Mastering JavaScript Interview Questions: Data Structures and Algorithms

Do you have a JavaScript interview coming up? Are you feeling nervous about answering questions related to data structures and algorithms? Don’t worry! In this article, we will cover some common JavaScript interview questions that are commonly asked in interviews. We will explore the internals of popular data structures, design algorithms, and provide code examples to help you understand the concepts better. So, let’s get started!

1. What are data structures and algorithms?

Before we dive into the interview questions, let’s quickly understand what data structures and algorithms are. Data structures are a way of organizing and storing data in computer memory. They define the relationship between the data, what operations can be performed on the data, and the rules for accessing and manipulating the data.

Algorithms, on the other hand, are step-by-step procedures or sets of rules for solving a specific problem. They define how the data structures are accessed, modified, and manipulated to solve a problem efficiently.

2. What is the difference between an array and a linked list?

Arrays and linked lists are both linear data structures, but they have some key differences.

An array is a contiguous block of memory that stores a fixed-size sequence of elements. It allows random access to any element using an index, which makes accessing and updating elements efficient. However, inserting or deleting elements in the middle of an array requires shifting all subsequent elements, making it inefficient.

A linked list, on the other hand, is a collection of nodes, where each node contains a value and a reference to the next node in the list. It allows efficient insertion and deletion of elements at any position as it only requires updating the references. However, accessing an element in a linked list requires traversing the list from the beginning, which can be inefficient for large lists.

3. Explain the working of a hash table.

A hash table is a data structure that implements an associative array abstract data type. It uses a hash function to compute an index, also known as a hash code, into an array of buckets or slots, from which the desired value can be found.

When a value is inserted into a hash table, its key is input into the hash function, which computes its hash code. The hash code is then used as an index to store the value in the corresponding bucket. If two keys result in the same hash code, a collision occurs. To resolve collisions, various techniques like separate chaining or open addressing are used.

Hash tables provide efficient lookup, insertion, and deletion operations, making them suitable for applications where the key-value pairs need to be stored and retrieved quickly.

Here’s an example of how a hash table can be implemented in JavaScript:

class HashTable {
  constructor(size) {
    this.size = size;
    this.buckets = new Array(size);
  }

  hash(key) {
    let hash = 0;
    for (let i = 0; i < key.length; i++) {
      hash += key.charCodeAt(i);
    }
    return hash % this.size;
  }

  insert(key, value) {
    const index = this.hash(key);
    if (!this.buckets[index]) {
      this.buckets[index] = [];
    }
    this.buckets[index].push({ key, value });
  }

  get(key) {
    const index = this.hash(key);
    if (!this.buckets[index]) {
      return null;
    }
    for (let item of this.buckets[index]) {
      if (item.key === key) {
        return item.value;
      }
    }
    return null;
  }
}

const hashtable = new HashTable(10);
hashtable.insert("name", "John");
hashtable.insert("age", 25);
console.log(hashtable.get("name")); // Output: John
console.log(hashtable.get("age")); // Output: 25
console.log(hashtable.get("city")); // Output: null

4. Implementing a stack using an array

A stack is a linear data structure that follows the LIFO (Last-In-First-Out) principle. It can be easily implemented using an array. Here’s an example of how to implement a stack in JavaScript:

class Stack {
  constructor() {
    this.stack = [];
  }

  push(element) {
    this.stack.push(element);
  }

  pop() {
    if (this.stack.length === 0) {
      return "Stack is empty.";
    }
    return this.stack.pop();
  }

  peek() {
    if (this.stack.length === 0) {
      return "Stack is empty.";
    }
    return this.stack[this.stack.length - 1];
  }

  isEmpty() {
    return this.stack.length === 0;
  }

  size() {
    return this.stack.length;
  }
}

const stack = new Stack();
stack.push(1);
stack.push(2);
stack.push(3);
console.log(stack.peek()); // Output: 3
console.log(stack.pop()); // Output: 3
console.log(stack.isEmpty()); // Output: false
console.log(stack.size()); // Output: 2

These are just a few examples of the kind of questions you might encounter in a JavaScript interview related to data structures and algorithms. It’s important to not only understand the concepts but also practice implementing them with code examples.

Remember to study the time and space complexities of different algorithms and their applications to make informed decisions during the interview. Good luck with your preparations and ace that interview!