Javascript small practical interview questions and answers

Javascript small practical interview questions and answers.

1. Reverse string JavaScript.

let name = "Ankit";

let revName = name.split("").reverse().join("");

console.log("revName---->", revName); // output : revName----> tiknA

let revNames = "";

for(let i = name.length-1; i >= 0; i--) {

    revNames += name[i];

}

console.log("revNames---->", revNames); // output : revNames----> tiknA

2. Remove duplicate string from array JavaScript.

let cityName = ["pune", "ahmedabad", "nagpur", "pune"];

let uniqName = [...new Set(cityName)];

console.log("uniqName--->", uniqName);

// output : uniqName---> [ 'pune', 'ahmedabad', 'nagpur' ]

let uniqeCityName = [];

let filterCityName = cityName.map(x => {

    uniqeCityName.indexOf(x) === -1 && uniqeCityName.push(x);

});

console.log("uniqeCityName--->", uniqeCityName);

// output : uniqeCityName---> [ 'pune', 'ahmedabad', 'nagpur' ]

3. Swapping of two numbers JavaScript.

let a = 10, b = 20, tmp;

tmp=a; a=b; b=tmp;

console.log("a,b--->", a,b);

// output : a,b---> 20 10

let swapNumber = [a,b] = [b,a]; // Destructuring es6 method

console.log("swapNumber---->", swapNumber);

// output : swapNumber----> [ 10, 20 ]

4. Find highest and Second highest JavaScript.

let numArray = [6,7,1,2,3,99,4,5,1,2,33,33,3];

let maxNumer = Math.max(...numArray);

console.log('maxNumer--->', maxNumer);

// output : maxNumer---> 99

let numArray = [6,7,1,2,3,99,4,5,1,2,33,33,3];

let highestNum, secondHigh;

numArray.map(x => {

    if(!highestNum) highestNum = x;

   

    if(highestNum && x > highestNum){

        secondHigh = highestNum ;

        highestNum = x; }

    else {

        if(x > secondHigh) secondHigh = x;

    }

});

console.log('highestNum, secondHigh--->', highestNum, secondHigh);

// output : highestNum, secondHigh---> 99 33

5. Sorting array in Ascending and Descending order JavaScript.

let numArray = [6,7,1,2,3,99,4,5,1,2,33,33,3];

// Sort in ascending order

let ascending = [...numArray].sort((a, b) => a - b);

console.log("ascending :", ascending);

// output : ascending : [ 1, 1, 2, 2,  3, 3, 4, 5, 6, 7, 33, 33, 99 ]

// Sort in descending order

let descending = [...numArray].sort((a, b) => b - a);

console.log("descending :", descending);

// output : descending : [ 99, 33, 33, 7, 6, 5, 4,  3, 3, 2, 2, 1, 1 ]

let sortString = ["mumbai", "pune", "ahmedabad", "botad"];

let ascStringSortFun = sortString.sort();

console.log("ascStringSortFun---->", ascStringSortFun);

// output : ascStringSortFun----> [ 'ahmedabad', 'botad', 'mumbai', 'pune' ]

let descStringSortFun = sortString.sort().reverse();

console.log("descStringSortFun---->", descStringSortFun);

// output : descStringSortFun----> [ 'pune', 'mumbai', 'botad', 'ahmedabad' ]

6. Find Even and Odd Numbers in an Array in JavaScript.

let numArray = [6,7,1,2,3,99,4,5,1,2,33,33,3];

let evenNum = numArray.filter(x => {

    return x % 2 == 0 && x;    

});

console.log('evenNum--->', evenNum);

// output : evenNum---> [ 6, 2, 4, 2 ]

let oddNum = numArray.filter(x => {    

    return x % 2 != 0 && x;

});

console.log('oddNum--->', oddNum);

// output : oddNum---> [7,  1,  3, 99, 5, 1, 33, 33, 3]

7. Unique Values - Remove duplicates in an Array JavaScript.

let numArray = [6,7,1,2,3,99,4,5,1,2,33,33,3];

let unqArr = [...new Set(numArray)];

console.log("unqArr--->", unqArr);

// output : unqArr---> [6, 7, 1, 2, 3, 99, 4, 5, 33]

let numArray = [6,7,1,2,3,99,4,5,1,2,33,33,3];

let uniqeArray = [];

let filterNumArray = numArray.map(x => {

    uniqeArray.indexOf(x) === -1 && uniqeArray.push(x);

});

console.log("uniqeArray--->", uniqeArray);

// output : uniqeArray---> [ 6, 7, 1,  2, 3, 99, 4, 5, 33]

8. Find unique values from two array JavaScript.

let num1 = [1,22,33,44,55,99];

let num2 = [55,66,22];

let uniqData = [...new Set([...num1, ...num2])];

console.log('uniqData--->', uniqData);

// output : uniqData---> [1, 22, 33, 44, 55, 99, 66]

9. Find, Map, Filter and Reduce JavaScript.

// find : return only one value, otherwise return undeifned.

// map : return : true, false and array data

// filter : condition match the return array data otherwise return [] array

// reduce : use for mathematic operation

let x = [1, 2, 33, 44, 55, 66];

let findData = x.find((val) => { return val > 44 });

console.log("findData--->", findData); // return single value

// output : findData---> 55

let mapData = x.map((val, index) => { return val + 100; });

console.log("mapData--->", mapData); // return array

// output : mapData---> [ 101, 102, 133, 144, 155, 166 ]

let filterData = x.filter((val, index, array) => { return val > 22; });

console.log("filterData---->", filterData); // return array  

// output : filterData----> [ 33, 44, 55, 66 ]

let reduceSumData = x.reduce((total, val, index, array) => { return total + val; });

console.log("reduceSumData--->", reduceSumData); // return number value

// output : reduceSumData---> 201

10. Assume the output of example in JavaScript.

// ********example 1**********

console.log("1");

setTimeout(() => {

    console.log("2");

},0);

setTimeout(() => {

    console.log("3");

});

console.log("4");

// output : 1 4 2 3

// ********example 2**********

function logger(a,b,a) {

    console.log('a,b,a--->', a,b,a);

}

logger(1,2,3,4);

// output : a,b,a---> 3 2 3

// ********example 3**********

let aa = 5;

console.log('aa++--->',aa++);

// output : aa++---> 5

aa = 5;

console.log('++aa--->',++aa);

// output : ++aa---> 6

let bb = 5;

console.log('bb-- --->',bb--);

// output : bb-- ---> 5

bb = 5;

console.log('--bb --->',--bb);

// output : --bb ---> 4

// ********example 4**********

console.log(1);

for(let i=2;i<=10;i++){

  console.log(i);

}

setTimeout(() => {

  console.log(11)

}, 3000)

let promise = new Promise((resolve, reject) => {

  setTimeout(() => {

    resolve('success')

  }, 500);

})

setTimeout(() => {

  console.log(12)

}, 500)

promise.then(res => {

  console.log(res)

})

console.log(13);

// output : 1 2 3 4 5 6 7 8 9 10 13 success 12 11

11. Callbacks - Promises - Async/Await in JavaScript.

// *******callback********

function test3(callback) {

    console.log('test3---->');

    callback();

}

function test4() {

    console.log('test4---->');

}

test3(test4);

// output :

// test3---->

// test4---->

// *******Promises***********

function test8() {

    console.log('test8--->');    

    return new Promise((resolve, reject) => {        

        resolve();

        // reject('reject error---->');

    })

}

function test9() {    

    console.log('test9--->');

}

test8().then(test9).catch(e => console.log('e--->', e));

// output :

// test8--->

// test9--->

// *******Async/Await********

async function test1() {

    console.log('test1---->');

    await test2();

}

function test2() {

    console.log('test2---->');

}

test1();

// output :

// test1---->

// test2---->

12. Event loop in JavaScript.

// ****event loop******

function test5() {

    console.log('test5---->');    

}

function test6() {

    setTimeout(() => {

        console.log('test6---->'); // execute after 5 second

    }, 5000);    

}

function test7() {

    console.log('test7---->');

}

test5();

test6();

test7();

// output :

// test5---->

// test7---->

// test6---->

13. Event loop in JavaScript. Implement deepMerge, Arrays → concatenate, Objects → recursive merge

const a = {

  user: { name: "Akshay", roles: ["admin"] },

  settings: { theme: "dark" },

};

const b = {

  user: { roles: ["editor"], age: 25 },

  settings: { lang: "en" },

};

console.log(deepMerge(a, b));

// output :

{

  user: { name: "Akshay", roles: ["admin", "editor"], age: 25 },

  settings: { theme: "dark", lang: "en" }

}

// Solution :

function deepMerge(obj1, obj2) {

  // If both are arrays → concatenate

  if (Array.isArray(obj1) && Array.isArray(obj2)) {

    return [...obj1, ...obj2];

  }

  // If both are objects → recursive merge

  if (isObject(obj1) && isObject(obj2)) {

    const result = {};

    const keys = new Set([...Object.keys(obj1), ...Object.keys(obj2)]);

    for (const key of keys) {

      if (key in obj1 && key in obj2) {

        result[key] = deepMerge(obj1[key], obj2[key]);

      } else if (key in obj1) {

        result[key] = obj1[key];

      } else {

        result[key] = obj2[key];

      }

    }

    return result;

  }

  // Primitive values → obj2 overrides obj1

  return obj2;

}

function isObject(value) {

  return value !== null && typeof value === "object" && !Array.isArray(value);

}

14. Correct and solve this function

function getUser(id) {

  return setTimeout(() => {

    return { id: id, name: "John" };

  }, 500);

}

function getOrders(userId) {

  return setTimeout(() => {

    return [

      { id: 1, amount: 200 },

      { id: 2, amount: 300 },

    ];

  }, 500);

}

async function getUserData() {

  const user = await getUser(1);

  const orders = await getOrders(1);

  console.log({

    user,

    orders,

  });

}

getUserData();

// Solution

function getUser(id) {

  return new Promise((resolve) => {

    setTimeout(() => {

      resolve({ id: id, name: "John" });

    }, 500);

  });

}

function getOrders(userId) {

  return new Promise((resolve) => {

    setTimeout(() => {

      resolve([

        { id: 1, amount: 200 },

        { id: 2, amount: 300 },

      ]);

    }, 500);

  });

}

async function getUserData() {

  const user = await getUser(1);

  const orders = await getOrders(1);

  console.log("user---->", user);

  console.log("orders---->", orders);

  // Combined Output:

  // console.log({ user, orders });

}

getUserData();

// Output ::

// user----> { id: 1, name: "John" }

// orders----> [ { id: 1, amount: 200 }, { id: 2, amount: 300 } ]

15. Flat this array without any inbuilt method

const arr = [1, 2, [3, [4, 5], 6], 7];

function flattenArray(input) {

  let result = [];

  function helper(element) {

    // If the element is NOT an array → push directly

    if (typeof element !== "object") {

      result.push(element);

      return;

    }

    // Manually loop array elements (no built-ins)

    for (let i = 0; i < element.length; i++) {

      helper(element[i]);

    }

  }

  helper(input);

  return result;

}

console.log(flattenArray(arr));

// Output: [1, 2, 3, 4, 5, 6, 7]