Closures, Higher-Order Functions, and Prototypal Inheritance in JavaScript

This content originally appeared on DEV Community and was authored by francesco agati

JavaScript is a versatile language that mix functional programming and object-oriented programming paradigms. This flexibility allows developers to create powerful abstractions. We can try to mix concepts such as closures, higher-order functions, prototypal inheritance, and the this keyword to create elegant solutions.

Closures

A closure is a function that retains access to its lexical scope, even when the function is executed outside that scope. This means a closure “remembers” the environment in which it was created.

Example:

function outerFunction(outerVariable) {
  return function innerFunction(innerVariable) {
    console.log('Outer Variable:', outerVariable);
    console.log('Inner Variable:', innerVariable);
  };
}

const newFunction = outerFunction('outside');
newFunction('inside'); // Outputs: Outer Variable: outside, Inner Variable: inside

In this example, innerFunction forms a closure, capturing the outerVariable from its lexical scope.

Higher-Order Functions

A higher-order function is a function that either takes another function as an argument or returns a function as its result.

Example:

function higherOrderFunction(callback) {
  return function(value) {
    return callback(value);
  };
}

const addTen = higherOrderFunction(function(num) {
  return num + 10;
});

console.log(addTen(5)); // Outputs: 15

Here, higherOrderFunction is a higher-order function that returns a new function applying the callback.

Prototypal Inheritance

JavaScript uses prototypal inheritance, where objects inherit properties and methods from other objects. This is achieved through the prototype chain.

Example:

function Animal(name) {
  this.name = name;
}

Animal.prototype.speak = function() {
  console.log(this.name + ' makes a noise.');
};

const dog = new Animal('Dog');
dog.speak(); // Outputs: Dog makes a noise.

In this example, dog inherits the speak method from Animal.prototype.

The this Keyword

In JavaScript, this refers to the context in which a function is called. Its value can change depending on how the function is invoked.

Example:

const obj = {
  name: 'Object',
  getName: function() {
    return this.name;
  }
};

console.log(obj.getName()); // Outputs: Object

Here, this refers to obj within getName method.

Combining Concepts: A Code Example

Now, let’s combine these concepts in a practical example that uses closures, higher-order functions, prototypal inheritance, and dynamic this binding.

Example:

function multiplier(x) {
  return function(y) {
    return x * y * this.z;
  };
}

const mul5 = multiplier(5);

const Obj = function(z) {
  this.z = z;
};

Obj.prototype.mul5 = mul5;

const obj = new Obj(10);
console.log(obj.mul5(15)); // Outputs: 750

Explanation:

1. Closure and Higher-Order Function: The multiplier function returns another function, creating a closure that captures the value of x.
2. Dynamic this Binding: The inner function returned by multiplier uses this.z. When mul5 is called as a method of obj, this refers to obj.
3. Prototypal Inheritance: Obj is a constructor function, and mul5 is assigned to its prototype. This means every instance of Obj will have access to the mul5 method.

Functional and Prototype-Based Abstractions

Both functional programming (closures, higher-order functions) and prototype-based programming, can create powerful abstractions. Here are some benefits:

• Modularity: Functions and methods can be easily reused and composed.
• Encapsulation: Closures help in encapsulating private variables and functions.
• Inheritance: Prototypal inheritance allows for shared methods and properties, reducing redundancy.

Combining functional programming techniques with JavaScript’s prototypal inheritance system provides a robust way to write clean, maintainable, and efficient code. Understanding and utilizing closures, higher-order functions, dynamic this binding, and prototypes together can significantly enhance your programming toolkit, leading to elegant and powerful abstractions.

This content originally appeared on DEV Community and was authored by francesco agati