First-Class Functions

A first-class function (or first-class citizen) refers to a function that is treated like any other variable in a programming language. This means that functions can:

• Be assigned to variables.
• Be passed as arguments to other functions.
• Be returned from other functions.
• Be stored in data structures (arrays, objects, etc.). This feature allows for functional programming paradigms and is commonly found in languages like JavaScript, Python, and Lisp.

See also Higher-Order Functions

Benefits

• Higher-Order Functions: Enables map, filter, and reduce operations.
• Callbacks & Event Handling: Used in asynchronous programming (e.g., JavaScript’s event loop).
• Functional Programming: Encourages immutability and declarative code.
• Code Reusability & Composition: Functions can be composed dynamically.

Examples

JavaScript

Assigning a Function to a Variable

const greet = function(name) {
    return `Hello, ${name}!`;
};
console.log(greet("Alice")); // Output: Hello, Alice!

Passing a Function as an Argument

function callFunction(fn, value) {
    return fn(value);
}
 
console.log(callFunction(greet, "Bob")); // Output: Hello, Bob!

Returning a Function from Another Function

function createMultiplier(factor) {
    return function(number) {
        return number * factor;
    };
}
 
const double = createMultiplier(2);
console.log(double(5)); // Output: 10

Python

def greet(name):
    return f"Hello, {name}!"
 
# Assign function to a variable
say_hello = greet
print(say_hello("Alice"))  # Output: Hello, Alice!
 
# Pass function as argument
def execute_function(fn, value):
    return fn(value)
 
print(execute_function(greet, "Bob"))  # Output: Hello, Bob!
 
# Return function from another function
def create_multiplier(factor):
    return lambda x: x * factor
 
double = create_multiplier(2)
print(double(5))  # Output: 10