Introduction to Stacks and Queues in JavaScript

Lesson 5

Introduction: Stacks and Queues

Welcome to an exciting exploration of two fundamental data structures: Stacks and Queues! Remember, data structures store and organize data in a manner that is structured and efficient. Stacks and Queues are akin to stacking plates and standing in a line, respectively. Intriguing, isn't it? Let's dive in!

Stacks: Last In, First Out (LIFO)

A Stack adheres to the "Last In, First Out" or LIFO principle. It's like a pile of plates where the last plate added is the first one to be removed. JavaScript uses arrays to create a stack, with push() used for insert (push), and pop() used for remove (pop, return the element that has been removed).

Let's explore this using a pile of plates.

JavaScript
1class StackOfPlates {
2  constructor() {
3    this.stack = [];
4  }
5
6  // Inserts a plate at the top of the stack
7  addPlate(plate) {
8    this.stack.push(plate);
9  }
10
11  // Removes the top plate from the stack
12  removePlate() {
13    if (this.stack.length === 0) {
14      return "No plates left to remove!";
15    }
16    return this.stack.pop();
17  }
18}
19
20// Create a stack of plates
21const plates = new StackOfPlates();
22plates.addPlate('Plate'); // Pushing a plate
23plates.addPlate('Another Plate'); // Pushing another plate
24// Let's remove a plate; it should be the last one we added.
25console.log('Removed:', plates.removePlate());  // Outputs: Removed: Another Plate

The last plate added was removed first, demonstrating the LIFO property of a stack.

Queues: First In, First Out (FIFO)

A Queue represents the "First In, First Out" or FIFO principle, much like waiting in line at the grocery store. In JavaScript, we can use arrays to achieve this behavior, using push() for enqueue (insert at the end) and shift() for dequeue (remove from the front, return the element that has been removed).

Let's examine this through a queue of people.

JavaScript
1class QueueOfPeople {
2  constructor() {
3    this.queue = [];
4  }
5
6  // Add a person to the end of the queue
7  enqueuePerson(person) {
8    this.queue.push(person);
9  }
10
11  // Remove the first person from the queue (who has been waiting the longest)
12  dequeuePerson() {
13    if (this.queue.length === 0) {
14      return "No people left to dequeue!";
15    }
16    return this.queue.shift();
17  }
18}
19
20// Create a queue of people
21const people = new QueueOfPeople();
22people.enqueuePerson('Person 1'); // Person 1 enters the queue
23people.enqueuePerson('Person 2'); // Person 2 arrives after Person 1
24// Who's next in line? It must be Person 1!
25console.log('Removed:', people.dequeuePerson());  // Outputs: Removed: Person 1

Here, Person 1, the first to join the queue, left before Person 2, demonstrating the FIFO property of a queue.

Stacks and Queues: When and Where to Use?

Stacks handle ordered data efficiently, much like your web browser's history. Queues, on the other hand, are optimal when the order of arrival is essential, such as in a store queue.

Mastering Stack and Queue Operations With a Class

Let's depict the two structures in a text editor that features an Undo mechanism (a Stack) and a Print Queue.

JavaScript
1class TextEditor {
2  constructor() {
3    this.stack = [];
4    this.queue = [];
5  }
6
7  // Make a change (e.g., edit text, insert image, change font)
8  makeChange(action) {
9    this.stack.push(action); // Add to the stack of actions
10  }
11
12  // Undo the most recent change
13  undoChange() {
14    if (this.stack.length === 0) {
15      return "No changes to undo!";
16    }
17    return this.stack.pop(); // Remove the last action from the stack
18  }
19
20  // Add a document to the queue for printing
21  addToPrint(doc) {
22    this.queue.push(doc);
23  }
24
25  // Print the document that has been waiting the longest
26  printDoc() {
27    if (this.queue.length === 0) {
28      return "No documents in print queue!";
29    }
30    return this.queue.shift(); // Remove the first document from the queue
31  }
32}
33
34// Use our text editor!
35const editor = new TextEditor();
36editor.makeChange("Changed font size"); // Make a change
37editor.makeChange("Inserted image"); // Make another change
38// Let's undo a change. It should be the last change we made.
39console.log('Undo:', editor.undoChange());  // Undo: Inserted image
40
41editor.addToPrint("Proposal.docx"); // Queue a document for printing
42editor.addToPrint("Report.xlsx"); // Queue another document
43// Let's print a document. It should be the document that has been waiting the longest.
44console.log('Print:', editor.printDoc());  // Print: Proposal.docx

This code reintroduces the concepts of a Stack (Undo feature) and Queue (Print queue) in the context of a real-life scenario.

Lesson Summary

Great work! You have examined the mechanics of Stacks and Queues, both integral data structures. Remember to practice what you've learned. Happy coding!

Enjoy this lesson? Now it's time to practice with Cosmo!

Practice is how you turn knowledge into actual skills.