Issue #1639 - Implement object.fromEntries()

Adds a self-hosted implementation of this map->object conversion.
This resolves #1639.

js/src/builtin/Object.cpp

@@ -1239,6 +1239,7 @@ static const JSFunctionSpec object_static_methods[] = {
     JS_FN("isFrozen",                  obj_isFrozen,                1, 0),
     JS_FN("seal",                      obj_seal,                    1, 0),
     JS_FN("isSealed",                  obj_isSealed,                1, 0),
+    JS_SELF_HOSTED_FN("fromEntries",   "ObjectFromEntries",         1, 0),
     JS_FS_END
 };
 

js/src/builtin/Object.js

@@ -202,3 +202,21 @@ function ObjectLookupGetter(name) {
 
     // Step 3.d. (implicit)
 }
+
+// Stage 4 draft 2020-09-06 https://tc39.github.io/proposal-object-from-entries/
+// Object.fromEntries (iterable)
+function ObjectFromEntries(iter) {
+    // We omit the usual step number comments here because they don't help.
+    // This implementation inlines AddEntriesFromIterator and
+    // CreateDataPropertyOnObject, so it looks more like the polyfill
+    // than the step-by-step spec algorithm.
+    const obj = {};
+
+    for (const pair of allowContentIter(iter)) {
+        if (!IsObject(pair))
+            ThrowTypeError(JSMSG_INVALID_MAP_ITERABLE, "Object.fromEntries");
+        _DefineDataProperty(obj, pair[0], pair[1]);
+    }
+
+    return obj;
+}

js/xpconnect/tests/chrome/test_xrayToJS.xul

@@ -190,9 +190,9 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=933681
     constructorProps(["setPrototypeOf", "getOwnPropertyDescriptor", "getOwnPropertyDescriptors",
                       "keys", "is", "defineProperty", "defineProperties", "create",
                       "getOwnPropertyNames", "getOwnPropertySymbols",
-                      "preventExtensions", "freeze", "isFrozen", "seal",
+                      "preventExtensions", "freeze", "fromEntries", "isFrozen", "seal",
                       "isSealed", "assign", "getPrototypeOf", "values",
-                      "entries", "isExtensible"])
+                      "entries", "isExtensible"]);
   gPrototypeProperties['Array'] =
     ["length", "toSource", "toString", "toLocaleString", "join", "reverse", "sort", "push",
       "pop", "shift", "unshift", "splice", "concat", "slice", "lastIndexOf", "indexOf",