diff --git a/js/src/frontend/BinASTTokenReaderContext.cpp b/js/src/frontend/BinASTTokenReaderContext.cpp
index dc283b9ba3cf..6c3ef05200ee 100644
--- a/js/src/frontend/BinASTTokenReaderContext.cpp
+++ b/js/src/frontend/BinASTTokenReaderContext.cpp
@@ -511,8 +511,11 @@ class HuffmanPreludeReader {
   struct Boolean : EntryBase {
     // The C++ type of values for this grammar type.
     using Type = bool;
+
     Boolean(const NormalizedInterfaceAndField identity) : EntryBase(identity) {}
-    size_t maxMumberOfSymbols() {
+
+    // The max number of symbols in a table for this type.
+    size_t maxMumberOfSymbols() const {
       // Sadly, to this day, there are only two known booleans.
       return 2;
     }
@@ -543,11 +546,18 @@ class HuffmanPreludeReader {
 
   // An optional value of a given interface.
   struct MaybeInterface : EntryBase {
+    // The C++ type of values for this grammar type.
+    using Type = Nullable;
+
     // The kind of the interface.
     const BinASTKind kind;
+
     MaybeInterface(const NormalizedInterfaceAndField identity, BinASTKind kind)
         : EntryBase(identity), kind(kind) {}
 
+    // The max number of symbols in a table for this type.
+    size_t maxMumberOfSymbols() const { return 2; }
+
     // Utility struct, used in macros to call the constructor as
     // `MaybeInterface::Maker(kind)(identity)`.
     struct Maker {
@@ -803,9 +813,13 @@ class HuffmanPreludeReader {
     MOZ_TRY_VAR(headerByte, reader.readByte());
     switch (headerByte) {
       case TableHeader::SingleValue: {
-        // The table contains a single value.
+        // Construct in-place.
         table = {mozilla::VariantType<Table>{}, cx_};
-        return readSingleValueTable<Table, Entry>(table.as<Table>(), entry);
+        auto& tableRef = table.as<Table>();
+
+        // The table contains a single value.
+        MOZ_TRY((readSingleValueTable<Table, Entry>(tableRef, entry)));
+        return Ok();
       }
       case TableHeader::MultipleValues: {
         // Table contains multiple values.
@@ -814,8 +828,12 @@ class HuffmanPreludeReader {
         if (numberOfSymbols > entry.maxMumberOfSymbols()) {
           return raiseInvalidTableData(entry.identity);
         }
-        return readMultipleValuesTable<Table, Entry>(table.as<Table>(), entry,
-                                                     numberOfSymbols);
+        // Construct in-place.
+        table = {mozilla::VariantType<Table>{}, cx_};
+        auto& tableRef = table.as<Table>();
+        MOZ_TRY((readMultipleValuesTable<Table, Entry>(tableRef, entry,
+                                                       numberOfSymbols)));
+        return Ok();
       }
       case TableHeader::Unreachable:
         // Table is unreachable, nothing to do.
@@ -825,6 +843,15 @@ class HuffmanPreludeReader {
     }
   }
 
+  // Reading a single boolean.
+  // 0 -> False
+  // _ -> True
+  template <>
+  MOZ_MUST_USE JS::Result<bool> readSymbol<Boolean>() {
+    BINJS_MOZ_TRY_DECL(symbol, reader.readByte());
+    return symbol != 0;
+  }
+
   // Reading tables of booleans
   // 0 -> False
   // _ -> True
@@ -842,13 +869,31 @@ class HuffmanPreludeReader {
     return Ok();
   }
 
-  // Reading a single boolean.
-  // 0 -> False
-  // _ -> True
+  // Reading a single `Foo?` where `Foo` is an interface.
+  // 0 -> null
+  // 1 -> Foo
   template <>
-  MOZ_MUST_USE JS::Result<bool> readSymbol<Boolean>() {
+  MOZ_MUST_USE JS::Result<Nullable> readSymbol<MaybeInterface>() {
     BINJS_MOZ_TRY_DECL(symbol, reader.readByte());
-    return symbol != 0;
+    return symbol == 0 ? Nullable::Null : Nullable::NonNull;
+  }
+
+  // Reading a table for `Foo?` where `Foo` is an interface.
+  // 0 -> null
+  // 1 -> Foo
+  template <>
+  MOZ_MUST_USE JS::Result<Ok> readSingleValueTable<
+      HuffmanTableIndexedSymbolsMaybeInterface, MaybeInterface>(
+      HuffmanTableIndexedSymbolsMaybeInterface& table, MaybeInterface entry) {
+    uint8_t indexByte;
+    MOZ_TRY_VAR(indexByte, reader.readByte());
+    if (indexByte >= 2) {
+      return raiseInvalidTableData(entry.identity);
+    }
+
+    MOZ_TRY(table.impl.initWithSingleValue(
+        cx_, indexByte == 0 ? Nullable::Null : Nullable::NonNull));
+    return Ok();
   }
 
  private:
@@ -885,11 +930,26 @@ class HuffmanPreludeReader {
       return Ok();
     }
 
+    // Optional Interface.
+    // Values: [null, non-null].
     MOZ_MUST_USE JS::Result<Ok> operator()(const MaybeInterface& entry) {
-      // FIXME: Enqueue fields.
-      // FIXME: Read table.
-      // FIXME: Initialize table.
-      MOZ_CRASH("Unimplemented");
+      // First, we need a table to determine whether the value is `null`.
+      MOZ_TRY((owner.readTable<HuffmanTableIndexedSymbolsMaybeInterface,
+                               MaybeInterface>(entry)));
+
+      // Then, if the table contains `true`, we need the fields of the
+      // interface.
+      // FIXME: readTable could return a reference to the table, eliminating an
+      // array lookup.
+      const auto& table = owner.dictionary.tableForField(entry.identity);
+      if (table.is<HuffmanTableUnreachable>()) {
+        return Ok();
+      }
+      const auto& tableRef =
+          table.as<HuffmanTableIndexedSymbolsMaybeInterface>();
+      if (!tableRef.isAlwaysNull()) {
+        owner.pushFields(entry.kind);
+      }
       return Ok();
     }
 
diff --git a/js/src/frontend/BinASTTokenReaderContext.h b/js/src/frontend/BinASTTokenReaderContext.h
index 54a7fd5cfa2c..dcc1e0b553c6 100644
--- a/js/src/frontend/BinASTTokenReaderContext.h
+++ b/js/src/frontend/BinASTTokenReaderContext.h
@@ -417,8 +417,18 @@ struct HuffmanEntry {
   const T value;
 };
 
+// The default inline buffer length for instances of HuffmanTable.
+// Specific type (e.g. booleans) will override this to provide something
+// more suited to their type.
 const size_t HUFFMAN_TABLE_DEFAULT_INLINE_BUFFER_LENGTH = 8;
 
+// A flag that determines only whether a value is `null`.
+// Used for optional interface.
+enum class Nullable {
+  Null,
+  NonNull,
+};
+
 template <typename T, int N = HUFFMAN_TABLE_DEFAULT_INLINE_BUFFER_LENGTH>
 class HuffmanTableImpl {
  public:
@@ -439,6 +449,10 @@ class HuffmanTableImpl {
   HuffmanTableImpl() = delete;
   HuffmanTableImpl(HuffmanTableImpl&) = delete;
 
+  size_t length() const { return values.length(); }
+  const HuffmanEntry<T>* begin() const { return values.begin(); }
+  const HuffmanEntry<T>* end() const { return values.end(); }
+
  private:
   // The entries in this Huffman table.
   // Entries are always ranked by increasing bit_length, and within
@@ -475,6 +489,24 @@ struct HuffmanTableIndexedSymbolsBool {
   HuffmanTableIndexedSymbolsBool(JSContext* cx) : impl(cx) {}
 };
 
+struct HuffmanTableIndexedSymbolsMaybeInterface {
+  HuffmanTableImpl<Nullable, 2> impl;
+  HuffmanTableIndexedSymbolsMaybeInterface(JSContext* cx) : impl(cx) {}
+
+  // `true` if this table only contains values for `null`.
+  bool isAlwaysNull() const {
+    MOZ_ASSERT(impl.length() > 0);
+
+    // By definition, we have either 1 or 2 values.
+    // By definition, if we have 2 values, one of them is not null.
+    if (impl.length() != 1) {
+      return false;
+    }
+    // Otherwise, check the single value.
+    return impl.begin()->value == Nullable::Null;
+  }
+};
+
 struct HuffmanTableIndexedSymbolsStringEnum {
   HuffmanTableImpl<uint8_t> impl;
 };
@@ -491,8 +523,8 @@ struct HuffmanTableIndexedSymbolsOptionalLiteralString {
 using HuffmanTable = mozilla::Variant<
     HuffmanTableUnreachable,  // Default value.
     HuffmanTableExplicitSymbolsF64, HuffmanTableExplicitSymbolsU32,
-    HuffmanTableIndexedSymbolsTag, HuffmanTableIndexedSymbolsBool,
-    HuffmanTableIndexedSymbolsStringEnum,
+    HuffmanTableIndexedSymbolsTag, HuffmanTableIndexedSymbolsMaybeInterface,
+    HuffmanTableIndexedSymbolsBool, HuffmanTableIndexedSymbolsStringEnum,
     HuffmanTableIndexedSymbolsLiteralString,
     HuffmanTableIndexedSymbolsOptionalLiteralString>;