Functional vs OOP in JS: What to Choose in 2024?

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Two Sides of the Same Coin: Functional vs Object-Oriented Approach in JavaScript

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If you've ever wondered why some developers write code that looks like a mathematical formula, while others write code like an IKEA furniture assembly manual, you've stumbled upon the age-old debate: functional programming (FP) vs object-oriented programming (OOP) in JavaScript.

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JavaScript is a unique language. It was born as a scripting language for buttons, but grew into a monster that can do everything. It is multi-paradigm: you can write in it using procedural, OOP, or functional styles. And that's where the fun begins. Which approach should a beginner choose? And an experienced developer? Let's figure it out without fanaticism, using real examples.

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Briefly About the Main Thing: What is What?

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Object-Oriented Approach (OOP) is about "entities". You have objects (cat, car, user), they have properties (color, speed, name) and methods (meow, drive, log in). You group data and behavior together. Classic: class Car { drive() {} }.

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Functional Approach (FP) is about "actions". Data and functions are separated. Functions are pure transformers: input data in, different data out, with no side effects. No this, mutations, or hidden state.

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The irony is that JavaScript is not ideal for either "out of the box", but it allows you to elegantly use both styles. The question is only when to apply which.

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OOP in JavaScript: From Prototypes to ES6 Classes

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Once upon a time, in the era of IE6, OOP in JS looked frightening. Everything was built on prototypes — a mechanism where objects inherit properties from other objects. It was powerful, but not intuitive.

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With the arrival of ES6 (ES2015), syntactic sugar was added to the language — class. But it's important to understand: under the hood, it's still the same prototypes. Classes made the code cleaner and more understandable for those coming from Java or C++.

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OOP Example: "User" Class

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class User {\n  constructor(name, email) {\n    this.name = name;\n    this.email = email;\n    this.loggedIn = false;\n  }\n\n  login() {\n    this.loggedIn = true;\n    return `${this.name} logged in`;\n  }\n\n  logout() {\n    this.loggedIn = false;\n    return `${this.name} logged out`;\n  }\n}\n\nconst admin = new User('Anna', 'anna@example.com');\nconsole.log(admin.login()); // Anna logged in\n

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Everything is logical: data (name, email) lives together with methods (login, logout). The object's state changes through its own methods. This is classic encapsulation.

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Problems with OOP in JS

The this context. Losing context is a scourge for beginners. If you pass a method as a callback, this "breaks". You have to use .bind(), arrow functions, or closures.
Inheritance. Deep class hierarchies ("Diamond Problem") lead to confusion. Often composition (mixins) turns out to be better than inheritance.
State. Mutating state (when we change object properties) is the main source of bugs in large projects. One method can accidentally change something it shouldn't.

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