Accessing JS arrays from Rust

[TilBlechschmidt]

According to the documentation it is possible to convert a Rust Vector into JavaScript arrays.

I wasn't able to find any documentation on how to do that the other way around e.g. iterate a JS array or possibly convert it into a Rust vector if possible (obviously requires that all array entries can be converted to the same type).

Trying to pass an Array to Rust

fn print_vec(vec: stdweb::Array) {
    let x: Vec<stdweb::Value> = Vec::from(&vec);
    println!("This is a vector: {:?}", x);
}

fn main() {
    stdweb::initialize();

    js! {
        Module.exports.printArray = @{print_vec};
    }
}

const vector = require( './target/wasm32-unknown-unknown/release/vector.js' );
vector..printArray([1, 2, 3, 4]);

I get the following error twice:

error[E0277]: the trait bound `stdweb::Array: stdweb::unstable::TryFrom<stdweb::Value>` is not satisfied
  --> src/main.rs:21:5
   |
21 | /     js! {
22 | |         Module.exports.printArray = @{print_vec};
23 | |     }
   | |_____^ the trait `stdweb::unstable::TryFrom<stdweb::Value>` is not implemented for `stdweb::Array`

   = note: required because of the requirements on the impl of `stdweb::private::JsSerializableOwned` for `stdweb::private::Newtype<(stdweb::webcore::serialization::FunctionTag, (stdweb::Array,)), fn(stdweb::Array) {print_vec}>`
   = note: required by `stdweb::private::JsSerializableOwned::memory_required_owned`
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

[koute]

You can convert either a Value or an Array into a Vec<T> using the TryFrom trait (Specifically this and this impl.)

This should work:

let array: Vec< String > = js! { return ["foo", "bar"]; }.try_into().unwrap();

You can't currently access a JavaScript array in Rust without converting it to a vector first.

[TilBlechschmidt]

Thanks for the quick answer!

I wasn't aware of the .try_into() function! Looks good.

The above error seems to persist though:

error[E0277]: the trait bound `std::vec::Vec<std::string::String>: std::convert::From<stdweb::Value>` is not satisfied
  --> src/main.rs:25:63
   |
25 |     let array: Vec< String > = js! { return ["foo", "bar"]; }.try_into().unwrap();
   |                                                               ^^^^^^^^ the trait `std::convert::From<stdweb::Value>` is not implemented for `std::vec::Vec<std::string::String>`
   |
   = help: the following implementations were found:
             <std::vec::Vec<stdweb::Value> as std::convert::From<&'a stdweb::Array>>
             <std::vec::Vec<u8> as std::convert::From<&'a stdweb::web::TypedArray<u8>>>
             <std::vec::Vec<stdweb::Value> as std::convert::From<&'a mut stdweb::Array>>
             <std::vec::Vec<stdweb::Value> as std::convert::From<stdweb::Array>>
           and 10 others
   = note: required because of the requirements on the impl of `std::convert::TryFrom<stdweb::Value>` for `std::vec::Vec<std::string::String>`
   = note: required because of the requirements on the impl of `std::convert::TryInto<std::vec::Vec<std::string::String>>` for `stdweb::Value`

Seems like I am missing a piece of the puzzle there ...

[koute]

You're using the wrong TryFrom. (:

The std::convert::TryFrom is still unstable, so we have our own in stdweb::unstable::TryFrom/stdweb::unstable::TryInto.

[TilBlechschmidt]

That did the trick!
Thanks for your swift help 😃

This might as well be the next big thing for Rust ;)
Props to you good sir and this project! Keep up the amazing work!

[koute]

No problem; thanks. (: