First-class Functions and Closures: JavaScript and Python Side by Side

Anton Ioffe - August 18th 2023 - 3 minutes read


Over my extensive 15-year journey in the development world, I've come across many paradigms, patterns, and programming concepts. Among these, first-class functions and closures have always held a special place in my heart. Their beauty and complexity are akin to a well-orchestrated symphony, played out across different languages. Today, we'll venture into a side-by-side analysis of these concepts in two of my favorite languages: JavaScript and Python. Ready to deep dive? Let's get started!

1. First-class Functions: What Are They?

Before delving into the nitty-gritty, let's demystify what we mean by first-class functions. In simple terms, if a programming language treats functions as first-class citizens, it means the language supports passing functions as arguments to other functions, returning them as the values from other functions, and assigning them to variables.

1.1. JavaScript

In JavaScript, everything is an object, and that includes functions. Here's a basic illustration:

// Defining a function
function greet() {
  return "Hello!";

// Assigning a function to a variable
let sayHello = greet;

// Calling the function through the variable
console.log(sayHello()); // Outputs: Hello!

1.2. Python

Python too treats functions as first-class citizens, providing similar flexibility as JavaScript:

# Defining a function
def greet():
    return "Hello!"

# Assigning a function to a variable
say_hello = greet

## Calling the function through the variable
print(say_hello())  # Outputs: Hello!

2. Passing Functions as Arguments

Being able to pass functions as arguments opens the door to higher-order functions, which simply means a function that takes one or more functions as arguments.

2.1. JavaScript

In JavaScript, array methods like map, filter, and reduce are prime examples of higher-order functions:

const numbers = [1, 2, 3, 4];

// Using the map method
const doubled = {
  return num * 2;

console.log(doubled); // Outputs: [2, 4, 6, 8]

2.2. Python

Python's built-in functions like map and filter are also examples of higher-order functions:

numbers = [1, 2, 3, 4]

# Using the map function
doubled = list(map(lambda num: num * 2, numbers))

print(doubled)  # Outputs: [2, 4, 6, 8]

3. Closures

Now, onto closures. At a high level, a closure is a function that has access to the parent scope, even after the parent function has closed. This is a concept that can be challenging even for seasoned developers, but its power is unparalleled.

3.1. JavaScript

Let's illustrate closures with a classic example:

function outer() {
  let count = 0;

  // Inner function accessing parent scope variable
  return function inner() {
    count += 1;
    return count;

const incrementer = outer();

console.log(incrementer()); // Outputs: 1
console.log(incrementer()); // Outputs: 2

3.2. Python

The essence of closures remains the same in Python:

def outer():
    count = 0

    # Inner function accessing parent scope variable
    def inner():
        nonlocal count
        count += 1
        return count

    return inner

incrementer = outer()

print(incrementer())  # Outputs: 1
print(incrementer())  # Outputs: 2

4. When and Why: The Power of First-class Functions and Closures

First-class functions and closures are more than just fancy concepts. They have practical applications:

  • Modularity and Reusability: Breaking down functionalities into functions makes code modular and reusable.

  • Encapsulation: Closures help in data hiding and encapsulation, ensuring internal states cannot be modified unexpectedly.

  • Callbacks and Asynchronous operations: In JavaScript, they enable us to handle asynchronous operations using callbacks.

  • Decorators and Middleware: In Python, closures form the backbone of decorators, which add functionalities to existing functions without modifying their structure.

5. Common Mistakes and Best Practices

5.1. Overusing Closures

It's easy to get carried away and use closures everywhere, but remember:

  • Memory Usage: Every closure retains its own copy of the outer function's variables, leading to higher memory usage.

  • Complexity: Overusing closures can lead to code that's hard to understand and debug.

5.2. Not Leveraging First-class Functions

Many developers, especially beginners, tend to rewrite the same logic multiple times. Leveraging first-class functions for reusability can drastically reduce redundancy.


First-class functions and closures are incredibly powerful tools in a developer's arsenal. When wielded correctly, they can lead to elegant, modular, and efficient code. Both JavaScript and Python, despite their differences, offer a rich platform to explore and harness these concepts.


Time to flex those muscles! Create a utility function in both JavaScript and Python that memoizes (caches) the result of another function. Ensure that if the function is called with the same arguments again, the cached result is returned instead of recalculating.

I'm eager to see the innovative solutions you come up with. Remember, it's not just about getting it to work, but about writing clean, efficient, and reusable code. Best of luck, and happy coding!

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