webassembly/objjsproxy: Lookup attributes without testing they exist.

In JavaScript when accessing an attribute such as `obj.attr` a value of
`undefined` is returned if the attribute does not exist.  This is unlike
Python semantics where an `AttributeError` is raised.  Furthermore, in some
cases in JavaScript (eg a Proxy instance) `attr in obj` can return false
yet `obj.attr` is still valid and returns something other than `undefined`.
So the source of truth for whether a JavaScript attribute exists is to just
right away attempt `obj.attr`.

To more closely match these JavaScript semantics when proxying a JavaScript
object through to Python, change the attribute lookup logic on a `JsProxy`
so that it immediately attempts `obj.attr` instead of first testing if the
attribute exists via `attr in obj`.

This allows JavaScript objects which dynamically create attributes to work
correctly on the Python side, with both `obj.attr` and `obj["attr"]`.  Note
that `obj["attr"]` already works in all cases because it immediately does
the subscript access without first testing if the attribute exists.

As a benefit, this new behaviour matches the Pyodide behaviour.

Signed-off-by: Damien George <damien@micropython.org>

1 files changed, 8 insertions, 2 deletions

diff --git a/ports/webassembly/objjsproxy.c b/ports/webassembly/objjsproxy.c
index cbfe8be49..167d4382b 100644
--- a/ports/webassembly/objjsproxy.c
+++ b/ports/webassembly/objjsproxy.c
@@ -46,8 +46,14 @@ EM_JS(bool, has_attr, (int jsref, const char *str), {
 EM_JS(bool, lookup_attr, (int jsref, const char *str, uint32_t * out), {
     const base = proxy_js_ref[jsref];
     const attr = UTF8ToString(str);
-    if (attr in base) {
-        let value = base[attr];
+
+    // Attempt to lookup the requested attribute from the base object:
+    // - If the value is not `undefined` then the attribute exists with that value.
+    // - Otherwise if the value is `undefined` and the `in` operator returns true, then
+    //   that attribute does exist and is intended to have a value of `undefined`.
+    // - Otherwise, the attribute does not exist.
+    let value = base[attr];
+    if (value !== undefined || attr in base) {
         if (typeof value === "function") {
             if (base !== globalThis) {
                 if ("_ref" in value) {