Struct neon::types::JsBox[][src]

pub struct JsBox<T: Send + 'static> { /* fields omitted */ }
Expand description

A smart pointer for Rust data managed by the JavaScript engine.

The type JsBox<T> provides shared ownership of a value of type T, allocated in the heap. The data is owned by the JavaScript engine and the lifetime is managed by the JavaScript garbage collector.

Shared references in Rust disallow mutation by default, and JsBox is no exception: you cannot generally obtain a mutable reference to something inside a JsBox. If you need to mutate through a JsBox, use Cell, RefCell, or one of the other types that provide interior mutability.

Values contained by a JsBox must implement the Finalize trait. Finalize::finalize will execute with the value in a JsBox immediately before the JsBox is garbage collected. If no additional finalization is necessary, an emply implementation may be provided.

Deref behavior

JsBox<T> automatically dereferences to T (via the Deref trait), so you can call T’s method on a value of type JsBox<T>.

let vec: Handle<JsBox<Vec<_>>> = cx.boxed(vec![1, 2, 3]);

println!("Length: {}", vec.len());


Passing some immutable data between Rust and JavaScript.

fn create_path(mut cx: FunctionContext) -> JsResult<JsBox<PathBuf>> {
    let path = cx.argument::<JsString>(0)?.value(&mut cx);
    let path = Path::new(&path).to_path_buf();


fn print_path(mut cx: FunctionContext) -> JsResult<JsUndefined> {
    let path = cx.argument::<JsBox<PathBuf>>(0)?;

    println!("{}", path.display());


Passing a user defined struct wrapped in a RefCell for mutability. This pattern is useful for creating classes in JavaScript.

type BoxedPerson = JsBox<RefCell<Person>>;

struct Person {
     name: String,

impl Finalize for Person {}

impl Person {
    pub fn new(name: String) -> Self {
        Person { name }

    pub fn set_name(&mut self, name: String) { = name;

    pub fn greet(&self) -> String {
        format!("Hello, {}!",

fn person_new(mut cx: FunctionContext) -> JsResult<BoxedPerson> {
    let name = cx.argument::<JsString>(0)?.value(&mut cx);
    let person = RefCell::new(Person::new(name));


fn person_set_name(mut cx: FunctionContext) -> JsResult<JsUndefined> {
    let person = cx.argument::<BoxedPerson>(0)?;
    let mut person = person.borrow_mut();
    let name = cx.argument::<JsString>(1)?.value(&mut cx);



fn person_greet(mut cx: FunctionContext) -> JsResult<JsString> {
    let person = cx.argument::<BoxedPerson>(0)?;
    let person = person.borrow();
    let greeting = person.greet();



Values contained by a JsBox must be Finalize + Send + 'static


The neon::prelude::Finalize trait provides a finalize method that will be called immediately before the JsBox is garbage collected.


JsBox may be moved across threads. It is important to guarantee that the contents is also safe to move across threads.


The lifetime of a JsBox is managed by the JavaScript garbage collector. Since Rust is unable to verify the lifetime of the contents, references must be valid for the entire duration of the program. This does not mean that the JsBox will be valid until the application terminates, only that its lifetime is indefinite.

Constructs a new JsBox containing value.

Trait Implementations

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

Formats the value using the given formatter. Read more

The resulting type after dereferencing.

Dereferences the value.

This is supported on crate feature napi-6 only.

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more

Immutably borrows from an owned value. Read more

Mutably borrows from an owned value. Read more

Performs the conversion.

Performs the conversion.

The resulting type after obtaining ownership.

Creates owned data from borrowed data, usually by cloning. Read more

🔬 This is a nightly-only experimental API. (toowned_clone_into)

recently added

Uses borrowed data to replace owned data, usually by cloning. Read more

The type returned in the event of a conversion error.

Performs the conversion.

The type returned in the event of a conversion error.

Performs the conversion.