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For dedicated programmers looking to learn javascript. All the the core concepts of JavaScript can be found here with coding challenges to solidify your knowledge.

"Get only questions here "

Best of luck on your coding journey ; )

improvement contributions are appreciated.

I am sharing here some important questions and solution below for practice:

1. Merge Two Objects in JavaScript

Write a function that takes two objects and returns a new object with combined key-value pairs. If both objects contain the same key, the value from the second object should overwrite the value in the first object.

Example

Given two objects:

const obj1 = { a: 1, b: 2, c: 3 };
const obj2 = { b: 2, c: 4, d: 5 };

The merged output should be:

{ a: 1, b: 2, c: 4, d: 5 }

Solution

Way 1: Using a for...in Loop
function mergeObjects(obj1, obj2) {
    for (let key in obj2) {
        obj1[key] = obj2[key];
    }
    return obj1;
}

const mergedResult1 = mergeObjects(obj1, obj2);
console.log(mergedResult1); // Output: { a: 1, b: 2, c: 4, d: 5 }
Way 2: Using the Spread Operator
function mergeObjects1(obj1, obj2) {
    return { ...obj1, ...obj2 };
}

const mergedResult2 = mergeObjects1(obj1, obj2);
console.log(mergedResult2); // Output: { a: 1, b: 2, c: 4, d: 5 }
Way 3: Using the Object.assign
function mergeObjects3(obj1, obj2) {
    return Object.assign({}, obj1, obj2)
}

// console.log(mergeObjects3(obj1, obj2));  // Output: { a: 1, b: 2, c: 4, d: 5 }

2. Transform Array of Objects in JavaScript

Convert an array of objects, where each object contains an id and name property, into an object where each key is an id from the original array, and the corresponding value is an object with the name property.

Example

Input

let arr1 = [{ id: 1, name: 'A' }, { id: 2, name: 'B' }];

Output

{ 1: { name: 'A' }, 2: { name: 'B' } }

Solution

Way 1: Using a for...of Loop
function transformArrayOfObject(arr) {
    let obj = {};
    for (const element of arr) {
        obj[element.id] = { name: element.name };
    }
    return obj;
}

console.log(transformArrayOfObject(arr1)); 
// Output: { 1: { name: 'A' }, 2: { name: 'B' } }
Way 2: Using Array.prototype.reduce
function transformArrayOfObject1(arr) {
    return arr.reduce((acc, item) => {
        acc[item.id] = { name: item.name };
        return acc;
    }, {});
}

console.log(transformArrayOfObject1(arr1)); 
// Output: { 1: { name: 'A' }, 2: { name: 'B' }

3. Convert Array of Strings to Object Format in JavaScript

Convert an array of strings, such as ["apple", "banana", "cherry"], into an object where each string becomes a key, and each key has a default value of 0.

Example

Input

let arr2 = ["apple", "banana", "cherry"];

Output

{ "apple": 0, "banana": 0, "cherry": 0 }

Solution

Way 1: Using a for...of Loop
function arrayToObject(arr) {
    let res = {};
    for (const element of arr) {
        res[element] = 0;
    }
    return res;
}

console.log(arrayToObject(arr2)); 
// Output: { "apple": 0, "banana": 0, "cherry": 0 }
Way 2: Using Array.prototype.reduce
function arrayToObject1(arr) {
    return arr.reduce((acc, item) => {
        acc[item] = 0;
        return acc;
    }, {});
}

console.log(arrayToObject1(arr2)); 
// Output: { "apple": 0, "banana": 0, "cherry": 0 }

4. Array Mapping

Write a function to take an array of numbers and return an array of squared values..

Example

Input

let arr3= [1,2,3,4,5];

Output

[ 1, 4, 9, 16, 25 ]

Solution

function arrayMap(arr) {
    return arr.map((data) => {
        return data * data;
    });
}

console.log(arrayMap(arr3)); 
// Output: [1, 4, 9, 16, 25]

5. Group by Property in JavaScript

Given an array of students, group them by their city.

Example

Input

let arr4 = [
    { name: 'A', age: 20, city: 'BLR' },
    { name: 'B', age: 20, city: 'DEL' },
    { name: 'C', age: 21, city: 'BLR' }
];

Output

{
    "BLR": [{ name: 'A', age: 20 }, { name: 'C', age: 21 }],
    "DEL": [{ name: 'B', age: 20 }]
}

Solution

function groupByCategory(arr, property) {
    return arr.reduce((acc, item) => {
        if (!acc[item[property]]) {
            acc[item[property]] = [];
        }
        acc[item[property]].push({ name: item.name, age: item.age });
        return acc;
    }, {});
}

console.log(groupByCategory(arr4, 'city')); 
// Output: { "BLR": [{ name: 'A', age: 20 }, { name: 'C', age: 21 }], "DEL": [{ name: 'B', age: 20 }] }

6. Find Duplicates in Array

Write a function to identify duplicates in an array and return them as a new array.

Example

Input

let arr5= [1,2,3,1,3,4,2,5];

Output

[ 1, 2, 3 ]

Solution

Way 1: Using two for loop Loop
function findDuplicate(arr){
    let res=[];
    for(let i=0; i<arr.length; i++){
        for(let j=i+1; j<arr.length; j++){
            if(arr[i]===arr[j]){
                res.push(arr[i]);
            }
        }
    }
    return res;
}
console.log( findDuplicate(arr5));
Way 2: Using hashmap with object
function findDuplicate1(arr){
    let res= {}
    arr.forEach(element => {
        if(!res[element]){
            res[element]= 1;
        }
        else{
            res[element]= res[element]+1;
        }
    });

    return res;
}

7. Count Frequency of Elements

Write a function to count the frequency of each element in an array and return an object:

Example

Input

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

Output

{ '1': 1, '2': 2, '3': 3, '4': 1 }

Solution

Way 1: Using HashMap
function countFrequency(arr) {
    let frequencyMap = {};
    for (let element of arr) {
        if (frequencyMap[element]) {
            frequencyMap[element]++;
        } else {
            frequencyMap[element] = 1;
        }
    }
    return frequencyMap;
}

let arr = [1, 2, 2, 3, 3, 3, 4];
console.log(countFrequency(arr));

8. Longest String in Array

Write a function to find and return the first occurrence of the longest string in an array of strings.

Example

Input

const strings = ["apple", "banana", "cherry", "orange", "grape"];

Output

"banana"

Solution

Way 1:
function findFirstLongestStr(arr) {
  let longest = ""; 
  for (let data of arr) {
    if (data.length > longest.length) {
      longest = data;
    }
  }
  return longest;
}
Way 2:
function findLongestStr(arr){
	return arr.reduce((acc, data)=>{
  	if(data.length>acc.length){
    	return data;
    }
    else{
    	return acc;
    }
  }, "")
}

9. Write a function to count the number of vowels in a string.

Example

Given two objects:

const str = "Bittu bittu"

Output

4   // ('i', 'u', 'i', 'u')

Solution

Way 1: Using a for...of Loop
function countVowels(str) {
    let count = 0;
    for (let char of str) {
        if ("aeiou".includes(char.toLowerCase())) {
            count++;
        }
    }
    return count;
}

console.log(countVowels(str)); // Output: 4
Way 2: Using reduce
function countVowels(str) {
    return str.split("").reduce((count, char) => {
        if ("aeiou".includes(char.toLowerCase())) {
            count++;
        }
        return count;
    }, 0);
}

console.log(countVowels(str)); // Output: 4

10. Implement a function to flatten a nested array.

We can solve this in different way ( for..of, reduce, flat, flatMap)

Example

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

Output

[1, 2, 3, 4, 5]

Solution

Way 1: Using a for...of Loop
function flattenArr(arr) {
  let res = []
  for (let data of arr) {
    if (Array.isArray(data)) {
      let flatData = flattenArr(data)
      res.push(...flatData)
    } else {
      res.push(data)
    }
  }
  return res
}

console.log(flattenArr(arr))  // [1, 2, 3, 4, 5]
Way 2: Using reduce
function flattenArr(arr){
  return arr.reduce((acc, data)=>{
    if(Array.isArray(data)){
      return acc.concat(flattenArr(data));
    }
    else{
    return acc.concat(data)
    }
  }, [])
}

console.log(flattenArr(arr))  // [1, 2, 3, 4, 5]
Way 3: Using reduce
console.log(arr.flat(Infinity))  // [1, 2, 3, 4, 5]

11. Combine Arrays Without Duplicates

Write a function that merges two arrays into one, ensuring there are no duplicate values.

Example

mergeArrays([1, 2, 3], [2, 3, 4])

should return

[1, 2, 3, 4]

Solution

let arr1= [1,2,3];
let arr2= [2,3,4];

function mergeArrays(arr1, arr2){
  return [...new Set(arr1.concat(arr2))]
}

console.log(mergeArrays(arr1, arr2))

12. Write a function to remove specified keys from an object.

Example

Input

 { a: 1, b: 2, c: 3 }, Keys to remove: ['b', 'c']

Output

{ a: 1 }

Solution

Way 1: Using a Object.keys and reduce
function keysToRemove(obj, keys) {
  return Object.keys(obj).reduce((res, key) => {
    if (!keys.includes(key)) {
      res[key] = obj[key];
    }
    return res;
  }, {});
}

let obj = { a: 1, b: 2, c: 3 };
let keys = ['b', 'c'];
console.log(keysToRemove(obj, keys)); // { a: 1 }
Way 2: Using for...in
function keysToRemove(obj, keys) {
  const res = {}; 
  for (const key in obj) { 
    if (!keys.includes(key)) { 
      res[key] = obj[key]; 
    }
  }
  return res;
}

let obj = { a: 1, b: 2, c: 3 };
let keys = ['b', 'c'];
console.log(keysToRemove(obj, keys)); // { a: 1 }

13. Write a function to check if two strings are anagrams of each other.

Example

 isAnagram('listen', 'silent') Should return true

Solution

Way 1: Using a Object.keys and reduce
function anagramChecker(str1, str2) {
    if (str1.length !== str2.length) {
        return false;
    }

    const formatStr = str => str.toLowerCase().split("").sort().join("");

    return formatStr(str1) === formatStr(str2);
}

console.log(anagramChecker('listen', 'silent')) // true

14. Find the missing number in an array containing n−1 integers, where the numbers are in the range 1 to n.

Example

 [1, 2, 4, 5]

Should return

3

Solution

Way 1:
function findMissing(arr){
  arr.sort((a, b) => a - b);
	for(let i=0;i<arr.length; i++){
  	if(arr[i]!==i+1){
    	return i+1;
    }
  }
  return arr.length + 1;
}


let arr= [1,2,4,5];
let arr1= [1,2,3,4,5]

console.log(findMissing(arr))  // 3
console.log(findMissing(arr1)) // 6
Way 2:

we can get sum of all number in that range (1 to max) and then get sum of input array- total sum - array sum

15. Majority Element- Find the element that appears more than n/2 times in an array. If no such element exists, return null.

.

Example

 [3, 3, 4, 2, 3, 3] should return 3 
 
 As n= arr.length= 6, element count should be greater than n/2
 
 First will find all numbers count using hashmap, then will check whose count is greater than n/2

Solution

Way 1:
let arr = [3, 3, 4, 2, 3, 3]

function findMajorityElement(array) {
  let majorityThreshold = array.length / 2

  let count = array.reduce((acc, ele) => {
    acc[ele] = (acc[ele] || 0) + 1
    return acc
  }, {})

  for (let element in count) {
    if (count[element] > majorityThreshold) {
      return parseInt(element)
    }
  }
  return null
}

console.log(findMajorityElement(arr)) // 3