wasm-bindgen/guide/src/reference/passing-data.md

Passing arbitrary data to JS

It's possible to pass data from Rust to JS not explicitly supported in the Feature Reference by serializing via Serde.

wasm-bindgen includes the JsValue type, which streamlines serializing and deserializing.

In order accomplish this, we must include the serde and serde_derive crates in Cargo.toml, and configure wasm-bindgen to work with this feature:

Cargo.toml

[dependencies]
serde = "^1.0.59"
serde_derive = "^1.0.59"

[dependencies.wasm-bindgen]
version = "^0.2"
features = ["serde-serialize"]

In our top-level Rust file (eg lib.rs or main.rs), we enable the Serialize macro:

#[macro_use]
extern crate serde_derive;

The data we pass at all nesting levels must be supported by serde, or be a struct or enum that derives the Serialize trait. For example, let's say we'd like to pass this struct to JS; doing so is not possible in bindgen directly due to the use of public fields, HashMaps, arrays, and nested Vecs. Note that we do not need to use the #[wasm_bindgen] macro.

#[derive(Serialize)]
pub struct Example {
    pub field1: HashMap<u32, String>,
    pub field2: Vec<Vec<f32>>,
    pub field3: [f32; 4],
}

Here's a function that will pass an instance of this struct to JS:

#[wasm_bindgen]
pub fn pass_example() -> JsValue {
    let mut field1 = HashMap::new();
    field1.insert(0, String::from("ex"));
    let example = Example {
        field1,
        field2: vec![vec![1., 2.], vec![3., 4.]],
        field3: [1., 2., 3., 4.]
    };

    JsValue::from_serde(&example).unwrap()
}

When calling this function from JS, its output will automatically be deserialized. In this example, fied1 will be a JS object (Not a JS Map), field2 will be a 2d JS array, and field3 will be a 1d JS array. Example calling code:

const rust = import("./from_rust");

rust.then(
    r => {
        console.log(r.pass_example())
    }
)