9. Coding Questions Practice

Coding questions are extremely important in frontend and JavaScript interviews.

Interviewers check:

• Problem-solving
• JavaScript fundamentals
• Array manipulation
• Logic building
• Optimization skills

Practice these questions regularly.

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Most Important Coding Questions

• Reverse String
• Palindrome
• Remove Duplicates
• Flatten Array
• Debounce Function
• Throttle Function
• Custom map()
• Custom filter()
• Custom reduce()
• Fibonacci
• Deep Clone
• Group By Object Key

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1. Reverse String

Reverse characters of a string.

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Example

Input:

"hello";

Output:

"olleh";

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Solution 1 — Using Built-in Methods

function reverseString(str) {
  return str.split("").reverse().join("");
}

console.log(reverseString("hello"));

---

Output

olleh;

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Explanation

• split("") converts string → array
• reverse() reverses array
• join("") converts array → string

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Solution 2 — Without Built-in Reverse

function reverseString(str) {
  let reversed = "";

  for (let i = str.length - 1; i >= 0; i--) {
    reversed += str[i];
  }

  return reversed;
}

console.log(reverseString("hello"));

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2. Palindrome

Palindrome reads same forward and backward.

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Example

madam;
racecar;

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Solution

function isPalindrome(str) {
  const reversed = str.split("").reverse().join("");

  return str === reversed;
}

console.log(isPalindrome("madam"));

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Output

true;

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3. Remove Duplicates from Array

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Example

Input:

[1, 2, 2, 3, 4, 4];

Output:

[1, 2, 3, 4];

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Solution Using Set

function removeDuplicates(arr) {
  return [...new Set(arr)];
}

console.log(removeDuplicates([1, 2, 2, 3, 4, 4]));

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Output

[1, 2, 3, 4];

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4. Flatten Array (VERY IMPORTANT)

Convert nested array into single array.

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Example

Input:

[1, [2, 3], [4, [5]]];

Output:

[1, 2, 3, 4, 5];

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Solution Using flat()

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

console.log(arr.flat(Infinity));

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Recursive Solution

function flattenArray(arr) {
  let result = [];

  for (let item of arr) {
    if (Array.isArray(item)) {
      result = result.concat(flattenArray(item));
    } else {
      result.push(item);
    }
  }

  return result;
}

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

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5. Debounce Function (VERY IMPORTANT)

Delays function execution until user stops triggering event.

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Example

function debounce(fn, delay) {
  let timer;

  return function (...args) {
    clearTimeout(timer);

    timer = setTimeout(() => {
      fn(...args);
    }, delay);
  };
}

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Common Uses

• Search bar
• API optimization
• Resize events

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6. Throttle Function (VERY IMPORTANT)

Limits function execution rate.

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Example

function throttle(fn, delay) {
  let lastCall = 0;

  return function (...args) {
    const now = Date.now();

    if (now - lastCall >= delay) {
      lastCall = now;

      fn(...args);
    }
  };
}

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Common Uses

• Scroll events
• Resize events
• Mouse movement

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Difference Between Debounce and Throttle

Debounce	Throttle
Waits before executing	Executes at intervals
Best for search input	Best for scroll events
Reduces unnecessary calls	Limits execution frequency

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7. Custom map() Polyfill (VERY IMPORTANT)

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Example

Array.prototype.myMap = function (callback) {
  const result = [];

  for (let i = 0; i < this.length; i++) {
    result.push(callback(this[i], i, this));
  }

  return result;
};

const nums = [1, 2, 3];

const output = nums.myMap((num) => num * 2);

console.log(output);

---

Output

[2, 4, 6];

---

8. Custom filter() Polyfill

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Example

Array.prototype.myFilter = function (callback) {
  const result = [];

  for (let i = 0; i < this.length; i++) {
    if (callback(this[i], i, this)) {
      result.push(this[i]);
    }
  }

  return result;
};

const nums = [1, 2, 3, 4];

const even = nums.myFilter((num) => num % 2 === 0);

console.log(even);

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Output

[2, 4];

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9. Custom reduce() Polyfill

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Example

Array.prototype.myReduce = function (callback, initialValue) {
  let accumulator = initialValue;

  for (let i = 0; i < this.length; i++) {
    accumulator = callback(accumulator, this[i], i, this);
  }

  return accumulator;
};

const nums = [1, 2, 3, 4];

const sum = nums.myReduce((acc, curr) => acc + curr, 0);

console.log(sum);

---

Output

10;

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10. Fibonacci Series

Each number is sum of previous two numbers.

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Example

0 1 1 2 3 5 8

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Solution

function fibonacci(n) {
  let result = [0, 1];

  for (let i = 2; i < n; i++) {
    result.push(result[i - 1] + result[i - 2]);
  }

  return result;
}

console.log(fibonacci(7));

---

Output

[0, 1, 1, 2, 3, 5, 8];

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11. Deep Clone (VERY IMPORTANT)

Creates completely independent object copy.

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Example

const user = {
  name: "Dipak",
  address: {
    city: "Kolkata",
  },
};

const deepCopy = structuredClone(user);

deepCopy.address.city = "Delhi";

console.log(user.address.city);

---

Output

Kolkata;

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Alternative Method

const copy = JSON.parse(JSON.stringify(user));

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Limitation

Does not properly copy:

• Functions
• Dates
• undefined

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12. Group By Object Key (VERY IMPORTANT)

Very common interview question.

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Example

Input:

const users = [
  { name: "Dipak", city: "Kolkata" },
  { name: "Rahul", city: "Delhi" },
  { name: "Amit", city: "Kolkata" },
];

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Expected Output

{
  Kolkata: [
    { name: "Dipak", city: "Kolkata" },
    { name: "Amit", city: "Kolkata" }
  ],

  Delhi: [
    { name: "Rahul", city: "Delhi" }
  ]
}

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Solution

function groupBy(arr, key) {
  return arr.reduce((acc, curr) => {
    const group = curr[key];

    if (!acc[group]) {
      acc[group] = [];
    }

    acc[group].push(curr);

    return acc;
  }, {});
}

const users = [
  { name: "Dipak", city: "Kolkata" },
  { name: "Rahul", city: "Delhi" },
  { name: "Amit", city: "Kolkata" },
];

console.log(groupBy(users, "city"));

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13. Count Character Frequency

Very common interview question.

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Example

Input:

"hello";

Output:

{
  h: 1,
  e: 1,
  l: 2,
  o: 1
}

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Solution

function countChars(str) {
  const result = {};

  for (let char of str) {
    result[char] = (result[char] || 0) + 1;
  }

  return result;
}

console.log(countChars("hello"));

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14. Find Maximum Number in Array

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Solution

const nums = [10, 20, 5, 40];

const max = Math.max(...nums);

console.log(max);

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Without Built-in Method

function findMax(arr) {
  let max = arr[0];

  for (let num of arr) {
    if (num > max) {
      max = num;
    }
  }

  return max;
}

console.log(findMax([10, 20, 5, 40]));

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15. Find Missing Number

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Example

Input:

[1, 2, 3, 5];

Output:

4;

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Solution

function findMissing(arr, n) {
  const total = (n * (n + 1)) / 2;

  const sum = arr.reduce((acc, curr) => acc + curr, 0);

  return total - sum;
}

console.log(findMissing([1, 2, 3, 5], 5));

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16. Check Anagram

Two strings contain same characters.

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Example

listen → silent

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Solution

function isAnagram(str1, str2) {
  const a = str1.split("").sort().join("");
  const b = str2.split("").sort().join("");

  return a === b;
}

console.log(isAnagram("listen", "silent"));

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Output

true;

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17. Reverse Number

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Example

Input:

1234

Output:

4321

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Solution

function reverseNumber(num) {
  return Number(num.toString().split("").reverse().join(""));
}

console.log(reverseNumber(1234));

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18. Capitalize First Letter of Each Word

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Example

Input:

hello world

Output:

Hello World

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Solution

function capitalize(str) {
  return str
    .split(" ")
    .map((word) => {
      return word[0].toUpperCase() + word.slice(1);
    })
    .join(" ");
}

console.log(capitalize("hello world"));

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Most Asked Coding Questions

Focus heavily on:

• Debounce
• Throttle
• Polyfills
• Flatten array
• Group by
• Array manipulation
• String problems

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Interview Tips

• Practice writing logic without copying
• Explain time complexity
• Explain approach before coding
• Write clean variable names
• Handle edge cases

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Best Practice for Preparation

Practice daily:

1. Array questions
2. Object manipulation
3. Async JavaScript
4. Polyfills
5. String problems
6. Output-based questions

These are repeatedly asked in frontend interviews.