Variables and Mutability

One of the most noticeable differences between Rust and JavaScript is how variables and mutability work. Let’s explore these differences to help you transition between the two languages.

Variables in JavaScript

In JavaScript, you can declare variables with var, let, or const:

// Variable declarations in JavaScript
var oldSchool = "I'm hoisted";
let mutable = "I can be changed";
const immutable = "I cannot be reassigned";

// Reassignment
mutable = "New value";  // ✅ Works
// immutable = "New value";  // ❌ TypeError: Assignment to constant variable

// But objects declared with const CAN be modified
const person = { name: "Alice" };
person.name = "Bob";  // ✅ Works! Only the binding is immutable

Variables in Rust

Rust takes a different approach, making variables immutable by default:

// Variable declarations in Rust
let immutable = "I cannot be changed"; // Immutable by default
let mut mutable = "I can be changed";  // Use 'mut' to make mutable

// Reassignment
// immutable = "New value";  // ❌ Error: cannot assign twice to immutable variable
mutable = "New value";      // ✅ Works

Key Differences

• In JavaScript, variables declared with let are mutable by default
• In Rust, variables are immutable by default and require mut for mutability
• JavaScript’s const prevents reassignment but not modification of object properties
• In Rust, immutability is more comprehensive (we’ll see more when we get to references)

Constants

Both languages have constants, but they work differently:

JavaScript Constants

const PI = 3.14159;
// PI = 3.0;  // ❌ Error: Assignment to constant variable

// But objects can be modified
const user = { name: "Alice" };
user.name = "Bob";  // ✅ Works - only the binding is constant, not the content

Rust Constants

const PI: f64 = 3.14159;
// PI = 3.0;  // ❌ Error: cannot assign to this expression

// Constants require type annotations and use SCREAMING_SNAKE_CASE by convention
const MAX_POINTS: u32 = 100_000;

Rust constants:

• Must have explicit type annotations
• Can only be set to a constant expression (evaluated at compile time)
• Can be declared in any scope, including the global scope
• Are inlined at compile time

Shadowing in Rust

Rust allows “shadowing” of variables, which is declaring a new variable with the same name as a previous variable:

let x = 5;
let x = x + 1;  // Shadows the previous 'x'
let x = x * 2;  // Shadows again

println!("The value of x is: {}", x);  // 12

This is different from mutation:

// With shadowing
let spaces = "   ";
let spaces = spaces.len();  // ✅ Works, type can change with shadowing

// With mutation
let mut spaces = "   ";
// spaces = spaces.len();  // ❌ Error: mismatched types

JavaScript Variable Shadowing

JavaScript has similar behavior with block-scoped variables:

let x = 5;
{
  let x = x + 1;  // Shadows the outer 'x'
  console.log(x);  // 6
}
console.log(x);  // 5

However, you can’t redeclare the same variable in the same scope in JavaScript:

let x = 10;
let x = 20;  // ❌ SyntaxError: Identifier 'x' has already been declared

Variable Scope

Both languages use curly braces to define scopes:

JavaScript

{
  let x = 10;
  console.log(x);  // 10
}
// console.log(x);  // ❌ ReferenceError: x is not defined

Rust

{
    let x = 10;
    println!("x: {}", x);  // 10
}
// println!("x: {}", x);  // ❌ Error: cannot find value `x` in this scope

Naming Conventions

JavaScript

• camelCase for variables and functions
• PascalCase for classes and constructor functions
• SCREAMING_SNAKE_CASE for constants

Rust

• snake_case for variables and functions
• PascalCase for types and traits
• SCREAMING_SNAKE_CASE for constants and static variables

Type Inference

Both Rust and JavaScript can infer types, but in very different ways:

JavaScript (Dynamic Typing)

let x = 5;        // Number
x = "hello";      // Now it's a String - no problem!

Rust (Static Typing with Inference)

let x = 5;        // Inferred as i32
// x = "hello";   // ❌ Error: mismatched types

// Need explicit type annotation for type changes
let x = 5;        // i32
let x: &str = "hello"; // Shadowing with a new type

Destructuring

Both languages support destructuring:

JavaScript

const point = { x: 10, y: 20 };
const { x, y } = point;
console.log(x, y);  // 10 20

const numbers = [1, 2, 3];
const [first, second] = numbers;
console.log(first, second);  // 1 2

Rust

struct Point {
    x: i32,
    y: i32,
}

let point = Point { x: 10, y: 20 };
let Point { x, y } = point;
println!("x: {}, y: {}", x, y);  // x: 10, y: 20

let numbers = [1, 2, 3];
let [first, second, _] = numbers;
println!("first: {}, second: {}", first, second);  // first: 1, second: 2

Best Practices

JavaScript

• Use const by default, let when you need to reassign
• Avoid var in modern JavaScript
• Use meaningful variable names
• Keep variables close to where they’re used

Rust

• Use immutable variables by default
• Only use mut when you need to change a value
• Use shadowing for type changes or when you want to reuse a name
• Follow Rust naming conventions
• Keep variables in the smallest scope possible

Memory Considerations

An important difference that isn’t immediately visible is how these variables are stored in memory:

JavaScript

JavaScript handles memory allocation and garbage collection automatically. You generally don’t need to worry about where values live.

Rust

In Rust, values can be allocated on the stack or heap, and the ownership system (which we’ll cover later) determines how memory is managed.

// Stack allocated
let x = 5;  

// Heap allocated, but Rust abstracts memory management
let s = String::from("hello");

Next Steps

Now that you understand variables and mutability in Rust, let’s move on to Data Types to learn about Rust’s rich type system.