Note: this is a very rough sketch of a proposal to serve as a reference in other issues, more design work is required to make it fully practical.

Proposal

• If a user type implements Externalizer interface, GopherJS will use the result of Externalize() method when exporting this object to JavaScript. Otherwise standard conversion rules are applied.
• If a user type implements Internalizer interface, GopherJS will create a new Go type instance and call Internalize() method on it, passing the original *js.Object in, and will use the instance as internalized value.

package js

type Externalizer interface {
	Externalize() *js.Object
}

type Internalizer interface {
	Internalize(o *js.Object)
}

The general idea is similar to that if Marshaler/Unmarshaler interfaces in package/json.

Motivation

• Interoperability with JavaScript BigInt libraries: U/Int64 conversion  #360.
• Importing configuration objects from JavaScript: Enable a way to pass/access a JSON object from backend to frontend more efficiently (skipping unmarshaling from a string). #359.