Data Types

Rust’s type system is one of the major differences from JavaScript. Let’s explore the various data types in Rust and how they compare to JavaScript.

Type Systems: Static vs Dynamic

JavaScript is dynamically typed, meaning variables can change types at runtime:

let value = 42;       // Number
value = "hello";      // Now it's a String
value = true;         // Now it's a Boolean

Rust is statically typed, meaning every variable’s type must be known at compile time:

let value = 42;       // i32 by default
// value = "hello";   // ❌ Error: mismatched types
// value = true;      // ❌ Error: mismatched types

In Rust, you can explicitly annotate types:

let age: u32 = 30;
let name: String = String::from("Alice");
let is_active: bool = true;

Scalar Types

Integers

JavaScript has a single numeric type (Number) for integers and floating-point values, plus BigInt for large integers.

Rust has multiple integer types based on size and signedness:

Type	Size (bits)	Range	JS Equivalent
i8	8	-128 to 127	Number
u8	8	0 to 255	Number
i16	16	-32,768 to 32,767	Number
u16	16	0 to 65,535	Number
i32	32	-2,147,483,648 to 2,147,483,647	Number
u32	32	0 to 4,294,967,295	Number
i64	64	-9,223,372,036,854,775,808 to 9,223,372,036,854,775,807	BigInt
u64	64	0 to 18,446,744,073,709,551,615	BigInt
i128	128	Very large range	BigInt
u128	128	Very large range	BigInt
isize	arch	Depends on architecture	Number
usize	arch	Depends on architecture	Number

let a: i32 = 42;        // 32-bit signed integer
let b: u64 = 1000000;   // 64-bit unsigned integer
let c = 98_222;         // Underscores for readability, like 98,222
let d = 0xff;           // Hex value
let e = 0o77;           // Octal value
let f = 0b1111_0000;    // Binary value

In JavaScript, all these would be:

const a = 42;           // Number
const b = 1000000;      // Number
const c = 98222;        // Number
const d = 0xff;         // Hex value (255)
const e = 0o77;         // Octal value (63)
const f = 0b11110000;   // Binary value (240)
const g = 9007199254740992n; // BigInt for numbers > 2^53

Floating-Point Numbers

Rust has two floating-point types:

Type	Size (bits)	Precision	JS Equivalent
f32	32	Single precision	Number
f64	64	Double precision	Number

let x: f64 = 2.0;       // Double-precision float (default)
let y: f32 = 3.0;       // Single-precision float

In JavaScript:

const x = 2.0;         // Number (always double precision)

Booleans

Both languages have boolean types:

let t: bool = true;
let f: bool = false;

const t = true;
const f = false;

Characters

JavaScript doesn’t have a dedicated character type; single characters are just strings of length 1.

Rust’s char type represents a Unicode scalar value:

let c: char = 'z';
let z: char = 'ℤ';
let heart: char = '❤';

In JavaScript:

const c = 'z';        // String of length 1
const z = 'ℤ';        // String of length 1 (but 2 bytes in UTF-16)
const heart = '❤';    // String of length 1 (but 2 bytes in UTF-16)

Compound Types

Tuples

Rust has tuples, which are fixed-length collections of values of different types:

let tup: (i32, f64, u8) = (500, 6.4, 1);

// Destructuring
let (x, y, z) = tup;
println!("y is: {}", y);  // 6.4

// Accessing by index
let five_hundred = tup.0;  // 500

JavaScript added tuple-like behavior with array destructuring:

// JavaScript doesn't have real tuples, but we can use arrays
const tup = [500, 6.4, 1];

// Destructuring
const [x, y, z] = tup;
console.log(y);  // 6.4

// Accessing by index
const fiveHundred = tup[0];  // 500

Arrays

Rust arrays have a fixed length and contain elements of the same type:

// Array with 5 elements of type i32
let a: [i32; 5] = [1, 2, 3, 4, 5];

// Initialize an array with the same value repeated
let b = [3; 5];  // Equivalent to [3, 3, 3, 3, 3]

// Access elements
let first = a[0];  // 1

JavaScript arrays are dynamic and can contain elements of different types:

// JavaScript arrays are dynamic
const a = [1, 2, 3, 4, 5];
a.push(6);  // Can add elements

// Can contain mixed types
const mixed = [1, "two", true, { four: 4 }];

// Access elements the same way
const first = a[0];  // 1

For dynamic arrays in Rust, you’d use a Vec (vector), which we’ll cover in the Collections section.

Strings

Strings are a complex topic in Rust, especially coming from JavaScript.

JavaScript Strings

In JavaScript, strings are simple:

const greeting = "Hello, world!";
const name = 'Alice';
const template = `Hello, ${name}!`;

// String methods
const upperName = name.toUpperCase();
const sub = greeting.substring(0, 5);  // "Hello"

Rust Strings

Rust has two string types: String and &str:

// String literals are &str (string slices)
let greeting: &str = "Hello, world!";

// String type is growable, heap-allocated
let mut name = String::from("Alice");
name.push_str(" Smith");  // Now "Alice Smith"

// Creating a String from a &str
let s = "initial".to_string();
let s = String::from("initial");

// Concatenation
let hello = String::from("Hello, ");
let world = String::from("world!");
let hello_world = hello + &world;  // Note: hello is moved here

// Format macro (like template literals)
let name = "Alice";
let greeting = format!("Hello, {}!", name);

The main differences:

• &str is immutable and often used for string literals or views into strings
• String is growable, heap-allocated, and owned
• Rust strings are UTF-8 encoded, which affects how they’re indexed and manipulated

Type Conversion

JavaScript Type Coercion

JavaScript often performs implicit type conversion:

console.log("5" + 1);      // "51" (string)
console.log("5" - 1);      // 4 (number)
console.log(5 + true);     // 6 (number)

Rust Explicit Conversion

Rust requires explicit conversion:

let x = 5;
// let y = x + "10";  // ❌ Error: cannot add `&str` to `{integer}`

// Explicit conversion
let y = x + 10;       // 15
let s = x.to_string() + "10";  // "510"
let z = x + "10".parse::<i32>().unwrap();  // 15

Special Types

The Unit Type

Rust has a unit type (), which represents the absence of a value:

// Functions with no return value implicitly return the unit type
fn do_something() {
    println!("Hello");
    // Implicitly returns () 
}

JavaScript doesn’t have an equivalent; functions that don’t return anything implicitly return undefined.

The Option Type

Instead of null or undefined, Rust uses Option<T> to represent a value that might be absent:

let some_number = Some(5);
let some_string = Some("a string");
let absent_number: Option<i32> = None;

// Using Option requires handling both cases
match some_number {
    Some(n) => println!("Got a number: {}", n),
    None => println!("No number"),
}

JavaScript equivalent:

const someNumber = 5;
const someString = "a string";
const absentNumber = null;

// Check for null/undefined
if (someNumber !== null && someNumber !== undefined) {
    console.log(`Got a number: ${someNumber}`);
} else {
    console.log("No number");
}

Type Inference

Both languages can infer types, but Rust’s inference is more sophisticated and happens at compile time:

// Rust infers these types
let x = 5;        // i32
let y = 3.0;      // f64
let active = true;  // bool

// JavaScript also has type inference, but at runtime
let x = 5;        // number
let y = 3.0;      // number
let active = true;  // boolean

Type Aliases

Rust allows you to create type aliases:

type Kilometers = i32;

let distance: Kilometers = 5;

In JavaScript with TypeScript:

type Kilometers = number;

let distance: Kilometers = 5;

Next Steps

Now that you understand Rust’s data types, you’re ready to move on to Functions, where we’ll explore how to define and use functions in Rust compared to JavaScript.