JavaScript Data Structure Interview Questions (and Answers)

In this article, we cover some of the most frequently asked questions about JavaScript data structure, related to arrays, strings, and objects, that are frequently asked during interviews at Tier 1 companies.

1. Custom sorting program in JS via Bubble Sort?

let unSortArr = [4,-1,34,09,-9,103]

const sortArr = (inputArr) => {
    for(let i=0; i<inputArr.length; i++)
    {
        for(let j=i+1; j<inputArr.length; j++)
        {
            let temp = inputArr[i];
            if(inputArr[i] > inputArr[j])
            {
                inputArr[i] = inputArr[j];
                inputArr[j] = temp;
            }
        }
    }
    return inputArr;
}

console.log(sortArr(unSortArr));

2. Write a program to test whether a string, word, or number is a palindrome?

Examples of palindromic words are: race car, sir.

const isPlaindrome = (inputChar) => {
    let str = inputChar.toString();
    let resultWord = '';
    for(let i=str.length-1; i>=0; i--)
    {
        resultWord += str[i];
    }
    return (resultWord == str) ? true : false;
}
console.log(isPlaindrome('racecar'))
console.log(isPlaindrome('abc'))
console.log(isPlaindrome(121))  

3. Write a program to check whether the value/target exists or not in an increasing array with time complexity O(log n)?

NOTE: For this you need to know the binary search algorithm.

const customInArray = (sortedArray, key) => {
    let start = 0;
    let end = sortedArray.length - 1;

    while (start <= end) {
        let middle = Math.floor((start + end) / 2);
        if (sortedArray[middle] === key) {
            return true;
        } else if (sortedArray[middle] < key) {
            start = middle + 1;
        } else {
            end = middle - 1;
        }
    }
 return false;
}

console.log(customInArray([1,3,5,6,9,14,29,57,89],29));

4. Write a program to find the total number of vowels in String?

const getVowelCount = (inputStr) => {
    let totalVowelCount = 0;
    for(let i=0; i<inputStr.length; i++)
    {
        let char = inputStr[i];
        if(char == 'a' || char == 'e' || char == 'i' || char == 'o' || char == 'u')
            totalVowelCount++;
    }
    return totalVowelCount;
}
console.log(getVowelCount('hello how are you today programiz website?'))

5. Write a program to generate factorials of an arbitrary number?

const getFactorial = (inputNum) => {
    let result = 1;
    for(let i=1; i<=inputNum; i++)
    {
        result *= i;
    }
    return result;
}
console.log(getFactorial(5));

6. Write a program to check whether a number is prime or not?

Prime numbers: those numbers that are divisible by themselves and only by 1.

const isPrime = (inputNum) => {
    let result = true;
    for(let i=2; i<inputNum; i++)
    {
        if(inputNum%i === 0)
            result = false;
            break;
    }
    return result;
}
console.log(isPrime(17));
console.log(isPrime(18));

7. Write a program to check whether the number is perfect or not?

Prime number: whose SUM of all the factors is equal to the expected value, the value of the factor itself.

const isPerfectNum = (inputNum) => {
    let result = true;
    let factSum = 0;
    for(let i=1; i<inputNum; i++)
    {
        if(inputNum % i == 0)
            factSum = factSum+i;
    }
    return (factSum == inputNum) ? true : false;
}
console.log(isPerfectNum(6));
console.log(isPerfectNum(10));
console.log(isPerfectNum(28));

8. Write a program to find duplicate numbers in an array of numbers?

const findDuplicateEle = (inputArr) => {
    let duplicateEleArr = [];
    let uniqueArr = [];
    for(let i=0; i<inputArr.length; i++)
    {
        if(!uniqueArr.includes(inputArr[i]))
            uniqueArr.push(inputArr[i])
        else
            duplicateEleArr.push(inputArr[i])
    }
    return duplicateEleArr;
}
console.log(findDuplicateEle([1,2,3,5,3,1,9]));    

9. How to remove duplicates from an array of integers?

const removeDuplicateEle = (inputArr) => {
    let uniqueArr = [];
    for(let i=0; i<inputArr.length; i++)
    {
        if(!uniqueArr.includes(inputArr[i]))
            uniqueArr.push(inputArr[i])
    }
    return uniqueArr;
}
console.log(removeDuplicateEle([1,2,3,5,3,1,9])); 

10. We have group of people in the form of array and you have to group people basis upon age ?

let peopleArr = [
    {name: 'A', age: 10},
    {name: 'B', age: 17},
    {name: 'C', age: 14},
    {name: 'D', age: 10},
];

let resultObj = {};
for(let i=0; i<peopleArr.length; i++)
{
    if(resultObj[peopleArr[i].age]){
       resultObj[peopleArr[i].age].push(peopleArr[i].name); 
    }else{
       resultObj[peopleArr[i].age] = [peopleArr[i].name]; 
    }
}
console.log(resultObj)