Data Structures - 33 JavaScript Concepts

Why does finding an item in an array take longer as it grows? Why can you look up an object property instantly regardless of how many properties it has? The answer lies in data structures.

// Array: searching gets slower as the array grows
const users = ['alice', 'bob', 'charlie', /* ...thousands more */]
users.includes('zara')  // Has to check every element - O(n)

// Object: lookup is instant regardless of size
const userMap = { alice: 1, bob: 2, charlie: 3, /* ...thousands more */ }
userMap['zara']  // Direct access - O(1)

A data structure is a way of organizing data so it can be used efficiently. As Donald Knuth emphasizes in The Art of Computer Programming, the choice of data structure often matters more than the choice of algorithm. The right structure makes your code faster and cleaner. The wrong one can make simple operations painfully slow.

What you’ll learn in this guide:

JavaScript’s built-in structures: Array, Object, Map, Set, WeakMap, WeakSet
When to use each built-in structure
How to implement: Stack, Queue, Linked List, Binary Search Tree
Choosing the right data structure for the job
Common interview questions and patterns

Prerequisites: This guide shows time complexity (like O(1) and O(n)) for operations. If you’re not familiar with Big O notation, check out our Algorithms & Big O guide first. We also use classes for implementations.

What Are Data Structures?

Think of data structures like different ways to organize a library. You could:

Stack books on a table — Easy to add/remove from the top, but finding a specific book means digging through the pile
Line them up on a shelf — Easy to browse in order, but adding a book in the middle means shifting everything
Organize by category with an index — Finding any book is fast, but you need to maintain the index

┌─────────────────────────────────────────────────────────────────────────┐
│                    DATA STRUCTURE TRADE-OFFS                             │
├─────────────────────────────────────────────────────────────────────────┤
│                                                                          │
│   ARRAY                    OBJECT/MAP               LINKED LIST          │
│   ┌─┬─┬─┬─┬─┐              ┌────────────┐           ┌───┐   ┌───┐        │
│   │0│1│2│3│4│              │ key: value │           │ A │──►│ B │──►     │
│   └─┴─┴─┴─┴─┘              │ key: value │           └───┘   └───┘        │
│                            └────────────┘                                │
│   ✓ Fast index access      ✓ Fast key lookup       ✓ Fast insert/delete │
│   ✓ Ordered                ✓ Flexible keys         ✗ Slow search        │
│   ✗ Slow insert in middle  ✗ No order (Object)     ✗ No index access    │
│                                                                          │
└─────────────────────────────────────────────────────────────────────────┘

Every data structure has trade-offs. Your job is to pick the one that makes your most frequent operations fast.

JavaScript’s Built-in Data Structures

JavaScript gives you several data structures out of the box. Let’s look at each one.

Arrays

An Array is an ordered collection of values, accessed by numeric index. It’s the most common data structure in JavaScript.

const fruits = ['apple', 'banana', 'cherry']

// Access by index - O(1)
fruits[0]  // 'apple'

// Add to end - O(1)
fruits.push('date')  // ['apple', 'banana', 'cherry', 'date']

// Remove from end - O(1)
fruits.pop()  // 'date'

// Add to beginning - O(n) - shifts all elements!
fruits.unshift('apricot')  // ['apricot', 'apple', 'banana', 'cherry']

// Search - O(n)
fruits.indexOf('banana')  // 3
fruits.includes('mango')  // false

Time Complexity:

Operation	Method	Complexity	Why
Access by index	`arr[i]`	O(1)	Direct memory access
Add/remove at end	`push()`, `pop()`	O(1)	No shifting needed
Add/remove at start	`unshift()`, `shift()`	O(n)	Must shift all elements
Search	`indexOf()`, `includes()`	O(n)	Must check each element
Insert in middle	`splice()`	O(n)	Must shift elements after

When to use Arrays:

You need ordered data
You access elements by position
You mostly add/remove from the end
You need to iterate over all elements

Objects

An Object stores key-value pairs where keys are strings or Symbols. It’s JavaScript’s fundamental way to group related data.

const user = {
  name: 'Alice',
  age: 30,
  email: '[email protected]'
}

// Access - O(1)
user.name        // 'Alice'
user['age']      // 30

// Add/Update - O(1)
user.role = 'admin'

// Delete - O(1)
delete user.email

// Check if key exists - O(1)
'name' in user           // true
user.hasOwnProperty('name')  // true

Limitations of Objects:

Keys are converted to strings (numbers become “1”, “2”, etc.)
Objects have a prototype chain (inherited properties)
No built-in .size property
As defined in the ECMAScript specification, property order is preserved in ES2015+, but with specific rules: integer keys are sorted numerically first, then string keys appear in insertion order

When to use Objects:

Storing entity data (user profiles, settings)
When keys are known strings
Configuration objects
JSON data

Map

A Map is like an Object but with superpowers: keys can be any type, it maintains insertion order, and has a .size property.

const map = new Map()

// Keys can be ANY type
map.set('string', 'works')
map.set(123, 'number key')
map.set({ id: 1 }, 'object key')
map.set(true, 'boolean key')

// Access - O(1)
map.get('string')  // 'works'
map.get(123)       // 'number key'

// Size is built-in
map.size  // 4

// Check existence - O(1)
map.has('string')  // true

// Delete - O(1)
map.delete(123)

// Iteration (maintains insertion order)
for (const [key, value] of map) {
  console.log(key, value)
}

Map vs Object:

Feature	Map	Object
Key types	Any	String or Symbol
Order	Guaranteed insertion order	Preserved (integer keys sorted first)
Size	`map.size`	`Object.keys(obj).length`
Iteration	Directly iterable	Need `Object.keys()`
Performance	Better for frequent add/delete	Better for static data
Prototype	None	Has prototype chain

When to use Map:

Keys aren’t strings (objects, functions, etc.)
You need to know the size frequently
You add/delete keys often
Order matters

// Common use: counting occurrences
function countWords(text) {
  const words = text.toLowerCase().split(/\s+/)
  const counts = new Map()
  
  for (const word of words) {
    counts.set(word, (counts.get(word) || 0) + 1)
  }
  
  return counts
}

countWords('the cat and the dog')
// Map { 'the' => 2, 'cat' => 1, 'and' => 1, 'dog' => 1 }

Set

A Set stores unique values. Duplicates are automatically ignored.

const set = new Set()

// Add values - O(1)
set.add(1)
set.add(2)
set.add(2)  // Ignored - already exists
set.add('hello')

set.size  // 3 (not 4!)

// Check existence - O(1)
set.has(2)  // true

// Delete - O(1)
set.delete(1)

// Iteration
for (const value of set) {
  console.log(value)
}

The classic use case: removing duplicates

const numbers = [1, 2, 2, 3, 3, 3, 4]
const unique = [...new Set(numbers)]  // [1, 2, 3, 4]

Set Operations (ES2024+):

These methods are part of ES2024 and are supported in all modern browsers as of late 2024. Check browser compatibility if you need to support older browsers.

const a = new Set([1, 2, 3])
const b = new Set([2, 3, 4])

// Union: elements in either set
a.union(b)  // Set {1, 2, 3, 4}

// Intersection: elements in both sets
a.intersection(b)  // Set {2, 3}

// Difference: elements in a but not in b
a.difference(b)  // Set {1}

// Symmetric difference: elements in either but not both
a.symmetricDifference(b)  // Set {1, 4}

// Subset check
new Set([1, 2]).isSubsetOf(a)  // true

When to use Set:

You need unique values
You check “does this exist?” frequently
Removing duplicates from arrays
Tracking visited items

WeakMap and WeakSet

WeakMap and WeakSet (Advanced)

WeakMap and WeakSet are special versions where keys (WeakMap) or values (WeakSet) are held “weakly.” This means they don’t prevent garbage collection.WeakMap:

Keys must be objects (or non-registered symbols)
If the key object has no other references, it gets garbage collected
Not iterable (no .keys(), .values(), .forEach())
No .size property

const privateData = new WeakMap()

class User {
  constructor(name, password) {
    this.name = name
    // Store private data that can't be accessed externally
    privateData.set(this, { password })
  }
  
  checkPassword(input) {
    return privateData.get(this).password === input
  }
}

const user = new User('Alice', 'secret123')
user.name  // 'Alice'
user.password  // undefined - it's private!
user.checkPassword('secret123')  // true

// When 'user' is garbage collected, the private data is too

WeakSet:

Values must be objects
Useful for tracking which objects have been processed

const processed = new WeakSet()

function processOnce(obj) {
  if (processed.has(obj)) {
    return  // Already processed
  }
  
  processed.add(obj)
  // Do expensive processing...
}

When to use Weak versions:

Caching computed data for objects
Storing private instance data
Tracking objects without preventing garbage collection

Implementing Common Data Structures

JavaScript doesn’t have built-in Stack, Queue, or Linked List classes, but they’re easy to implement and important to understand.

Stack (LIFO)

A Stack follows Last-In-First-Out: the last item added is the first removed. Think of a stack of plates.

┌─────────────────────────────────────────────────────────────────────────┐
│                              STACK (LIFO)                                │
├─────────────────────────────────────────────────────────────────────────┤
│                                                                          │
│     push(4)        pop()                                                 │
│        │             │                                                   │
│        ▼             ▼                                                   │
│      ┌───┐         ┌───┐                                                 │
│      │ 4 │ ◄─ top  │   │                                                 │
│      ├───┤         ├───┤                                                 │
│      │ 3 │         │ 3 │ ◄─ top                                          │
│      ├───┤         ├───┤                                                 │
│      │ 2 │         │ 2 │                                                 │
│      ├───┤         ├───┤                                                 │
│      │ 1 │         │ 1 │                                                 │
│      └───┘         └───┘                                                 │
│                                                                          │
│   "Last in, first out" - like a stack of plates                          │
│                                                                          │
└─────────────────────────────────────────────────────────────────────────┘

Real-world uses:

Browser history (back button)
Undo/redo functionality
Function call stack
Expression evaluation (parentheses matching)

Implementation:

class Stack {
  constructor() {
    this.items = []
  }
  
  push(item) {
    this.items.push(item)
  }
  
  pop() {
    return this.items.pop()
  }
  
  peek() {
    return this.items[this.items.length - 1]
  }
  
  isEmpty() {
    return this.items.length === 0
  }
  
  size() {
    return this.items.length
  }
}

// Usage
const stack = new Stack()
stack.push(1)
stack.push(2)
stack.push(3)
stack.peek()   // 3 (look at top without removing)
stack.pop()    // 3
stack.pop()    // 2
stack.size()   // 1

Time Complexity: All operations are O(1).

Queue (FIFO)

A Queue follows First-In-First-Out: the first item added is the first removed. Think of a line at a store. According to the Stack Overflow Developer Survey 2023, queues and other fundamental data structures remain among the most commonly tested topics in technical interviews.

┌─────────────────────────────────────────────────────────────────────────┐
│                              QUEUE (FIFO)                                │
├─────────────────────────────────────────────────────────────────────────┤
│                                                                          │
│   enqueue(4)                                        dequeue()            │
│       │                                                 │                │
│       ▼                                                 ▼                │
│     ┌───┬───┬───┬───┐                             ┌───┬───┬───┐          │
│     │ 4 │ 3 │ 2 │ 1 │  ───────────────────────►   │ 4 │ 3 │ 2 │          │
│     └───┴───┴───┴───┘                             └───┴───┴───┘          │
│     back          front                           back      front        │
│                                                                          │
│   "First in, first out" - like a line at a store                         │
│                                                                          │
└─────────────────────────────────────────────────────────────────────────┘

Real-world uses:

Task scheduling
Print queue
BFS graph traversal
Message queues

Implementation:

class Queue {
  constructor() {
    this.items = []
  }
  
  enqueue(item) {
    this.items.push(item)
  }
  
  dequeue() {
    return this.items.shift()  // Note: O(n) with arrays!
  }
  
  front() {
    return this.items[0]
  }
  
  isEmpty() {
    return this.items.length === 0
  }
  
  size() {
    return this.items.length
  }
}

// Usage
const queue = new Queue()
queue.enqueue('first')
queue.enqueue('second')
queue.enqueue('third')
queue.dequeue()  // 'first'
queue.front()    // 'second'

Performance note: Using shift() on an array is O(n) because all remaining elements must be re-indexed. For performance-critical code, use a linked list implementation or an object with head/tail pointers.

Linked List

A Linked List is a chain of nodes where each node points to the next. Unlike arrays, elements aren’t stored in contiguous memory.

┌─────────────────────────────────────────────────────────────────────────┐
│                            LINKED LIST                                   │
├─────────────────────────────────────────────────────────────────────────┤
│                                                                          │
│   head                                                     tail          │
│     │                                                        │           │
│     ▼                                                        ▼           │
│   ┌──────────┐     ┌──────────┐     ┌──────────┐     ┌──────────┐       │
│   │ value: 1 │     │ value: 2 │     │ value: 3 │     │ value: 4 │       │
│   │ next: ───────► │ next: ───────► │ next: ───────► │ next: null│      │
│   └──────────┘     └──────────┘     └──────────┘     └──────────┘       │
│                                                                          │
│   Nodes can be anywhere in memory - connected by references              │
│                                                                          │
└─────────────────────────────────────────────────────────────────────────┘

Linked List vs Array:

Operation	Array	Linked List
Access by index	O(1)	O(n)
Insert at beginning	O(n)	O(1)
Insert at end	O(1)	O(1) with tail pointer
Insert in middle	O(n)	O(1) if you have the node
Search	O(n)	O(n)

Implementation:

class Node {
  constructor(value) {
    this.value = value
    this.next = null
  }
}

class LinkedList {
  constructor() {
    this.head = null
    this.size = 0
  }
  
  // Add to beginning - O(1)
  prepend(value) {
    const node = new Node(value)
    node.next = this.head
    this.head = node
    this.size++
  }
  
  // Add to end - O(n)
  append(value) {
    const node = new Node(value)
    
    if (!this.head) {
      this.head = node
    } else {
      let current = this.head
      while (current.next) {
        current = current.next
      }
      current.next = node
    }
    this.size++
  }
  
  // Find a value - O(n)
  find(value) {
    let current = this.head
    while (current) {
      if (current.value === value) {
        return current
      }
      current = current.next
    }
    return null
  }
  
  // Convert to array for easy viewing
  toArray() {
    const result = []
    let current = this.head
    while (current) {
      result.push(current.value)
      current = current.next
    }
    return result
  }
}

// Usage
const list = new LinkedList()
list.prepend(1)
list.append(2)
list.append(3)
list.prepend(0)
list.toArray()  // [0, 1, 2, 3]
list.find(2)    // Node { value: 2, next: Node }

When to use Linked Lists:

Frequent insertions/deletions at the beginning
You don’t need random access by index
Implementing queues (for O(1) dequeue)

Binary Search Tree

A Binary Search Tree (BST) is a hierarchical structure where each node has at most two children. The left child is smaller, the right child is larger.

┌─────────────────────────────────────────────────────────────────────────┐
│                        BINARY SEARCH TREE                                │
├─────────────────────────────────────────────────────────────────────────┤
│                                                                          │
│                              ┌────┐                                      │
│                              │ 10 │  ◄─ root                             │
│                              └────┘                                      │
│                             /      \                                     │
│                        ┌────┐      ┌────┐                                │
│                        │ 5  │      │ 15 │                                │
│                        └────┘      └────┘                                │
│                       /    \           \                                 │
│                  ┌────┐  ┌────┐      ┌────┐                              │
│                  │ 3  │  │ 7  │      │ 20 │                              │
│                  └────┘  └────┘      └────┘                              │
│                                                                          │
│   Rule: left child < parent < right child                                │
│   This makes searching fast: just go left or right!                      │
│                                                                          │
└─────────────────────────────────────────────────────────────────────────┘

Time Complexity:

Operation	Average	Worst (unbalanced)
Search	O(log n)	O(n)
Insert	O(log n)	O(n)
Delete	O(log n)	O(n)

Implementation:

class TreeNode {
  constructor(value) {
    this.value = value
    this.left = null
    this.right = null
  }
}

class BinarySearchTree {
  constructor() {
    this.root = null
  }
  
  insert(value) {
    const node = new TreeNode(value)
    
    if (!this.root) {
      this.root = node
      return
    }
    
    let current = this.root
    while (true) {
      if (value < current.value) {
        // Go left
        if (!current.left) {
          current.left = node
          return
        }
        current = current.left
      } else {
        // Go right
        if (!current.right) {
          current.right = node
          return
        }
        current = current.right
      }
    }
  }
  
  search(value) {
    let current = this.root
    
    while (current) {
      if (value === current.value) {
        return current
      }
      current = value < current.value ? current.left : current.right
    }
    
    return null
  }
  
  // In-order traversal: left, root, right (gives sorted order)
  inOrder(node = this.root, result = []) {
    if (node) {
      this.inOrder(node.left, result)
      result.push(node.value)
      this.inOrder(node.right, result)
    }
    return result
  }
}

// Usage
const bst = new BinarySearchTree()
bst.insert(10)
bst.insert(5)
bst.insert(15)
bst.insert(3)
bst.insert(7)
bst.insert(20)

bst.search(7)   // TreeNode { value: 7, ... }
bst.search(100) // null
bst.inOrder()   // [3, 5, 7, 10, 15, 20] - sorted!

When to use BST:

You need fast search, insert, and delete (O(log n) average)
Data needs to stay sorted
Implementing autocomplete, spell checkers

Graph

Graph (Brief Overview)

A Graph consists of nodes (vertices) connected by edges. Think social networks (people connected by friendships) or maps (cities connected by roads).

┌─────────────────────────────────────────────────────────────────────────┐
│                              GRAPH                                       │
├─────────────────────────────────────────────────────────────────────────┤
│                                                                          │
│              A ─────── B                                                 │
│             /│\        │                                                 │
│            / │ \       │                                                 │
│           /  │  \      │                                                 │
│          C   │   D ────┘                                                 │
│           \  │  /                                                        │
│            \ │ /                                                         │
│             \│/                                                          │
│              E                                                           │
│                                                                          │
│   Adjacency List representation:                                         │
│   A: [B, C, D, E]                                                        │
│   B: [A, D]                                                              │
│   C: [A, E]                                                              │
│   ...                                                                    │
│                                                                          │
└─────────────────────────────────────────────────────────────────────────┘

Basic Implementation (Adjacency List):

class Graph {
  constructor() {
    this.adjacencyList = new Map()
  }
  
  addVertex(vertex) {
    if (!this.adjacencyList.has(vertex)) {
      this.adjacencyList.set(vertex, [])
    }
  }
  
  addEdge(v1, v2) {
    this.adjacencyList.get(v1).push(v2)
    this.adjacencyList.get(v2).push(v1)  // For undirected graph
  }
  
  // Breadth-First Search - uses Queue (FIFO)
  bfs(start) {
    const visited = new Set()
    const queue = [start]
    const result = []
    
    while (queue.length) {
      const vertex = queue.shift()
      if (visited.has(vertex)) continue
      
      visited.add(vertex)
      result.push(vertex)
      
      for (const neighbor of this.adjacencyList.get(vertex)) {
        if (!visited.has(neighbor)) {
          queue.push(neighbor)
        }
      }
    }
    
    return result
  }
  
  // Depth-First Search - uses Stack (LIFO) via recursion
  dfs(start, visited = new Set(), result = []) {
    if (visited.has(start)) return result
    
    visited.add(start)
    result.push(start)
    
    for (const neighbor of this.adjacencyList.get(start)) {
      this.dfs(neighbor, visited, result)
    }
    
    return result
  }
}

// Usage
const graph = new Graph()
graph.addVertex('A')
graph.addVertex('B')
graph.addVertex('C')
graph.addEdge('A', 'B')
graph.addEdge('A', 'C')
graph.addEdge('B', 'C')
graph.bfs('A')  // ['A', 'B', 'C'] - level by level
graph.dfs('A')  // ['A', 'B', 'C'] - goes deep first

Real-world uses:

Social networks (friend connections)
Maps and navigation (shortest path)
Recommendation systems
Dependency resolution (package managers)

Choosing the Right Data Structure

┌─────────────────────────────────────────────────────────────────────────┐
│                    WHICH DATA STRUCTURE SHOULD I USE?                    │
├─────────────────────────────────────────────────────────────────────────┤
│                                                                          │
│   Need ordered data with index access?                                   │
│      └──► ARRAY                                                          │
│                                                                          │
│   Need key-value pairs with string keys?                                 │
│      └──► OBJECT (static data) or MAP (dynamic)                          │
│                                                                          │
│   Need key-value with any type as key?                                   │
│      └──► MAP                                                            │
│                                                                          │
│   Need unique values only?                                               │
│      └──► SET                                                            │
│                                                                          │
│   Need LIFO (last in, first out)?                                        │
│      └──► STACK                                                          │
│                                                                          │
│   Need FIFO (first in, first out)?                                       │
│      └──► QUEUE                                                          │
│                                                                          │
│   Need fast insert/delete at beginning?                                  │
│      └──► LINKED LIST                                                    │
│                                                                          │
│   Need fast search + sorted data?                                        │
│      └──► BINARY SEARCH TREE                                             │
│                                                                          │
│   Modeling relationships/connections?                                    │
│      └──► GRAPH                                                          │
│                                                                          │
└─────────────────────────────────────────────────────────────────────────┘

Use Case	Best Structure	Why
Todo list	Array	Ordered, index access
User settings	Object	String keys, static
Word frequency counter	Map	Easy increment, any key
Tag system	Set	Unique values
Browser back button	Stack	LIFO
Task scheduler	Queue	FIFO
Playlist with prev/next	Linked List (doubly)	O(1) traversal
Dictionary/autocomplete	Trie	Fast prefix search
Social network	Graph	Connections

Common Interview Questions

Interview questions often test your understanding of data structures. Here are patterns you’ll encounter:

Array: Two Sum

Problem: Find two numbers in an array that add up to a target.Approach: Use a Map to store numbers you’ve seen. For each number, check if target - number exists in the Map.

function twoSum(nums, target) {
  const seen = new Map()
  
  for (let i = 0; i < nums.length; i++) {
    const complement = target - nums[i]
    
    if (seen.has(complement)) {
      return [seen.get(complement), i]
    }
    
    seen.set(nums[i], i)
  }
  
  return []
}

twoSum([2, 7, 11, 15], 9)  // [0, 1]

Why Map? O(1) lookup turns O(n²) brute force into O(n).

Stack: Valid Parentheses

Problem: Check if a string of brackets is valid: ()[]{}.Approach: Push opening brackets onto stack. When you see a closing bracket, pop and check if it matches.

function isValid(s) {
  const stack = []
  const pairs = { ')': '(', ']': '[', '}': '{' }
  
  for (const char of s) {
    if (char in pairs) {
      // Closing bracket - check if it matches
      if (stack.pop() !== pairs[char]) {
        return false
      }
    } else {
      // Opening bracket - push to stack
      stack.push(char)
    }
  }
  
  return stack.length === 0
}

isValid('([{}])')  // true
isValid('([)]')    // false

Linked List: Reverse

Problem: Reverse a linked list.Approach: Keep track of previous, current, and next. Reverse pointers as you go.

function reverseList(head) {
  let prev = null
  let current = head
  
  while (current) {
    const next = current.next  // Save next
    current.next = prev        // Reverse pointer
    prev = current             // Move prev forward
    current = next             // Move current forward
  }
  
  return prev  // New head
}

Key insight: You need three pointers to avoid losing references.

Linked List: Detect Cycle

Problem: Determine if a linked list has a cycle.Approach: Floyd’s Tortoise and Hare - use two pointers, one fast (2 steps) and one slow (1 step). If they meet, there’s a cycle.

function hasCycle(head) {
  let slow = head
  let fast = head
  
  while (fast && fast.next) {
    slow = slow.next
    fast = fast.next.next
    
    if (slow === fast) {
      return true  // They met - cycle exists
    }
  }
  
  return false  // Fast reached end - no cycle
}

Why this works: In a cycle, the fast pointer will eventually “lap” the slow pointer.

Tree: Maximum Depth

Problem: Find the maximum depth of a binary tree.Approach: Recursively find the depth of left and right subtrees, take the max.

function maxDepth(root) {
  if (!root) return 0
  
  const leftDepth = maxDepth(root.left)
  const rightDepth = maxDepth(root.right)
  
  return Math.max(leftDepth, rightDepth) + 1
}

Base case: Empty tree has depth 0.

Queue: Implement with Two Stacks

Problem: Implement a queue using only stacks.Approach: Use two stacks. Push to stack1. For dequeue, if stack2 is empty, pour all of stack1 into stack2 (reversing order), then pop from stack2.

class QueueFromStacks {
  constructor() {
    this.stack1 = []  // For enqueue
    this.stack2 = []  // For dequeue
  }
  
  enqueue(item) {
    this.stack1.push(item)
  }
  
  dequeue() {
    if (this.stack2.length === 0) {
      // Pour stack1 into stack2
      while (this.stack1.length) {
        this.stack2.push(this.stack1.pop())
      }
    }
    return this.stack2.pop()
  }
}

Amortized O(1): Each element is moved at most twice.

Key Takeaways

The key things to remember:

Arrays are great for ordered data with index access. Push/pop are O(1), but shift/unshift are O(n).
Objects store string-keyed data. Use them for static configuration and entity data.
Map is the better choice when keys aren’t strings, you need .size, or you add/delete frequently.
Set stores unique values. The [...new Set(arr)] trick removes duplicates instantly.
Stack (LIFO) is perfect for undo/redo, parsing expressions, and DFS traversal.
Queue (FIFO) is ideal for task scheduling and BFS traversal. Use a linked list for O(1) dequeue.
Linked Lists excel at insertions/deletions but lack random access. Use when you frequently modify the beginning.
Binary Search Trees give O(log n) search/insert/delete on average. They keep data sorted.
Choose based on your most frequent operation. What makes one structure fast makes another slow.
Interview tip: When you need O(1) lookup, think Map or Set. When you need to track order of operations, think Stack or Queue.

Test Your Knowledge

Question 1: When would you use a Map instead of an Object?

Answer:Use Map when:

Keys are not strings (objects, numbers, etc.)
You need to know the size frequently (.size vs Object.keys().length)
You add/delete keys often (Map is optimized for this)
You need guaranteed insertion order
You want to avoid prototype chain issues

Use Object when:

Keys are known strings
You’re working with JSON data
You need object destructuring or spread syntax

Question 2: Why is array shift() O(n) but pop() O(1)?

Answer:pop() removes from the end. No other elements need to move.shift() removes from the beginning. Every remaining element must be re-indexed:

Element at index 1 moves to 0
Element at index 2 moves to 1
…and so on

This is why Queue implementations with arrays have O(n) dequeue. For O(1), use a linked list or object with head/tail pointers.

Question 3: What's the difference between a Stack and a Queue?

Answer:Stack (LIFO): Last In, First Out

Like a stack of plates - you take from the top
push() and pop() operate on the same end
Use for: undo/redo, back button, recursion

Queue (FIFO): First In, First Out

Like a line at a store - first person in line is served first
enqueue() adds to back, dequeue() removes from front
Use for: task scheduling, BFS, print queues

Question 4: When would a Linked List be better than an Array?

Answer:Linked List wins when:

You frequently insert/delete at the beginning (O(1) vs O(n))
You don’t need random access by index
You’re implementing a queue (O(1) dequeue)
Memory is fragmented (nodes can be anywhere)

Array wins when:

You need index-based access
You iterate sequentially often
You mostly add/remove from the end
You need .length, .map(), .filter(), etc.

Question 5: What makes Binary Search Trees fast?

Answer:BSTs use the rule: left < parent < right. This means:

To find a value, compare with root
If smaller, go left; if larger, go right
Each comparison eliminates half the remaining nodes

This gives O(log n) search, insert, and delete (on average).Catch: If you insert sorted data, the tree becomes a linked list (all nodes on one side), and operations become O(n). Self-balancing trees (AVL, Red-Black) solve this.

Question 6: How would you remove duplicates from an array?

Answer:The cleanest way is with Set:

const unique = [...new Set(array)]

This works because:

new Set(array) creates a Set (which only keeps unique values)
[...set] spreads the Set back into an array

Time complexity: O(n) - each element is processed once.

Frequently Asked Questions

What are data structures in JavaScript?

Data structures are ways of organizing and storing data so it can be accessed and modified efficiently. JavaScript provides several built-in structures — Array, Object, Map, Set, WeakMap, and WeakSet — and you can implement others like stacks, queues, and linked lists using classes. Choosing the right structure depends on your most frequent operations.

When should I use a Map instead of an Object in JavaScript?

Use a Map when your keys are not strings (objects, functions, numbers), when you need guaranteed insertion order, when you frequently add and delete keys, or when you need the .size property. MDN recommends Map over Object for frequent additions and removals of key-value pairs due to better performance characteristics.

Are JavaScript arrays actually arrays?

Not in the traditional computer science sense. As the V8 blog explains, JavaScript engines use multiple internal representations. Dense arrays with sequential numeric keys are stored as contiguous memory (like C arrays), but sparse arrays or arrays with mixed types may be stored as hash tables internally. This is why V8 optimizes differently based on how you use arrays.

How do I choose the right data structure for my problem?

Consider your most frequent operations. If you need fast lookups by key, use a Map or Object. If you need to track unique values, use a Set. If order matters and you access by index, use an Array. If you need fast insertion and deletion at both ends, consider a linked list or deque implementation.

What is the time complexity of common JavaScript data structure operations?

Array access by index is O(1), but indexOf() is O(n). Object, Map, and Set lookups are all O(1). Array push()/pop() are O(1), but shift()/unshift() are O(n) because all elements must be re-indexed. Understanding these complexities, as outlined in the ECMAScript specification, helps you avoid performance bottlenecks.

Algorithms & Big O

Understanding time complexity helps you choose the right data structure

Factories & Classes

The class syntax used to implement data structures

Higher-Order Functions

Array methods like map, filter, and reduce

Recursion

Essential for tree and graph traversal algorithms

Reference

Array — MDN

Complete reference for JavaScript arrays

Object — MDN

Documentation for Object methods and properties

Map — MDN

Guide to the Map collection type

Set — MDN

Documentation for Set and its new ES2024 methods

WeakMap — MDN

When to use WeakMap for memory management

Data Structures Guide — MDN

MDN’s overview of JavaScript data types and structures

Articles

Map and Set — JavaScript.info

The clearest explanation of Map and Set with interactive examples. Covers WeakMap and WeakSet too.

JavaScript Algorithms and Data Structures

Oleksii Trekhleb’s legendary GitHub repo with implementations of every data structure and algorithm in JavaScript. Over 180k stars for good reason.

Data Structures in JavaScript

freeCodeCamp’s practical guide covering arrays through graphs with real-world examples you can follow along with.

Itsy Bitsy Data Structures

Jamie Kyle’s annotated source code explaining data structures in ~200 lines. Perfect if you learn by reading well-commented code.

Videos

Data Structures and Algorithms in JavaScript

freeCodeCamp’s complete 8-hour course covering everything from Big O to graph algorithms. Great for interview prep.

JavaScript Data Structures: Getting Started

Academind’s beginner-friendly introduction focusing on when and why to use each structure, not just how.

Data Structures Easy to Advanced

William Fiset’s comprehensive course with animations that make complex structures like trees and graphs click.

Getting Started

Fundamentals

Functions & Execution

Web Platform

Object-Oriented JS

Async JavaScript

Functional Programming

Advanced Topics

What's Next?

​What Are Data Structures?

​JavaScript’s Built-in Data Structures

​Arrays

​Objects

​Map

​Set

​WeakMap and WeakSet

​Implementing Common Data Structures

​Stack (LIFO)

​Queue (FIFO)

​Linked List

​Binary Search Tree

​Graph

​Choosing the Right Data Structure

​Common Interview Questions

​Key Takeaways

​Test Your Knowledge

​Frequently Asked Questions

​Related Concepts

Algorithms & Big O

Factories & Classes

Higher-Order Functions

Recursion

​Reference

Array — MDN

Object — MDN

Map — MDN

Set — MDN

WeakMap — MDN

Data Structures Guide — MDN

​Articles

Map and Set — JavaScript.info

JavaScript Algorithms and Data Structures

Data Structures in JavaScript

Itsy Bitsy Data Structures

​Videos

Data Structures and Algorithms in JavaScript

JavaScript Data Structures: Getting Started

Data Structures Easy to Advanced

What Are Data Structures?

JavaScript’s Built-in Data Structures

Arrays

Objects

Map

Set

WeakMap and WeakSet

Implementing Common Data Structures

Stack (LIFO)

Queue (FIFO)

Linked List

Binary Search Tree

Graph

Choosing the Right Data Structure

Common Interview Questions

Key Takeaways

Test Your Knowledge

Frequently Asked Questions

Related Concepts

Reference

Articles

Videos