To read and write arbitrary properties from any untyped JavaScript value regardless if it is an instanceof some JavaScript class or not, use the js_sys::Reflect APIs. These APIs are bindings to the JavaScript builtin Reflect object and its methods.

You might also benefit from using duck-typed interfaces instead of working with untyped values.

API documentation for js_sys::Reflect::get.

A function that returns the value of a property.

# #![allow(unused_variables)]
#fn main() {
let value = js_sys::Reflect::get(&target, &property_key)?;
#}

let value = target[property_key];

API documentation for js_sys::Reflect::set.

A function that assigns a value to a property. Returns a boolean that is true if the update was successful.

# #![allow(unused_variables)]
#fn main() {
js_sys::Reflect::set(&target, &property_key, &value)?;
#}

target[property_key] = value;

API documentation for js_sys::Reflect::has.

The JavaScript in operator as function. Returns a boolean indicating whether an own or inherited property exists on the target.

# #![allow(unused_variables)]
#fn main() {
if js_sys::Reflect::has(&target, &property_key)? {
    // ...
} else {
    // ...
}
#}

if (property_key in target) {
    // ...
} else {
    // ...
}

See the js_sys::Reflect API documentation for the full listing of JavaScript value reflection and introspection capabilities.