Variables in Rust - Rust for TypeScript Developers
Jacob Asper
Posted on February 11, 2023
Table Of Contents
Naming
In Rust, variables are snake cased, while constants are conventially in screaming snake case
const CONSTANT_VALUE: i32 = 2;
let non_constant_value = 32;
Note that variables defined with const must be explicitly typed
Scope
In Rust, variables are block scoped just like variables declared with let and const in TypeScript
fn main() {
let x = 32;
{
let x = 12;
println!("{x}") // 12
}
println!("{x}") // 32
}
Initialization
In TypeScript, the initialization of variables declared with var is hoisted
console.log(x); // undefined
var x = 12;
console.log(x); // 12
The initialization of variables declared with let or const is not hoisted
console.log(x);
let x = 12;
Uncaught ReferenceError: can't access lexical declaration 'x' before initialization
However, they are assigned undefined if not given an initializer
let x;
console.log(x) // undefined
Rust takes the more modern behavior of let and const and takes it one step further. Variables are not assigned a value if not given an initializer
This works
fn main() {
let x = 12;
println!("{x}"); // 12
}
This does not
fn main() {
let x: i32;
println!("{x}");
}
error[E0381]: used binding `x` isn't initialized
--> src/main.rs:4:16
|
2 | let x: i32;
| - binding declared here but left uninitialized
|
3 | println!("{x}");
| ^ `x` used here but it isn't initialized
Declaration
Variables declared with const in Rust can be global, but variables declared with let cannot
So this works,
const X: i32 = 12;
fn main() {
}
But this does not
let x = 12;
fn main() {
}
error: expected item, found keyword `let`
--> src/main.rs:1:1
|
1 | let x = 12;
| ^^^ consider using `const` or `static` instead of `let` for global variables
In TypeScript, variables declared with var, let, or const can be declared anywhere and aren't required to be explicitly typed
let x = 12;
Redeclaration
In TypeScript, variables assigned with var can be redeclared locally
var x = 12;
console.log(x); // 12
var x = 32;
console.log(x); // 32
Variables declared with let or const cannot be redeclared locally
const x = 12;
console.log(x);
const x = 32;
console.log(x);
Cannot redeclare block-scoped variable 'x'.
Rust's const works the same as TypeScript's in this sense
fn main() {
const X: i32 = 12;
const X: i32 = 12;
}
error[E0428]: the name `X` is defined multiple times
--> src/main.rs:5:3
|
4 | const X: i32 = 12;
| ------------------ previous definition of the value `X` here
5 | const X: i32 = 12;
| ^^^^^^^^^^^^^^^^^^ `X` redefined here
|
= note: `X` must be defined only once in the value namespace of this block
Rust's let works like var here—it can be redeclared in the same namespace
fn main() {
let x = 12;
println!("{x}"); // 12
let x = 43;
println!("{x}"); // 43
}
However, variables declared with let can be redeclared with a different type!
fn main() {
let x = 32;
println!("{x}");
let x = "hi";
println!("{x}");
}
While in TypeScript, variable declarations can't change type
var x = 1
var x = 'hi'
Subsequent variable declarations must have the same type.
Variable 'x' must be of type 'number', but here has type 'string'.
Reassignment
In TypeScript, variables declared with const cannot be reassigned
const x = 12;
x = 93;
Cannot assign to 'x' because it is a constant.
Variables declared with var or let can be reassigned
let x = 12;
console.log(x); // 12
x = 32;
console.log(x); // 32
In Rust, variables assigned with both let and const cannot be reassigned
fn main() {
let x = 12;
x = 23;
}
error[E0384]: cannot assign twice to immutable variable `x`
--> src/main.rs:6:3
|
4 | let x = 12;
| -
| |
| first assignment to `x`
| help: consider making this binding mutable: `mut x`
5 |
6 | x = 23;
| ^^^^^^ cannot assign twice to immutable variable
Mutability
As the compiler so kindly pointed out, we can use the mut keyword with let to make x mutable
fn main() {
let mut x = 12;
println!("{x}"); // 12
x = 23;
println!("{x}"); // 23
}
This differs from TypeScript, where objects and arrays are mutable by default
const arr = [];
arr.push(1);
console.log(arr); // [1]
In Rust, this does not compile without the mut keyword
fn main() {
let vec = Vec::new();
vec.push(1);
}
error[E0596]: cannot borrow `vec` as mutable, as it is not declared as mutable
--> src/main.rs:5:3
|
4 | let vec = Vec::new();
| --- help: consider changing this to be mutable: `mut vec`
5 | vec.push(1);
| ^^^^^^^^^^^ cannot borrow as mutable
Outro
Thank you for reading! Please let me know if you have any suggestions.
Posted on February 11, 2023
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