What version of Go are you using (go version)?

go version go1.11.1 linux/amd64

Does this issue reproduce with the latest release?

Yes (same on tip)

What operating system and processor architecture are you using (go env)?

GOOS=js
GOARCH=wasm

What did you do?

var v js.Value
fmt.Println(v == js.Undefined())

What did you expect to see?

true

What did you see instead?

false

Description

Currently, ref type codes of JS values in wasm are arranged in a way that makes a zero value a NaN number instead of undefined.

This breaks established Go semantics where a zero pointer value of a type is assumed to have a nil value (JS default value is undefined).

To illustrate the difference, consider the case:

var a *Obj // a == nil

var m map[string]*Obj
b := m["non existent"] // b == nil

a == b // true

However, this code does not work in the current implementation:

var a js.Value // NaN !

m := js.Global()
b := m.Get("non existent") // undefined

a == b // false !

Proposed solution

Remap JS value type enum to list undefined as a first type code (= 0) and move NaN to a different type code.

CC @neelance