The documentation for js.Func currently says:

Func.Release must be called to free up resources when the function will not be used any more.

Release frees up resources allocated for the function. The function must not be invoked after calling Release.

Based on a comment from @neelance and my understanding of the code, it seems the intention is for it to be safe to release a js.Func at a time when no future invocations of the js.Func will be made. An invocation here means the time the execution begins. So, if a js.Func will only be invoked once, it is safe to release a js.Func right away in the Go function that will be executing:

var f js.Func
f = js.FuncOf(func(this js.Value, args []js.Value) interface{} {
    f.Release() // okay, even though this Go function is still in the middle of executing
    ...

I believe the documentation needs to make this more clear, so that users can rely on this behavior.

/cc @neelance @hajimehoshi @johanbrandhorst