If you have a map with a struct type as the key type, then the values are printed as <nil> when you print the entire map.

    type T struct {
        i int
    }

    s := T{
        i: 1,
    }

    m := map[T]string{
        s: "foo",
    }

    // This correctly prints: m[s] = foo
    fmt.Printf("m[s] = %v\n", m[s])

    // This INCORRECTLY prints: m = map[{1}:<nil>]
    fmt.Printf("m = %v\n", m)

Possibly related:

I've been tracking down a bug involving the initialization of maps with struct keys, but I haven't found a clean repro case yet.

It has something to do with a struct-keyed map object being defined in package-level var or init function. In these cases it seems that the definitions of the map values are not included early enough in the generated JS file.

var (
  Foo = someTypeConstructor()
  m = map[otherType]thirdType{
    otherType{}: Foo.bar(),
  }
)

This version is too simple, and works fine. In my more complicated example, the compilation succeeds, but javascript complains that "Foo is undefined" when it tries to call Foo.bar(). The definition of Foo occurs 10 or so lines below in the generated JS.

I'm going to keep digging on this bug. Hopefully I can get a clean repro case soon.