}
console.log(res); //[ 4, 5, 8, 10, 12, 13 ]

10. Shuffle an array

let randomizeArr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
randomizeArr.sort((a, b) => 0.5-Math.random(), 0)
console.log(randomizeArr); // [ 7, 6, 9, 5, 1, 0, 4, 8, 3, 2 ]

11. Binary search

let arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];

const binarySearch = (arr, k) => {
    let start = 0, end = arr.length-1;
    while(start<end){
        let mid = parseInt((end - start) / 2)
        if(arr[mid] < k)
            start = mid+1
        else if(arr[mid] > k)
            end = mid-1
        else
            return mid 
    }
}

console.log(binarySearch(arr, 3));

12. Return all leap years under the given range

const isLeapYear = (year) => {
    if( (year % 4 === 0 && year%100 !==0) 
        || (year%100 === 0 && year%400 === 0)
    ){
        return year
    }
    return false
}
const leapYearRange = (startDate, endDate) => {
    let res = []
    for(let x=startDate; x<=endDate; x++){
        if(isLeapYear(x))
            res.push(x)
    }
    return res
}

console.log(leapYearRange(2000, 2012)); // [ 2000, 2004, 2008, 2012 ]

13. Flatten an array

const flatten = (arr, shallow, res) => {
    if(!res) res = []

    if(shallow) {return res.concat.apply(res, a);}

    for(let x=0; x<arr.length; x++){
        if (Array.isArray(arr[x])){
            flatten(arr[x], shallow, res)
        }else{
            res.push(arr[x])
        }
    }
    return res
} 
let arr = [1, [2], [3, [[4]]],[5,6]]
console.log(flatten([1, [2], [3, [[4]]],[5,6]])); // [ 1, 2, 3, 4, 5, 6 ]


// A LAME METHOD
console.log(arr.flat(10)) // [ 1, 2, 3, 4, 5, 6 ]

//ANOTHER AMAZING METHOD ⚡
console.log(arr.toString().split(",").map(Number));// [ 1, 2, 3, 4, 5, 6 ]

14. Union of two array / find the unique elements from two arrays

let arr1 = [1,2,3,3]
let arr2 = [4,5,6]

// METHOD 1
let res = [... new Set(arr1.concat(arr2))]
res; // [ 1, 2, 3, 4, 5, 6 ]


// METHOD 2
let res1 = []
arr1.forEach(x => !res1.includes(x) ? res1.push(x): "")
arr2.forEach(x => !res1.includes(x) ? res1.push(x): "")
res1 // [ 1, 2, 3, 4, 5, 6 ]

15. Find the difference of two arrays.

let arr1 = [1,2,3,3]
let arr2 = [4,[[5]],6]

const difference = (arr1, arr2) => {
    let res = []

    // flatten array (precautions🩺)
    arr1 = arr1.toString().split(',').map(Number)
    arr2 = arr2.toString().split(',').map(Number)

    arr1.forEach(x => !arr2.includes(x) ? res.push(x): "")
    arr2.forEach(x => !arr1.includes(x) ? res.push(x): "")

    return res
}

console.log(difference([1, 2,3, 3], [4,[[5]],6])) // [ 3, 100, 10 ] 

console.log(difference([1, 2, 3], [100, 2, 1, 10])) // [ 3, 100, 10 ] 

console.log(difference([1, 2, 3, 4, 5], [1, [2], [3, [[4]]],[5,6]])); // [6]

16. Function to remove. 'null', '0', '""', 'false', 'undefined' and 'NaN' values from an array

const cleansing = (arr) => {

    // 1st WAY
    /**
     * let res = []
     * for(let x of arr){
     *     if(x){
     *         res.push(x);
     *     }
     * }
     * return res;
     * 
     */


    //2nd WAY

    return arr.filter(i => i)
}

console.log(cleansing([NaN, 0, 15, false, -22, '',undefined, 47, null]));
//[ 15, -22, 47 ]

17. Sort the specified array of objects by title value

var library = [ 
    { author: 'Bill Gates', title: 'The Road Ahead', libraryID: 1254},
    { author: 'Steve Jobs', title: 'Walter Isaacson', libraryID: 4264},
    { author: 'Suzanne Collins', title: 'Mockingjay: The Final Book of The Hunger Games', libraryID: 3245}
]

library.sort((a, b) => {
    if(a.title < b.title) return -1
    if(a.title > b.title) return 1
    return 0
})
library
/*
[ { author: 'Suzanne Collins',
    title: 'Mockingjay: The Final Book of The Hunger Games',
    libraryID: 3245 },
  { author: 'Bill Gates',
    title: 'The Road Ahead',
    libraryID: 1254 },
  { author: 'Steve Jobs',
    title: 'Walter Isaacson',
    libraryID: 4264 } ]

*/

18. Pair Sum

const pairSum = (arr, k) => {
    let m = {}
    for (const [i, x] of arr.entries()){
        if(k-x in m){
            return [m[k-x], i].toString()
        }
        else{
            m[x] = i
        }
    }
}

console.log(pairSum([1,2,3,4,5], 5)); // [1,2]

19. Retrieve the value of a given property from all elements in an array

const library = [ 
    { author: 'Bill Gates', title: 'The Road Ahead', libraryID: 1254},
    { author: 'Steve Jobs', title: 'Walter Isaacson', libraryID: 4264},
    { author: 'Suzanne Collins', title: 'Mockingjay: The Final Book of The Hunger Games', libraryID: 3245}
    ];


console.log(library.map(x => Object.values(x)));
// OUTPUT
// [ [ 'Bill Gates', 'The Road Ahead', 1254 ],
//   [ 'Steve Jobs', 'Walter Isaacson', 4264 ],
//   [ 'Suzanne Collins','Mockingjay: The Final Book of The Hunger Games', 3245 ]
// ]

20. Fill the array

const fillArray = (start, end, freq) => {
    let res= []
    if (typeof start === "string"){
        start = start.charCodeAt(0)
        end = end.charCodeAt(0)
        for(let i=start; i< end; i+= freq){
            res.push(String.fromCharCode(i));
        }
    }
    else{
        for(let i=start; i< end; i+= freq){
            res.push(i);
        }
    }
    return res;
}

console.log(fillArray(1,10,2)) // [ 1, 3, 5, 7, 9 ]
console.log(fillArray('a','z',2)) // [ 'a', 'c', 'e', 'g', 'i', 'k', 'm', 'o', 'q', 's', 'u', 'w', 'y' ]

21. Remove specific element from array

let arr = [2, 5, 9, 6], k= 5;
arr.splice(arr.indexOf(k),1);
console.log(arr); // [ 2, 9, 6 ]

22. Return an array of specified length, filled with integer numbers, increase by one from starting position

// 1st WAY : SIMPLE FOR LOOP AND ADD VALUE TO ARRAY
const fillArray = (start, freq) => {
    let res = []
    for(let i=start; i<start+freq; i++){
        res.push(i)
    }
    return res
}

console.log(fillArray(1,4)); //[ 1, 2, 3, 4 ]
console.log(fillArray(-6,4)); // [ -6, -5, -4, -3 ]


// 2nd WAY : MANIPULATE HOW Array.from USUABILITY ⚡
const fillArray1 = (start, freq) => {
    return Array.from({length:freq}, (v, i) => i+start); 
}

console.log(fillArray1(1,4)); //[ 1, 2, 3, 4 ]
console.log(fillArray1(-6,4)); // [ -6, -5, -4, -3 ]

23. Return an array between two integers of 1 step length

const fillArray = (start, end) => {
    return Array.from({length: end-start+1}, (v, i)=> i+start)
}

console.log(fillArray(1,4)); //[ 1, 2, 3, 4 ]
console.log(fillArray(-6,4)); // [ -6, -5, -4, -3, -2, -1, 0, 1, 2, 3, 4 ]

24. Find the unique elements from two arrays

const findUnique = (arr1, arr2) => {
    arr1 = arr1.toString().split(',').map(Number);
    arr2 = arr2.toString().split(',').map(Number);
    console.log( [... new Set(arr1.concat(arr2))] );
}

findUnique([1, 2, 3], [100, 2, 1, 10]) // [ 1, 2, 3, 100, 10 ]
findUnique([1, 2, 3, 4, 5], [1, [2], [3, [[4]]],[5,6]])//[ 1, 2, 3, 4, 5, 6 ]

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

function unzip1(arr) {
  let res = [];
  let maxLen = Math.max(... arr.map(a => a.length))
  for(let i=0; i< maxLen; i++){
    res.push(arr.map(a => a[i]))
  }
  return res
}

console.log(unzip([['a', 1, true], ['b', 2, false]])) 
// [ [ 'a', 'b' ], [ 1, 2 ], [ true, false ] ] 
console.log(unzip([['a', 1, true], ['b', 2]])) 
// [ [ 'a', 'b' ], [ 1, 2 ], [ true, undefined ] ]

26. Remove falsy values from object

function cleaning(input){
  const data = Array.isArray(input) ? input.filter(Boolean): input;
  return Object.keys(data).reduce((acc, key) =>{
    const value = data[key]
    if(Boolean(value)){
      acc[key] = typeof value == 'object' ? cleaning(value) : value;
    }
    return acc
  },
  Array.isArray(input) ? [] : {}
  );
}


const obj = {
  a: null,
  b: false,
  c: true,
  d: 0,
  e: 1,
  f: '',
  g: 'a',
  h: [null, false, '', true, 1, 'a'],
  i: { j: 0, k: false, l: 'a' }
}

let arr = [1,3,''. null, false, 9, 0]

console.log(cleaning(arr)); // [ 1, 3, 9 ] 
console.log(cleaning(obj)); 
//{ c: true, e: 1, g: 'a', h: [ true, 1, 'a' ], i: { l: 'a' } }

27. Take an array of integers and returns false if every number is not prime.

const isAllPrime = (arr) => {

  const isPrime = (x) =>{
    for(let i=2; i <= Math.sqrt(x); i++){
      if(x%i === 0)
      return false
    }
    return x > 1
  }
  return arr.map(isPrime).every(Boolean)
}

let arr = [2,3,5,7]
console.log(isAllPrime([2,3,5,7]))// true
console.log(isAllPrime([2,3,5,7,8]))// false

28. Return the third smallest number

let arr = [2,3,5,7,1]
const thirdSmallest= (arr) => arr.length > 2 ? arr.sort((a,b) => a-b)[2] : -1

console.log(thirdSmallest([2,3,5,7,1])); //3
console.log(thirdSmallest([2,3,0,5,7,8,-2,-4])); // 0

29. Return sum of all numbers in array

const sumUp = arr => arr.reduce((total, curr) => {
  total = typeof curr === 'number' ? total + curr : total;
  return total
}, 0)

console.log(sumUp(['',10,1,2,'3'])); // 13
console.log(sumUp([2, "11", 3, "a2", false, 5, 7, 1])); // 18
console.log(sumUp([2, 3, 0, 5, 7, 8, true, false])); // 25

30. Get all indexes where NaN is found

const indexesOfNaN = (arr) => {
  let res = []
  for(let [i,x] of arr.entries()){
    if (isNaN(x))
      res.push(i)
  }


  return res
}

console.log(indexesOfNaN([2, NaN, 8, 16, 32])); // [1]
console.log(indexesOfNaN([2, 4, NaN, 16, 32, NaN])); // [2,5]
console.log(indexesOfNaN([2, 4, 16, 32])); // []

31. Count of the number of arrays inside given array

let ls = [2,8,[6],3,3,5,3,4,[5,4]]
const getCountOfInternalArrays = (ls) => ls.filter(
                                    l => Array.isArray(l)
                                ).length
console.log(getCountOfInternalArrays(ls)); // 2

Please do let me know if you solve with a better solution, OR If you find these solutions better then let me know in the comments 💌