Quick Reference: OOP vs. Functional Programming in JavaScript

🚨 The Problem with Singletons in JavaScript

❌ Singleton Example (Creates Shared State)

class TempTool {
  constructor() {
    this.data = [];
  }

  add(item) {
    this.data.push(item);
  }

  listAll() {
    return this.data;
  }
}

export default { instance: new TempTool() };

🔥 Issues:

Shared state across imports → One test modifying state affects all subsequent tests.
Difficult to reset state → Requires manual instance resets.
Mocking is harder → Singleton is instantiated before tests, making it difficult to override.

✅ Solution: Use Pure Functions or Dependency Injection Instead

✅ Functional Programming: Predictable & Testable Code

1. What is a Pure Function?

Same input → Always same output
No side effects
Easier to test and debug

✅ Example: A Pure Function

function add(a, b) {
  return a + b;
}

console.log(add(5, 4)); // 9
console.log(add(5, 4)); // 9
console.log(add(5, 4)); // 9

module.exports = { add };

🧪 Testing a Pure Function

const { add } = require("./demo");

describe("Add function", () => {
  it("adds two inputs", () => {
    expect(add(4, 5)).toEqual(9);
  });
});

❌ Impure Functions: Why They Make Testing Harder

Example: An Impure Function

function add(a, b) {
  return a + b + Math.random();
}

⚠️ Testing Problem:

const { add } = require("./demo");

describe("Add function", () => {
  it("adds two inputs", () => {
    const output = add(4, 5);
    expect(output).toEqual(); // ❌ What is the expected output?
  });
});

🛠 Fixing Impure Functions Using Mocks

✅ Stabilizing Tests by Mocking Randomness

const { add } = require("./demo");

describe("Add function", () => {
  it("adds two inputs", () => {
    jest.spyOn(Math, "random").mockImplementation(() => 10);

    const output = add(4, 5);
    expect(output).toEqual(19);
  });
});

🏆 Key Takeaways

✔ Avoid Singletons – They introduce shared state issues and make testing difficult.
✔ Use Pure Functions – They make your code more predictable and testable.
✔ Mock Impure Functions – When external dependencies exist, mock them to create repeatable results.

loraxx753/quick-reference.md