By default, structs exported from Rust become JavaScript classes with a single ptr property. All other properties are implemented as getters, which are not displayed when calling toJSON.

The inspectable attribute can be used on Rust structs to provide a toJSON and toString implementation that display all readable fields. For example:

# #![allow(unused_variables)]
#fn main() {
#[wasm_bindgen(inspectable)]
pub struct Baz {
    pub field: i32,
    private: i32,
}

#[wasm_bindgen]
impl Baz {
    #[wasm_bindgen(constructor)]
    pub fn new(field: i32) -> Baz {
        Baz { field, private: 13 }
    }
}
#}

Provides the following behavior as in this JavaScript snippet:

const obj = new Baz(3);
assert.deepStrictEqual(obj.toJSON(), { field: 3 });
obj.field = 4;
assert.strictEqual(obj.toString(), '{"field":4}');

One or both of these implementations can be overridden as desired. Note that the generated toString calls toJSON internally, so overriding toJSON will affect its output as a side effect.

# #![allow(unused_variables)]
#fn main() {
#[wasm_bindgen]
impl Baz {
    #[wasm_bindgen(js_name = toJSON)]
    pub fn to_json(&self) -> i32 {
        self.field
    }

    #[wasm_bindgen(js_name = toString)]
    pub fn to_string(&self) -> String {
        format!("Baz: {}", self.field)
    }
}
#}

Note that the output of console.log will remain unchanged and display only the ptr field in browsers. It is recommended to call toJSON or JSON.stringify in these situations to aid with logging or debugging. Node.js does not suffer from this limitation, see the section below.

When the nodejs target is used, an additional [util.inspect.custom] implementation is provided which calls toJSON internally. This method is used for console.log and similar functions to display all readable fields of the Rust struct.