webassembly/proxy_js: Create a special "undefined" type for Python.

This adds a new undefined singleton to Python, that corresponds directly to
JavaScript `undefined`.  It's accessible via `js.undefined`.
Signed-off-by: Damien George <damien@micropython.org>

ports/webassembly/README.md

@@ -160,6 +160,18 @@ context, created and returned by `loadMicroPython()`.
 - `replProcessCharWithAsyncify(chr)`: process an incoming character at the REPL,
   for use when ASYNCIFY is enabled.
 
+Type conversions
+----------------
+
+Read-only objects (booleanns, numbers, strings, etc) are converted when passed between
+Python and JavaScript.  The conversions are:
+
+- JavaScript `null` converts to/from Python `None`.
+- JavaScript `undefined` converts to/from Python `js.undefined`.
+
+The conversion between `null` and `None` matches the behaviour of the Python `json`
+module.
+
 Proxying
 --------
 

ports/webassembly/proxy_c.c

@@ -59,6 +59,16 @@ enum {
     PROXY_KIND_JS_PYPROXY = 7,
 };
 
+MP_DEFINE_CONST_OBJ_TYPE(
+    mp_type_undefined,
+    MP_QSTR_undefined,
+    MP_TYPE_FLAG_NONE
+    );
+
+static const mp_obj_base_t mp_const_undefined_obj = {&mp_type_undefined};
+
+#define mp_const_undefined (MP_OBJ_FROM_PTR(&mp_const_undefined_obj))
+
 MP_DEFINE_EXCEPTION(JsException, Exception)
 
 void proxy_c_init(void) {
@@ -80,7 +90,7 @@ static inline mp_obj_t proxy_c_get_obj(uint32_t c_ref) {
 
 mp_obj_t proxy_convert_js_to_mp_obj_cside(uint32_t *value) {
     if (value[0] == PROXY_KIND_JS_UNDEFINED) {
-        return mp_const_none;
+        return mp_const_undefined;
     } else if (value[0] == PROXY_KIND_JS_NULL) {
         return mp_const_none;
     } else if (value[0] == PROXY_KIND_JS_BOOLEAN) {
@@ -122,6 +132,9 @@ void proxy_convert_mp_to_js_obj_cside(mp_obj_t obj, uint32_t *out) {
         const char *str = mp_obj_str_get_data(obj, &len);
         out[1] = len;
         out[2] = (uintptr_t)str;
+    } else if (obj == mp_const_undefined) {
+        kind = PROXY_KIND_MP_JSPROXY;
+        out[1] = 1;
     } else if (mp_obj_is_jsproxy(obj)) {
         kind = PROXY_KIND_MP_JSPROXY;
         out[1] = mp_obj_jsproxy_get_ref(obj);

ports/webassembly/proxy_js.js

@@ -56,7 +56,7 @@ class PythonError extends Error {
 }
 
 function proxy_js_init() {
-    globalThis.proxy_js_ref = [globalThis];
+    globalThis.proxy_js_ref = [globalThis, undefined];
 }
 
 function proxy_call_python(target, argumentsList) {

tests/ports/webassembly/await_error_handling.mjs.exp

 1
 2
-(<JsProxy 6>, 'Error', 'test')
+(<JsProxy 7>, 'Error', 'test')
 3
 true Error test

tests/ports/webassembly/jsffi_create_proxy.mjs.exp

 1
-<JsProxy 1>
+<JsProxy 2>
 1
 1
 PyProxy { _ref: 3 }

tests/ports/webassembly/jsffi_to_js.mjs.exp

 1
-<JsProxy 1>
 <JsProxy 2>
+<JsProxy 3>
 false
 1
 true

tests/ports/webassembly/register_js_module.js.exp

-<JsProxy 1>
+<JsProxy 2>
 2

tests/ports/webassembly/run_python_async.mjs.exp

 = TEST 1 ==========
 1
-<JsProxy 1>
+<JsProxy 2>
 py 1
-<JsProxy 4>
+<JsProxy 5>
 py 2
 2
 resolved 123
 3
 = TEST 2 ==========
 1
-<JsProxy 5>
+<JsProxy 6>
 py 1
-<JsProxy 8>
+<JsProxy 9>
 py 2
 2
 setTimeout resolved