Functions

Functions are fundamental building blocks in any programming language. Let’s explore how functions work in Rust and how they compare to JavaScript.

Defining Functions

JavaScript Functions

In JavaScript, you can define functions in several ways:

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

// Function expression
const multiply = function(a, b) {
  return a * b;
};

// Arrow function
const subtract = (a, b) => a - b;

Rust Functions

In Rust, functions are defined with the fn keyword:

fn add(a: i32, b: i32) -> i32 {
    a + b
}

Key differences:

Rust requires type annotations for parameters
Return type is specified with -> after the parameter list
The final expression without a semicolon is implicitly returned
No function expressions or arrow functions in Rust

Function Body and Return Values

JavaScript Return Values

In JavaScript, the return keyword is used to return a value:

function square(x) {
  return x * x;
}

// Functions without return statements return undefined
function greet(name) {
  console.log(`Hello, ${name}!`);
  // implicitly returns undefined
}

Rust Return Values

In Rust, the final expression is returned without the return keyword:

fn square(x: i32) -> i32 {
    x * x  // No semicolon - this is an expression that returns a value
}

// Explicit return
fn early_return(x: i32) -> i32 {
    if x < 0 {
        return 0;  // Early return requires the return keyword
    }
    x * x
}

// Functions can return nothing (the unit type)
fn greet(name: &str) {
    println!("Hello, {}!", name);
    // Implicitly returns the unit type ()
}

The key concept here is that in Rust, statements end with a semicolon and don’t return values, while expressions don’t have a semicolon and do return values.

Function Parameters

JavaScript Parameters

JavaScript is flexible with parameters:

// Optional parameters with default values
function greet(name = "World") {
  return `Hello, ${name}!`;
}

// Rest parameters
function sum(...numbers) {
  return numbers.reduce((total, num) => total + num, 0);
}

// Destructuring parameters
function printPerson({ name, age }) {
  console.log(`${name} is ${age} years old`);
}

Rust Parameters

Rust parameters are more strictly typed:

// All parameters require type annotations
fn greet(name: &str) -> String {
    format!("Hello, {}!", name)
}

// Multiple parameters
fn calculate_rectangle_area(width: f64, height: f64) -> f64 {
    width * height
}

// Default parameters don't exist in the same way as JavaScript
// You can use Option<T> or implement multiple functions
fn greet_option(name: Option<&str>) -> String {
    let name = name.unwrap_or("World");
    format!("Hello, {}!", name)
}

// Call it with Some or None
let greeting = greet_option(Some("Alice"));  // "Hello, Alice!"
let default_greeting = greet_option(None);   // "Hello, World!"

Function Overloading

JavaScript doesn’t have true function overloading, though you can simulate it:

function process(arg) {
  if (typeof arg === 'number') {
    return arg * 2;
  } else if (typeof arg === 'string') {
    return arg.toUpperCase();
  }
  return null;
}

Rust doesn’t have function overloading either, but you can:

Use different function names
Use generic types (which we’ll cover later)
Use trait objects (which we’ll cover later)

fn double_number(x: i32) -> i32 {
    x * 2
}

fn uppercase_string(s: &str) -> String {
    s.to_uppercase()
}

Main Function

JavaScript doesn’t have a special main function - script execution starts at the top level.

In Rust, the main function is the entry point for executables:

fn main() {
    println!("Hello, world!");
}

Passing Functions as Arguments

JavaScript Higher-Order Functions

In JavaScript, functions are first-class citizens:

function applyFunction(x, operation) {
  return operation(x);
}

// Passing a function as an argument
const result = applyFunction(5, x => x * x);  // 25

Rust Function Pointers

Rust also supports passing functions:

fn apply_function(x: i32, operation: fn(i32) -> i32) -> i32 {
    operation(x)
}

fn square(x: i32) -> i32 {
    x * x
}

// Passing a function as an argument
let result = apply_function(5, square);  // 25

// Using closures (similar to lambda expressions)
let result = apply_function(5, |x| x * x);  // 25

Closures (Lambda Functions)

JavaScript Closures

JavaScript’s closure syntax is lightweight:

const add = (a, b) => a + b;

// Capturing variables from the environment
function makeCounter() {
  let count = 0;
  return function() {
    count += 1;
    return count;
  };
}

const counter = makeCounter();
console.log(counter());  // 1
console.log(counter());  // 2

Rust Closures

Rust closures use pipes (|) for parameters:

let add = |a: i32, b: i32| a + b;

// Type inference works with closures
let add_inferred = |a, b| a + b;
let sum = add_inferred(5, 10);  // 15

// Capturing variables from the environment
let x = 5;
let add_x = |y| x + y;  // Captures x from the environment
let result = add_x(10);  // 15

// Mutable closures
let mut counter = 0;
let mut increment = || {
    counter += 1;
    counter
};

println!("{}", increment());  // 1
println!("{}", increment());  // 2

Rust closures have three traits:

Fn: Captures by reference (similar to JS closures)
FnMut: Captures by mutable reference
FnOnce: Captures by value and can only be called once

This is related to Rust’s ownership system, which we’ll cover later.

Methods

JavaScript Methods

In JavaScript, methods are functions attached to objects:

class Rectangle {
  constructor(width, height) {
    this.width = width;
    this.height = height;
  }

  area() {
    return this.width * this.height;
  }
}

const rect = new Rectangle(10, 20);
console.log(rect.area());  // 200

Rust Methods

In Rust, methods are defined within impl blocks:

struct Rectangle {
    width: u32,
    height: u32,
}

impl Rectangle {
    // Static method (no self parameter)
    fn new(width: u32, height: u32) -> Rectangle {
        Rectangle { width, height }
    }

    // Instance method (uses &self - "self" is like "this" in JavaScript)
    fn area(&self) -> u32 {
        self.width * self.height
    }

    // Mutable method (uses &mut self)
    fn resize(&mut self, width: u32, height: u32) {
        self.width = width;
        self.height = height;
    }
}

// Usage
let mut rect = Rectangle::new(10, 20);
println!("Area: {}", rect.area());  // 200
rect.resize(30, 40);
println!("New area: {}", rect.area());  // 1200

Associated Functions

These are similar to static methods in JavaScript:

impl Rectangle {
    // This is an associated function (no self parameter)
    fn square(size: u32) -> Rectangle {
        Rectangle {
            width: size,
            height: size,
        }
    }
}

// Called with :: syntax, like JavaScript static methods
let square = Rectangle::square(10);

Function Documentation

JavaScript Documentation

JavaScript uses JSDoc comments:

/**
 * Calculates the sum of two numbers
 * @param {number} a - The first number
 * @param {number} b - The second number
 * @returns {number} The sum of a and b
 */
function add(a, b) {
  return a + b;
}

Rust Documentation

Rust uses doc comments:

/// Calculates the sum of two numbers
///
/// # Examples
///
/// ```
/// let sum = add(5, 10);
/// assert_eq!(sum, 15);
/// ```
fn add(a: i32, b: i32) -> i32 {
    a + b
}

Key Differences

Type Annotations: Rust requires explicit parameter and return types
Expressions vs. Statements: Rust’s final expression is returned without a return keyword
Default Parameters: Rust doesn’t have default parameters like JavaScript
Function Overloading: Neither language has true function overloading
Closures: Both languages support closures, but Rust’s have more nuanced capture behavior
Methods: Rust methods are defined in impl blocks and have explicit self parameters

Next Steps

Now that you understand functions in Rust, let’s explore Comments to see how to document your code effectively.