Function set_str 

pub fn set_str<T>(
    target: &Object<T>,
    property_key: &JsString,
    value: &T,
) -> Result<bool, JsValue>
where
    Object<T>: ErasableGenericBorrow<Object<JsValue>>,
    T: ErasableGenericBorrow<JsValue>,

The static Reflect.set() method works like setting a property on an object.

MDN documentation