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Javascript · Question Set

Arrays and Loops – Javascript interview questions

Arrays and Loops interview questions for placements and exams.

Subject: JavascriptDifficulty: Mixed15 questions in this set

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Javascript

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Javascript – Arrays and Loops questions (15)

Go through each question and its explanation. Use this set as a focused practice pack for Javascript.

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1. What does the forEach method return?

Difficulty: MediumType: MCQTopic: JS Arrays

The forEach method always returns undefined. It does not return a new array. ForEach is used for its side effects, like logging or updating DOM elements, not for transforming arrays. It executes a callback function for each element but does not create or return anything. If you need to transform an array, use map instead. You cannot break or continue out of forEach. Use a regular for loop if you need early exit.

Example code

const numbers = [1, 2, 3];

// forEach returns undefined
const result = numbers.forEach(num => num * 2);
console.log(result); // undefined

// Used for side effects
numbers.forEach(num => {
  console.log(num); // Just logging
});

// Accessing index and array
fruits.forEach((fruit, index, array) => {
  console.log(index, fruit, array.length);
});

// Compare with map (returns new array)
const doubled = numbers.map(num => num * 2);
console.log(doubled); // [2, 4, 6]

// Cannot break out of forEach
numbers.forEach(num => {
  // if (num === 2) break; // SyntaxError!
  console.log(num);
});

// Use for...of if you need break
for (const num of numbers) {
  if (num === 2) break; // Works!
  console.log(num);
}

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2. Explain the difference between find, findIndex, indexOf, and includes methods for searching arrays.

Difficulty: MediumType: SubjectiveTopic: JS Arrays

JavaScript provides several methods for searching arrays, each with different use cases. The find method returns the first element that satisfies a test function. It stops at the first match and returns the element itself. Returns undefined if nothing matches. Use find when you need the actual element and have complex search criteria. The findIndex method returns the index of the first element that satisfies a test function. Returns negative 1 if nothing matches. Use findIndex when you need the position, not the element. The indexOf method searches for a specific value using strict equality. Returns the index of the first occurrence, or negative 1 if not found. Use indexOf for simple value searches. It cannot search with conditions. The includes method checks if an array contains a specific value. Returns true or false. Use includes when you only need to know if something exists, not where it is. Key differences: Find and findIndex use callback functions for complex conditions. IndexOf and includes use direct value comparison. Find returns element, findIndex and indexOf return index, includes returns boolean. For objects, use find or findIndex because indexOf compares references, not properties.

Example code

const numbers = [5, 12, 8, 130, 44];

// find - returns element
const found = numbers.find(num => num > 10);
console.log(found); // 12

// findIndex - returns index
const index = numbers.findIndex(num => num > 10);
console.log(index); // 1

// indexOf - returns index (simple value)
const idx = numbers.indexOf(8);
console.log(idx); // 2
console.log(numbers.indexOf(100)); // -1 (not found)

// includes - returns boolean
const hasEight = numbers.includes(8);
console.log(hasEight); // true
console.log(numbers.includes(100)); // false

// Array of objects
const users = [
  { id: 1, name: 'John' },
  { id: 2, name: 'Jane' },
  { id: 3, name: 'Bob' }
];

// find - best for objects
const user = users.find(u => u.id === 2);
console.log(user); // { id: 2, name: 'Jane' }

// indexOf - doesn't work well for objects
const wrongIdx = users.indexOf({ id: 2, name: 'Jane' });
console.log(wrongIdx); // -1 (different reference!)

// findIndex - works with objects
const userIdx = users.findIndex(u => u.id === 2);
console.log(userIdx); // 1

// Check if user exists
const exists = users.some(u => u.name === 'Jane');
console.log(exists); // true

7. Explain the difference between for, while, and for of loops. When would you use each?

Difficulty: MediumType: SubjectiveTopic: JS Loops

JavaScript has multiple loop types suited for different scenarios. The for loop is best when you know the number of iterations in advance or need an index counter. It has initialization, condition, and increment in one line. Use for loops for iterating with indices, counting, or when you need fine control over iteration. The while loop is best when you do not know how many iterations are needed and want to continue until a condition becomes false. The condition is checked before each iteration. Use while loops for reading data until end, waiting for user input, or processing until a flag changes. The for of loop introduced in ES6 iterates directly over values of iterables like arrays, strings, maps, or sets. It gives you the values, not indices. Use for of when you need values but not indices, for cleaner and more readable code. It works with break and continue. For objects, use for in to iterate over properties, though Object keys or Object entries with for of is often cleaner. Choose based on: Whether you need indices. Whether iteration count is known. Whether you are working with arrays or objects.

Example code

// for loop - when you need index or known iterations
const arr = ['a', 'b', 'c'];
for (let i = 0; i < arr.length; i++) {
  console.log(i, arr[i]); // 0 'a', 1 'b', 2 'c'
}

// while loop - when iterations unknown
let data = getData();
while (data !== null) {
  process(data);
  data = getData(); // Keep going until null
}

// Practical while example
let attempts = 0;
let success = false;
while (!success && attempts < 3) {
  success = tryOperation();
  attempts++;
}

// for...of - when you need values, not indices
const fruits = ['apple', 'banana', 'cherry'];
for (const fruit of fruits) {
  console.log(fruit); // Clean and simple
}

// for...of with strings
for (const char of 'hello') {
  console.log(char); // h, e, l, l, o
}

// for...of with break
for (const num of [1, 2, 3, 4, 5]) {
  if (num === 3) break;
  console.log(num); // 1, 2
}

// for...in for objects (use carefully)
const person = { name: 'John', age: 30 };
for (const key in person) {
  console.log(key, person[key]);
}

// Better way for objects
for (const [key, value] of Object.entries(person)) {
  console.log(key, value);
}

13. Compare forEach, map, filter, and reduce. When should you use each?

Difficulty: MediumType: SubjectiveTopic: JS Arrays

These are the four most important array methods in JavaScript. Each serves a different purpose. ForEach executes a function for each element but returns undefined. Use it for side effects like logging, updating DOM, or sending data. It does not create a new array. You cannot break out of forEach. Map transforms each element and returns a new array of the same length. Use it when you need to convert or modify every element, like extracting properties, formatting data, or applying calculations. Original array stays unchanged. Filter creates a new array containing only elements that pass a test. Use it to select items matching criteria, remove unwanted elements, or find all matches. Returns empty array if nothing matches. Reduce accumulates array elements into a single value by applying a function. Use it for summing, finding max or min, flattening arrays, grouping data, or any operation combining all elements into one result. Takes accumulator and current value. When to use what: Use forEach for side effects without creating new array. Use map for transforming every element into new array. Use filter for selecting subset of elements. Use reduce for aggregating into single value. All methods are chainable and promote functional programming by avoiding mutations.

Example code

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

// forEach - side effects, returns undefined
let sum1 = 0;
numbers.forEach(num => {
  sum1 += num; // Updating external variable
});
console.log(sum1); // 15

// map - transform each, returns new array
const doubled = numbers.map(num => num * 2);
console.log(doubled); // [2, 4, 6, 8, 10]

// filter - select elements, returns new array
const evens = numbers.filter(num => num % 2 === 0);
console.log(evens); // [2, 4]

// reduce - accumulate to single value
const sum2 = numbers.reduce((acc, num) => acc + num, 0);
console.log(sum2); // 15

// Chaining methods
const result = numbers
  .filter(num => num > 2)     // [3, 4, 5]
  .map(num => num * 2)        // [6, 8, 10]
  .reduce((acc, num) => acc + num, 0); // 24

console.log(result); // 24

// Real world example
const users = [
  { name: 'John', age: 17, active: true },
  { name: 'Jane', age: 25, active: true },
  { name: 'Bob', age: 30, active: false }
];

// Get names of active adult users
const activeAdultNames = users
  .filter(user => user.active && user.age >= 18)
  .map(user => user.name);

console.log(activeAdultNames); // ['Jane']

14. Explain push, pop, shift, and unshift methods. What is the difference between them?

Difficulty: EasyType: SubjectiveTopic: JS Arrays

These are the four basic methods for adding and removing elements from arrays. Push adds one or more elements to the end of an array. It modifies the original array and returns the new length. Use push when building up an array or adding items to the end. Pop removes the last element from an array. It modifies the original array and returns the removed element. Use pop when you need to remove from the end, like implementing a stack. Shift removes the first element from an array. It modifies the original array, shifts all other elements down by one index, and returns the removed element. Use shift when you need to remove from the beginning, but note it is slower than pop. Unshift adds one or more elements to the beginning of an array. It modifies the original array, shifts all existing elements up by one index, and returns the new length. Use unshift to add to the beginning, but note it is slower than push. Performance note: Push and pop are fast operations. Shift and unshift are slower because they require reindexing all elements. For queue operations, consider using push and shift, or better yet, a proper queue data structure for large datasets.

Example code

const arr = [1, 2, 3];

// push - add to end
const newLength = arr.push(4, 5);
console.log(arr);       // [1, 2, 3, 4, 5]
console.log(newLength); // 5

// pop - remove from end
const last = arr.pop();
console.log(arr);  // [1, 2, 3, 4]
console.log(last); // 5

// unshift - add to beginning
const length2 = arr.unshift(0, -1);
console.log(arr);     // [-1, 0, 1, 2, 3, 4]
console.log(length2); // 6

// shift - remove from beginning
const first = arr.shift();
console.log(arr);   // [0, 1, 2, 3, 4]
console.log(first); // -1

// Stack (LIFO - Last In First Out)
const stack = [];
stack.push(1);    // Add to top
stack.push(2);
stack.push(3);
console.log(stack.pop());   // 3 (remove from top)

// Queue (FIFO - First In First Out)
const queue = [];
queue.push(1);    // Add to end
queue.push(2);
queue.push(3);
console.log(queue.shift()); // 1 (remove from beginning)

// All modify original array
const original = [1, 2, 3];
original.push(4);
console.log(original); // [1, 2, 3, 4] (changed)

15. Explain splice and slice methods. What is the difference and when would you use each?

Difficulty: MediumType: SubjectiveTopic: JS Arrays

Splice and slice are commonly confused because of similar names, but they do very different things. The splice method modifies the original array by adding, removing, or replacing elements. It takes start index, number of elements to delete, and optional elements to add. Returns an array of deleted elements. Use splice when you need to change the array in place. The slice method creates a shallow copy of a portion of an array. It takes start and end indices. It does not modify the original array. Returns a new array. Use slice when you need a copy without changing the original. Key differences: Splice mutates the array, slice does not. Splice can add and remove, slice only copies. Splice returns deleted elements, slice returns new array. Memory trick: Splice has p for permanent change. Slice has no p, so no permanent change. Common use cases: Splice for removing items, inserting items, or replacing items. Slice for copying arrays, getting subarrays, or implementing pagination.

Example code

// SPLICE - modifies original array
const arr1 = [1, 2, 3, 4, 5];

// Remove elements
const removed = arr1.splice(2, 2); // Start at index 2, remove 2 elements
console.log(arr1);    // [1, 2, 5] (modified!)
console.log(removed); // [3, 4] (deleted elements)

// Insert elements
arr1.splice(2, 0, 'a', 'b'); // Start at 2, remove 0, add 'a' and 'b'
console.log(arr1); // [1, 2, 'a', 'b', 5]

// Replace elements
arr1.splice(0, 2, 'x', 'y'); // Remove first 2, add 'x' and 'y'
console.log(arr1); // ['x', 'y', 'a', 'b', 5]

// Remove from end
const last = arr1.splice(-1); // Remove last element
console.log(last); // [5]

// SLICE - does NOT modify original
const arr2 = [1, 2, 3, 4, 5];

// Get portion
const portion = arr2.slice(1, 4); // From index 1 to 4 (not including 4)
console.log(portion); // [2, 3, 4]
console.log(arr2);    // [1, 2, 3, 4, 5] (unchanged!)

// Copy entire array
const copy = arr2.slice();
console.log(copy); // [1, 2, 3, 4, 5]

// Get last n elements
const lastTwo = arr2.slice(-2);
console.log(lastTwo); // [4, 5]

// Negative indices
const middle = arr2.slice(1, -1);
console.log(middle); // [2, 3, 4]

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Arrays and Loops – Javascript interview questions

Set overview

Javascript – Arrays and Loops questions (15)

1. What does the forEach method return?

Continue your preparation

2. Explain the difference between find, findIndex, indexOf, and includes methods for searching arrays.

3. Which method adds an element to the end of an array?

4. What does array.length return?

5. What does the find method return?

6. What does this code do: const [a, b] = [1, 2, 3]?

7. Explain the difference between for, while, and for of loops. When would you use each?

8. What does the map method do?

9. How do you create an array in JavaScript?

10. What is the correct syntax for a for loop?

11. What does the filter method return?

12. What does the reduce method do?

13. Compare forEach, map, filter, and reduce. When should you use each?

14. Explain push, pop, shift, and unshift methods. What is the difference between them?

15. Explain splice and slice methods. What is the difference and when would you use each?

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Arrays and Loops – Javascript interview questions

Set overview

1. What does the forEach method return?

Continue your preparation

2. Explain the difference between find, findIndex, indexOf, and includes methods for searching arrays.

3. Which method adds an element to the end of an array?

4. What does array.length return?

5. What does the find method return?

6. What does this code do: const [a, b] = [1, 2, 3]?

7. Explain the difference between for, while, and for of loops. When would you use each?

8. What does the map method do?

9. How do you create an array in JavaScript?

10. What is the correct syntax for a for loop?

11. What does the filter method return?

12. What does the reduce method do?

13. Compare forEach, map, filter, and reduce. When should you use each?

14. Explain push, pop, shift, and unshift methods. What is the difference between them?

15. Explain splice and slice methods. What is the difference and when would you use each?

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