The default set in the js file is overridden if you set a defaultValue in the hjson.
So it's one or the other.
Both are accessed using this.props.[propertyname]
The defaultValue from the hjson will show in the field within Wappler App Structure when you add the component, but the default from the js does not. It will only work at runtime.
If would be helpful if this was added to the documentation at the below locations.