Issue #1658 - Part 1: Implement support for optional chaining in the JS Parser

Partially based on: https://bugzilla.mozilla.org/show_bug.cgi?id=1566143

10 files changed, 483 insertions, 104 deletions

diff --git a/js/src/frontend/FoldConstants.cpp b/js/src/frontend/FoldConstants.cpp
index c348056c45..d0e88581ea 100644
--- a/js/src/frontend/FoldConstants.cpp
+++ b/js/src/frontend/FoldConstants.cpp
@@ -319,6 +319,7 @@ ContainsHoistedDeclaration(ExclusiveContext* cx, ParseNode* node, bool* result)
       case PNK_DELETEPROP:
       case PNK_DELETEELEM:
       case PNK_DELETEEXPR:
+      case PNK_DELETEOPTCHAIN:
       case PNK_POS:
       case PNK_NEG:
       case PNK_PREINCREMENT:
@@ -1304,7 +1305,7 @@ FoldElement(ExclusiveContext* cx, ParseNode** nodePtr, Parser<FullParseHandler>&
 {
     ParseNode* node = *nodePtr;
 
-    MOZ_ASSERT(node->isKind(PNK_ELEM));
+    MOZ_ASSERT(node->isKind(PNK_ELEM) || node->isKind(PNK_OPTELEM));
     MOZ_ASSERT(node->isArity(PN_BINARY));
 
     ParseNode*& expr = node->pn_left;
@@ -1348,9 +1349,15 @@ FoldElement(ExclusiveContext* cx, ParseNode** nodePtr, Parser<FullParseHandler>&
 
     // Optimization 3: We have expr["foo"] where foo is not an index.  Convert
     // to a property access (like expr.foo) that optimizes better downstream.
-    ParseNode* dottedAccess = parser.handler.newPropertyAccess(expr, name, node->pn_pos.end);
-    if (!dottedAccess)
+    ParseNode* dottedAccess;
+    if (node->isKind(PNK_OPTELEM)) {
+        dottedAccess = parser.handler.newOptionalPropertyAccess(expr, name, node->pn_pos.end);
+    } else {
+        dottedAccess = parser.handler.newPropertyAccess(expr, name, node->pn_pos.end);
+    }
+    if (!dottedAccess) {
         return false;
+    }
     dottedAccess->setInParens(node->isInParens());
     ReplaceNode(nodePtr, dottedAccess);
 
@@ -1599,13 +1606,13 @@ static bool
 FoldDottedProperty(ExclusiveContext* cx, ParseNode* node, Parser<FullParseHandler>& parser,
                    bool inGenexpLambda)
 {
-    MOZ_ASSERT(node->isKind(PNK_DOT));
+    MOZ_ASSERT(node->isKind(PNK_DOT) || node->isKind(PNK_OPTDOT));
     MOZ_ASSERT(node->isArity(PN_NAME));
 
     // Iterate through a long chain of dotted property accesses to find the
     // most-nested non-dotted property node, then fold that.
     ParseNode** nested = &node->pn_expr;
-    while ((*nested)->isKind(PNK_DOT)) {
+    while ((*nested)->isKind(PNK_DOT) || (*nested)->isKind(PNK_OPTDOT)) {
         MOZ_ASSERT((*nested)->isArity(PN_NAME));
         nested = &(*nested)->pn_expr;
     }
@@ -1632,6 +1639,9 @@ Fold(ExclusiveContext* cx, ParseNode** pnp, Parser<FullParseHandler>& parser, bo
     JS_CHECK_RECURSION(cx, return false);
 
     ParseNode* pn = *pnp;
+#ifdef DEBUG
+    ParseNodeKind kind = pn->getKind();
+#endif
 
     switch (pn->getKind()) {
       case PNK_NOP:
@@ -1713,6 +1723,8 @@ Fold(ExclusiveContext* cx, ParseNode** pnp, Parser<FullParseHandler>& parser, bo
         MOZ_ASSERT(pn->isArity(PN_BINARY));
         return Fold(cx, &pn->pn_left, parser, inGenexpLambda);
 
+      case PNK_DELETEOPTCHAIN:
+      case PNK_OPTCHAIN:
       case PNK_SEMI:
       case PNK_THIS:
         MOZ_ASSERT(pn->isArity(PN_UNARY));
@@ -1806,6 +1818,7 @@ Fold(ExclusiveContext* cx, ParseNode** pnp, Parser<FullParseHandler>& parser, bo
       case PNK_CLASS:
         return FoldClass(cx, pn, parser, inGenexpLambda);
 
+      case PNK_OPTELEM:
       case PNK_ELEM:
         return FoldElement(cx, pnp, parser, inGenexpLambda);
 
@@ -1896,6 +1909,7 @@ Fold(ExclusiveContext* cx, ParseNode** pnp, Parser<FullParseHandler>& parser, bo
         MOZ_ASSERT(pn->isArity(PN_NAME));
         return Fold(cx, &pn->pn_expr, parser, inGenexpLambda);
 
+      case PNK_OPTDOT:
       case PNK_DOT:
         return FoldDottedProperty(cx, pn, parser, inGenexpLambda);
 
diff --git a/js/src/frontend/FullParseHandler.h b/js/src/frontend/FullParseHandler.h
index f346351255..a5a5bd16a0 100644
--- a/js/src/frontend/FullParseHandler.h
+++ b/js/src/frontend/FullParseHandler.h
@@ -213,6 +213,18 @@ class FullParseHandler
         if (expr->isKind(PNK_ELEM))
             return newUnary(PNK_DELETEELEM, JSOP_NOP, begin, expr);
 
+        if (expr->isKind(PNK_OPTCHAIN)) {
+            ParseNode* kid = expr->pn_kid;
+            // Handle property deletion explicitly. OptionalCall is handled
+            // via DeleteExpr.
+            if (kid->isKind(PNK_DOT) ||
+                kid->isKind(PNK_OPTDOT) ||
+                kid->isKind(PNK_ELEM) ||
+                kid->isKind(PNK_OPTELEM)) {
+              return newUnary(PNK_DELETEOPTCHAIN, JSOP_NOP, begin, kid);
+            }
+        }
+
         return newUnary(PNK_DELETEEXPR, JSOP_NOP, begin, expr);
     }
 
@@ -319,6 +331,10 @@ class FullParseHandler
         return newList(PNK_CALL, JSOP_CALL);
     }
 
+    ParseNode* newOptionalCall() {
+        return newList(PNK_OPTCALL, JSOP_CALL);
+    }
+
     ParseNode* newTaggedTemplate() {
         return newList(PNK_TAGGED_TEMPLATE, JSOP_CALL);
     }
@@ -459,6 +475,11 @@ class FullParseHandler
         return new_<UnaryNode>(PNK_AWAIT, JSOP_AWAIT, pos, value);
     }
 
+    ParseNode* newOptionalChain(uint32_t begin, ParseNode* value) {
+        TokenPos pos(begin, value->pn_pos.end);
+        return new_<UnaryNode>(PNK_OPTCHAIN, JSOP_NOP, pos, value);
+    }
+
     // Statements
 
     ParseNode* newStatementList(const TokenPos& pos) {
@@ -678,6 +699,14 @@ class FullParseHandler
         return new_<PropertyByValue>(lhs, index, lhs->pn_pos.begin, end);
     }
 
+    ParseNode* newOptionalPropertyAccess(ParseNode* pn, PropertyName* name, uint32_t end) {
+        return new_<OptionalPropertyAccess>(pn, name, pn->pn_pos.begin, end);
+    }
+
+    ParseNode* newOptionalPropertyByValue(ParseNode* lhs, ParseNode* index, uint32_t end) {
+        return new_<OptionalPropertyByValue>(lhs, index, lhs->pn_pos.begin, end);
+    }
+
     inline MOZ_MUST_USE bool addCatchBlock(ParseNode* catchList, ParseNode* lexicalScope,
                                            ParseNode* catchName, ParseNode* catchGuard,
                                            ParseNode* catchBody);
diff --git a/js/src/frontend/ParseNode.cpp b/js/src/frontend/ParseNode.cpp
index 6233465630..dfbd724e2e 100644
--- a/js/src/frontend/ParseNode.cpp
+++ b/js/src/frontend/ParseNode.cpp
@@ -228,6 +228,8 @@ PushNodeChildren(ParseNode* pn, NodeStack* stack)
         return PushUnaryNodeChild(pn, stack);
 
       // Nodes with a single nullable child.
+      case PNK_OPTCHAIN:
+      case PNK_DELETEOPTCHAIN:
       case PNK_THIS:
       case PNK_SEMI: {
         MOZ_ASSERT(pn->isArity(PN_UNARY));
@@ -253,6 +255,7 @@ PushNodeChildren(ParseNode* pn, NodeStack* stack)
       case PNK_MODASSIGN:
       case PNK_POWASSIGN:
       // ...and a few others.
+      case PNK_OPTELEM:
       case PNK_ELEM:
       case PNK_IMPORT_SPEC:
       case PNK_EXPORT_SPEC:
@@ -449,6 +452,7 @@ PushNodeChildren(ParseNode* pn, NodeStack* stack)
       case PNK_POW:
       case PNK_COMMA:
       case PNK_NEW:
+      case PNK_OPTCALL:
       case PNK_CALL:
       case PNK_SUPERCALL:
       case PNK_GENEXP:
@@ -483,6 +487,7 @@ PushNodeChildren(ParseNode* pn, NodeStack* stack)
       }
 
       case PNK_LABEL:
+      case PNK_OPTDOT:
       case PNK_DOT:
       case PNK_NAME:
         return PushNameNodeChildren(pn, stack);
diff --git a/js/src/frontend/ParseNode.h b/js/src/frontend/ParseNode.h
index 8e5e74d3b7..aae2a691bb 100644
--- a/js/src/frontend/ParseNode.h
+++ b/js/src/frontend/ParseNode.h
@@ -34,6 +34,10 @@ class ObjectBox;
     F(POSTDECREMENT) \
     F(DOT) \
     F(ELEM) \
+    F(OPTDOT) \
+    F(OPTCHAIN) \
+    F(OPTELEM) \
+    F(OPTCALL) \
     F(ARRAY) \
     F(ELISION) \
     F(STATEMENTLIST) \
@@ -76,6 +80,7 @@ class ObjectBox;
     F(DELETEPROP) \
     F(DELETEELEM) \
     F(DELETEEXPR) \
+    F(DELETEOPTCHAIN) \
     F(TRY) \
     F(CATCH) \
     F(CATCHLIST) \
@@ -371,6 +376,29 @@ IsTypeofKind(ParseNodeKind kind)
  *                          for a more-specific PNK_DELETE* unless constant
  *                          folding (or a similar parse tree manipulation) has
  *                          occurred
+ * PNK_DELETEOPTCHAIN unary pn_kid: MEMBER expr that's evaluated, then the
+ *                          overall delete evaluates to true; If constant
+ *                          folding occurs, PNK_ELEM may become PNK_DOT.
+ *                          PNK_OPTELEM does not get folded into PNK_OPTDOT.
+ * PNK_OPTCHAIN unary       pn_kid: expression that is evaluated as a chain.
+ *                          Contains one or more optional nodes. Its first node
+ *                          is always an optional node, such as PNK_OPTELEM,
+ *                          PNK_OPTDOT, or PNK_OPTCALL. This will shortcircuit
+ *                          and return 'undefined' without evaluating the rest
+ *                          of the expression if any of the optional nodes it
+ *                          contains are nullish. An optional chain can also
+ *                          contain nodes such as PNK_DOT, PNK_ELEM, PNK_NAME,
+ *                          PNK_CALL, etc. These are evaluated normally.
+ * PNK_OPTDOT   name        pn_expr: MEMBER expr to left of .
+ *                          short circuits back to PNK_OPTCHAIN if nullish.
+ *                          pn_atom: name to right of .
+ * PNK_OPTELEM  binary      pn_left: MEMBER expr to left of [
+ *                          short circuits back to PNK_OPTCHAIN if nullish.
+ *                          pn_right: expr between [ and ]
+ * PNK_OPTCALL  list        pn_head: list of call, arg1, arg2, ... argN
+ *                          pn_count: 1 + N (where N is number of args)
+ *                          call is a MEMBER expr naming a callable object,
+ *                          short circuits back to PNK_OPTCHAIN if nullish.
  * PNK_DOT      name        pn_expr: MEMBER expr to left of .
  *                          pn_atom: name to right of .
  * PNK_ELEM     binary      pn_left: MEMBER expr to left of [
@@ -1182,11 +1210,12 @@ class RegExpLiteral : public NullaryNode
     }
 };
 
-class PropertyAccess : public ParseNode
+class PropertyAccessBase : public ParseNode
 {
   public:
-    PropertyAccess(ParseNode* lhs, PropertyName* name, uint32_t begin, uint32_t end)
-      : ParseNode(PNK_DOT, JSOP_NOP, PN_NAME, TokenPos(begin, end))
+    PropertyAccessBase(ParseNodeKind kind, ParseNode* lhs, PropertyName* name,
+                       uint32_t begin, uint32_t end)
+      : ParseNode(kind, JSOP_NOP, PN_NAME, TokenPos(begin, end))
     {
         MOZ_ASSERT(lhs != nullptr);
         MOZ_ASSERT(name != nullptr);
@@ -1195,7 +1224,8 @@ class PropertyAccess : public ParseNode
     }
 
     static bool test(const ParseNode& node) {
-        bool match = node.isKind(PNK_DOT);
+        bool match = node.isKind(PNK_DOT) ||
+                     node.isKind(PNK_OPTDOT);
         MOZ_ASSERT_IF(match, node.isArity(PN_NAME));
         return match;
     }
@@ -1214,18 +1244,56 @@ class PropertyAccess : public ParseNode
     }
 };
 
-class PropertyByValue : public ParseNode
+class PropertyAccess : public PropertyAccessBase
+{
+  public:
+    PropertyAccess(ParseNode* lhs, PropertyName* name, uint32_t begin,
+                   uint32_t end)
+      : PropertyAccessBase(PNK_DOT, lhs, name, begin, end)
+    {
+        MOZ_ASSERT(lhs != nullptr);
+        MOZ_ASSERT(name != nullptr);
+    }
+
+    static bool test(const ParseNode& node) {
+        bool match = node.isKind(PNK_DOT);
+        MOZ_ASSERT_IF(match, node.isArity(PN_NAME));
+        return match;
+    }
+};
+
+class OptionalPropertyAccess : public PropertyAccessBase
+{
+  public:
+    OptionalPropertyAccess(ParseNode* lhs, PropertyName* name, uint32_t begin,
+                           uint32_t end)
+      : PropertyAccessBase(PNK_OPTDOT, lhs, name, begin, end)
+    {
+        MOZ_ASSERT(lhs != nullptr);
+        MOZ_ASSERT(name != nullptr);
+    }
+
+    static bool test(const ParseNode& node) {
+        bool match = node.isKind(PNK_OPTDOT);
+        MOZ_ASSERT_IF(match, node.isArity(PN_NAME));
+        return match;
+    }
+};
+
+class PropertyByValueBase : public ParseNode
 {
   public:
-    PropertyByValue(ParseNode* lhs, ParseNode* propExpr, uint32_t begin, uint32_t end)
-      : ParseNode(PNK_ELEM, JSOP_NOP, PN_BINARY, TokenPos(begin, end))
+    PropertyByValueBase(ParseNodeKind kind, ParseNode* lhs, ParseNode* propExpr,
+                    uint32_t begin, uint32_t end)
+      : ParseNode(kind, JSOP_NOP, PN_BINARY, TokenPos(begin, end))
     {
         pn_u.binary.left = lhs;
         pn_u.binary.right = propExpr;
     }
 
     static bool test(const ParseNode& node) {
-        bool match = node.isKind(PNK_ELEM);
+        bool match = node.isKind(PNK_ELEM) ||
+                     node.isKind(PNK_OPTELEM);
         MOZ_ASSERT_IF(match, node.isArity(PN_BINARY));
         return match;
     }
@@ -1235,6 +1303,32 @@ class PropertyByValue : public ParseNode
     }
 };
 
+class PropertyByValue : public PropertyByValueBase {
+  public:
+    PropertyByValue(ParseNode* lhs, ParseNode* propExpr, uint32_t begin,
+                    uint32_t end)
+      : PropertyByValueBase(PNK_ELEM, lhs, propExpr, begin, end) {}
+
+    static bool test(const ParseNode& node) {
+        bool match = node.isKind(PNK_ELEM);
+        MOZ_ASSERT_IF(match, node.isArity(PN_BINARY));
+        return match;
+    }
+};
+
+class OptionalPropertyByValue : public PropertyByValueBase {
+ public:
+    OptionalPropertyByValue(ParseNode* lhs, ParseNode* propExpr,
+                            uint32_t begin, uint32_t end)
+      : PropertyByValueBase(PNK_OPTELEM, lhs, propExpr, begin, end) {}
+
+    static bool test(const ParseNode& node) {
+        bool match = node.isKind(PNK_OPTELEM);
+        MOZ_ASSERT_IF(match, node.isArity(PN_BINARY));
+        return match;
+    }
+};
+
 /*
  * A CallSiteNode represents the implicit call site object argument in a TaggedTemplate.
  */
diff --git a/js/src/frontend/Parser.cpp b/js/src/frontend/Parser.cpp
index 38bd77ead3..cced4a4ea9 100644
--- a/js/src/frontend/Parser.cpp
+++ b/js/src/frontend/Parser.cpp
@@ -8363,6 +8363,109 @@ Parser<ParseHandler>::unaryOpExpr(YieldHandling yieldHandling, ParseNodeKind kin
 
 template <typename ParseHandler>
 typename ParseHandler::Node
+Parser<ParseHandler>::optionalExpr(
+    YieldHandling yieldHandling, TripledotHandling tripledotHandling,
+    TokenKind tt, bool allowCallSyntax /* = true */,
+    PossibleError* possibleError /* = nullptr */,
+    InvokedPrediction invoked /* = PredictUninvoked */)
+{
+    JS_CHECK_RECURSION(context, return null());
+
+    uint32_t begin = pos().begin;
+
+    Node lhs = memberExpr(yieldHandling, tripledotHandling, tt,
+                          /* allowCallSyntax = */ true, possibleError, invoked);
+
+    if (!lhs) {
+        return null();
+    }
+
+    // Note: TokenStream::None is equivalent to TokenStream::SlashIsDiv
+    // in current Mozilla code, see bug 1539821.
+    if (!tokenStream.peekToken(&tt, TokenStream::None)) {
+        return null();
+    }
+
+    if (tt == TOK_EOF || tt != TOK_OPTCHAIN) {
+        return lhs;
+    }
+
+    while (true) {
+        if (!tokenStream.getToken(&tt)) {
+            return null();
+        }
+
+        if (tt == TOK_EOF) {
+            break;
+        }
+
+        Node nextMember;
+        if (tt == TOK_OPTCHAIN) {
+            if (!tokenStream.getToken(&tt)) {
+                return null();
+            }
+            if (TokenKindIsPossibleIdentifierName(tt)) {
+                nextMember = memberPropertyAccess(lhs, OptionalKind::Optional);
+                if (!nextMember) {
+                    return null();
+                }
+            } else if (tt == TOK_LB) {
+                nextMember = memberElemAccess(lhs, yieldHandling,
+                                              OptionalKind::Optional);
+                if (!nextMember) {
+                    return null();
+                }
+            } else if (tt == TOK_LP) {
+                nextMember = memberCall(tt, lhs, yieldHandling, possibleError,
+                                        OptionalKind::Optional);
+                if (!nextMember) {
+                    return null();
+                }
+            } else {
+                error(JSMSG_NAME_AFTER_DOT);
+                return null();
+            }
+        } else if (tt == TOK_DOT) {
+            if (!tokenStream.getToken(&tt)) {
+                return null();
+            }
+            if (TokenKindIsPossibleIdentifierName(tt)) {
+                nextMember = memberPropertyAccess(lhs);
+                if (!nextMember) {
+                    return null();
+                }
+            } else {
+                error(JSMSG_NAME_AFTER_DOT);
+                return null();
+            }
+        } else if (tt == TOK_LB) {
+            nextMember = memberElemAccess(lhs, yieldHandling);
+            if (!nextMember) {
+                return null();
+            }
+        } else if (allowCallSyntax && tt == TOK_LP) {
+            nextMember = memberCall(tt, lhs, yieldHandling, possibleError);
+            if (!nextMember) {
+                return null();
+            }
+        } else if (tt == TOK_TEMPLATE_HEAD || tt == TOK_NO_SUBS_TEMPLATE) {
+            error(JSMSG_BAD_OPTIONAL_TEMPLATE);
+            return null();
+        } else {
+            tokenStream.ungetToken();
+            break;
+        }
+
+        if (nextMember) {
+            lhs = nextMember;
+        }
+    }
+
+  return handler.newOptionalChain(begin, lhs);
+}
+
+template <typename ParseHandler>
+typename ParseHandler::Node
 Parser<ParseHandler>::unaryExpr(YieldHandling yieldHandling, TripledotHandling tripledotHandling,
                                 PossibleError* possibleError /* = nullptr */,
                                 InvokedPrediction invoked /* = PredictUninvoked */)
@@ -8454,8 +8557,9 @@ Parser<ParseHandler>::unaryExpr(YieldHandling yieldHandling, TripledotHandling t
         MOZ_FALLTHROUGH;
 
       default: {
-        Node expr = memberExpr(yieldHandling, tripledotHandling, tt, /* allowCallSyntax = */ true,
-                             possibleError, invoked);
+        Node expr = optionalExpr(yieldHandling, tripledotHandling, tt,
+                                 /* allowCallSyntax = */ true,
+                                 possibleError, invoked);
         if (!expr)
             return null();
 
@@ -8904,6 +9008,18 @@ Parser<ParseHandler>::memberExpr(YieldHandling yieldHandling, TripledotHandling
 
             handler.addList(lhs, ctorExpr);
 
+            // If we have encountered an optional chain, in the form of `new
+            // ClassName?.()` then we need to throw, as this is disallowed
+            // by the spec.
+            bool optionalToken;
+            if (!tokenStream.matchToken(&optionalToken, TOK_OPTCHAIN)) {
+                return null();
+            }
+            if (optionalToken) {
+                errorAt(newBegin, JSMSG_BAD_NEW_OPTIONAL);
+                return null();
+            }
+
             bool matched;
             if (!tokenStream.matchToken(&matched, TOK_LP))
                 return null();
@@ -8941,12 +9057,7 @@ Parser<ParseHandler>::memberExpr(YieldHandling yieldHandling, TripledotHandling
             if (!tokenStream.getToken(&tt))
                 return null();
             if (TokenKindIsPossibleIdentifierName(tt)) {
-                PropertyName* field = tokenStream.currentName();
-                if (handler.isSuperBase(lhs) && !checkAndMarkSuperScope()) {
-                    error(JSMSG_BAD_SUPERPROP, "property");
-                    return null();
-                }
-                nextMember = handler.newPropertyAccess(lhs, field, pos().end);
+                nextMember = memberPropertyAccess(lhs);
                 if (!nextMember)
                     return null();
             } else {
@@ -8954,17 +9065,7 @@ Parser<ParseHandler>::memberExpr(YieldHandling yieldHandling, TripledotHandling
                 return null();
             }
         } else if (tt == TOK_LB) {
-            Node propExpr = expr(InAllowed, yieldHandling, TripledotProhibited);
-            if (!propExpr)
-                return null();
-
-            MUST_MATCH_TOKEN(TOK_RB, JSMSG_BRACKET_IN_INDEX);
-
-            if (handler.isSuperBase(lhs) && !checkAndMarkSuperScope()) {
-                error(JSMSG_BAD_SUPERPROP, "member");
-                return null();
-            }
-            nextMember = handler.newPropertyByValue(lhs, propExpr, pos().end);
+            nextMember = memberElemAccess(lhs, yieldHandling);
             if (!nextMember)
                 return null();
         } else if ((allowCallSyntax && tt == TOK_LP) ||
@@ -9000,82 +9101,17 @@ Parser<ParseHandler>::memberExpr(YieldHandling yieldHandling, TripledotHandling
                 if (!thisName)
                     return null();
 
+                // XXX This was refactored out into memberSuperCall as part of
+                // bug 1566143, but was not used elsewhere aside from here, so
+                // I opted not to move this into a separate function. However,
+                // I'm unsure if this will be needed eventually in the future.
                 nextMember = handler.newSetThis(thisName, nextMember);
                 if (!nextMember)
                     return null();
             } else {
-                if (options().selfHostingMode && handler.isPropertyAccess(lhs)) {
-                    error(JSMSG_SELFHOSTED_METHOD_CALL);
-                    return null();
-                }
-
-                nextMember = tt == TOK_LP ? handler.newCall() : handler.newTaggedTemplate();
+                nextMember = memberCall(tt, lhs, yieldHandling, possibleError);
                 if (!nextMember)
                     return null();
-
-                JSOp op = JSOP_CALL;
-                bool maybeAsyncArrow = false;
-                if (PropertyName* prop = handler.maybeDottedProperty(lhs)) {
-                    // Use the JSOP_FUN{APPLY,CALL} optimizations given the
-                    // right syntax.
-                    if (prop == context->names().apply) {
-                        op = JSOP_FUNAPPLY;
-                        if (pc->isFunctionBox()) {
-                            pc->functionBox()->usesApply = true;
-                        }
-                    } else if (prop == context->names().call) {
-                        op = JSOP_FUNCALL;
-                    }
-                } else if (tt == TOK_LP) {
-                    if (handler.isAsyncKeyword(lhs, context)) {
-                        // |async (| can be the start of an async arrow
-                        // function, so we need to defer reporting possible
-                        // errors from destructuring syntax. To give better
-                        // error messages, we only allow the AsyncArrowHead
-                        // part of the CoverCallExpressionAndAsyncArrowHead
-                        // syntax when the initial name is "async".
-                        maybeAsyncArrow = true;
-                    } else if (handler.isEvalAnyParentheses(lhs, context)) {
-                        // Select the right EVAL op and flag pc as having a
-                        // direct eval.
-                        op = pc->sc()->strict() ? JSOP_STRICTEVAL : JSOP_EVAL;
-                        pc->sc()->setBindingsAccessedDynamically();
-                        pc->sc()->setHasDirectEval();
-
-                        // In non-strict mode code, direct calls to eval can
-                        // add variables to the call object.
-                        if (pc->isFunctionBox() && !pc->sc()->strict())
-                            pc->functionBox()->setHasExtensibleScope();
-
-                        // If we're in a method, mark the method as requiring
-                        // support for 'super', since direct eval code can use
-                        // it. (If we're not in a method, that's fine, so
-                        // ignore the return value.)
-                        checkAndMarkSuperScope();
-                    }
-                }
-
-                handler.setBeginPosition(nextMember, lhs);
-                handler.addList(nextMember, lhs);
-
-                if (tt == TOK_LP) {
-                    bool isSpread = false;
-                    PossibleError* asyncPossibleError = maybeAsyncArrow ? possibleError : nullptr;
-                    if (!argumentList(yieldHandling, nextMember, &isSpread, asyncPossibleError))
-                        return null();
-                    if (isSpread) {
-                        if (op == JSOP_EVAL)
-                            op = JSOP_SPREADEVAL;
-                        else if (op == JSOP_STRICTEVAL)
-                            op = JSOP_STRICTSPREADEVAL;
-                        else
-                            op = JSOP_SPREADCALL;
-                    }
-                } else {
-                    if (!taggedTemplate(yieldHandling, nextMember, tt))
-                        return null();
-                }
-                handler.setOp(nextMember, op);
             }
         } else {
             tokenStream.ungetToken();
@@ -9109,6 +9145,155 @@ Parser<ParseHandler>::newName(PropertyName* name, TokenPos pos)
     return handler.newName(name, pos, context);
 }
 
+template <class ParseHandler>
+typename ParseHandler::Node
+Parser<ParseHandler>::memberPropertyAccess(
+    Node lhs, OptionalKind optionalKind /* = OptionalKind::NonOptional */)
+{
+    MOZ_ASSERT(TokenKindIsPossibleIdentifierName(tokenStream.currentToken().type));
+    PropertyName* field = tokenStream.currentName();
+    if (handler.isSuperBase(lhs) && !checkAndMarkSuperScope()) {
+        error(JSMSG_BAD_SUPERPROP, "property");
+        return null();
+    }
+    if (optionalKind == OptionalKind::Optional) {
+        return handler.newOptionalPropertyAccess(lhs, field, pos().end);
+    }
+    return handler.newPropertyAccess(lhs, field, pos().end);
+}
+
+template <class ParseHandler>
+typename ParseHandler::Node
+Parser<ParseHandler>::memberElemAccess(
+    Node lhs, YieldHandling yieldHandling,
+    OptionalKind optionalKind /* = OptionalKind::NonOptional */)
+{
+    MOZ_ASSERT(tokenStream.currentToken().type == TOK_LB);
+    Node propExpr = expr(InAllowed, yieldHandling, TripledotProhibited);
+    if (!propExpr) {
+        return null();
+    }
+
+    MUST_MATCH_TOKEN(TOK_RB, JSMSG_BRACKET_IN_INDEX);
+
+    if (handler.isSuperBase(lhs) && !checkAndMarkSuperScope()) {
+        error(JSMSG_BAD_SUPERPROP, "member");
+        return null();
+    }
+
+    if (optionalKind == OptionalKind::Optional) {
+        return handler.newOptionalPropertyByValue(lhs, propExpr, pos().end);
+    }
+    return handler.newPropertyByValue(lhs, propExpr, pos().end);
+}
+
+template <class ParseHandler>
+typename ParseHandler::Node
+Parser<ParseHandler>::memberCall(
+    TokenKind tt, Node lhs, YieldHandling yieldHandling,
+    PossibleError* possibleError /* = nullptr */,
+    OptionalKind optionalKind /* = OptionalKind::NonOptional */)
+{
+    if (options().selfHostingMode && handler.isPropertyAccess(lhs)) {
+        error(JSMSG_SELFHOSTED_METHOD_CALL);
+        return null();
+    }
+
+    MOZ_ASSERT(tt == TOK_LP ||
+               tt == TOK_TEMPLATE_HEAD ||
+               tt == TOK_NO_SUBS_TEMPLATE,
+               "Unexpected token kind for member call");
+
+    Node nextMember;
+    if (tt == TOK_LP) {
+        if (optionalKind == OptionalKind::Optional) {
+            nextMember = handler.newOptionalCall();
+        } else {
+            nextMember = handler.newCall();
+        }
+    } else {
+        nextMember = handler.newTaggedTemplate();
+    }
+
+    JSOp op = JSOP_CALL;
+    bool maybeAsyncArrow = false;
+    if (PropertyName* prop = handler.maybeDottedProperty(lhs)) {
+        // Use the JSOP_FUN{APPLY,CALL} optimizations given the
+        // right syntax.
+        if (prop == context->names().apply) {
+            op = JSOP_FUNAPPLY;
+            if (pc->isFunctionBox()) {
+                pc->functionBox()->usesApply = true;
+            }
+        } else if (prop == context->names().call) {
+            op = JSOP_FUNCALL;
+        }
+    } else if (tt == TOK_LP && optionalKind == OptionalKind::NonOptional) {
+        if (handler.isAsyncKeyword(lhs, context)) {
+            // |async (| can be the start of an async arrow
+            // function, so we need to defer reporting possible
+            // errors from destructuring syntax. To give better
+            // error messages, we only allow the AsyncArrowHead
+            // part of the CoverCallExpressionAndAsyncArrowHead
+            // syntax when the initial name is "async".
+            maybeAsyncArrow = true;
+        } else if (handler.isEvalAnyParentheses(lhs, context)) {
+            // Select the right EVAL op and flag pc as having a
+            // direct eval.
+            op = pc->sc()->strict() ? JSOP_STRICTEVAL : JSOP_EVAL;
+            pc->sc()->setBindingsAccessedDynamically();
+            pc->sc()->setHasDirectEval();
+
+            // In non-strict mode code, direct calls to eval can
+            // add variables to the call object.
+            if (pc->isFunctionBox() && !pc->sc()->strict()) {
+                pc->functionBox()->setHasExtensibleScope();
+            }
+
+            // If we're in a method, mark the method as requiring
+            // support for 'super', since direct eval code can use
+            // it. (If we're not in a method, that's fine, so
+            // ignore the return value.)
+            checkAndMarkSuperScope();
+        }
+    }
+
+    handler.setBeginPosition(nextMember, lhs);
+    handler.addList(nextMember, lhs);
+
+    if (tt == TOK_LP) {
+        bool isSpread = false;
+        PossibleError* asyncPossibleError = maybeAsyncArrow ?
+                                            possibleError :
+                                            nullptr;
+
+        if (!argumentList(yieldHandling, nextMember, &isSpread, asyncPossibleError)) {
+            return null();
+        }
+
+        if (isSpread) {
+            if (op == JSOP_EVAL) {
+                op = JSOP_SPREADEVAL;
+            } else if (op == JSOP_STRICTEVAL) {
+                op = JSOP_STRICTSPREADEVAL;
+            } else {
+                op = JSOP_SPREADCALL;
+            }
+        }
+    } else {
+        if (!taggedTemplate(yieldHandling, nextMember, tt)) {
+            return null();
+        }
+        if (optionalKind == OptionalKind::Optional) {
+            error(JSMSG_BAD_OPTIONAL_TEMPLATE);
+            return null();
+        }
+    }
+    handler.setOp(nextMember, op);
+
+    return nextMember;
+}
+
 template <typename ParseHandler>
 bool
 Parser<ParseHandler>::checkLabelOrIdentifierReference(HandlePropertyName ident,
diff --git a/js/src/frontend/Parser.h b/js/src/frontend/Parser.h
index 0e2f28fe88..97f6917bd6 100644
--- a/js/src/frontend/Parser.h
+++ b/js/src/frontend/Parser.h
@@ -1321,6 +1321,11 @@ class Parser final : public ParserBase, private JS::AutoGCRooter
     Node unaryExpr(YieldHandling yieldHandling, TripledotHandling tripledotHandling,
                    PossibleError* possibleError = nullptr,
                    InvokedPrediction invoked = PredictUninvoked);
+    Node optionalExpr(YieldHandling yieldHandling,
+                      TripledotHandling tripledotHandling, TokenKind tt,
+                      bool allowCallSyntax = true,
+                      PossibleError* possibleError = nullptr,
+                      InvokedPrediction invoked = PredictUninvoked);
     Node memberExpr(YieldHandling yieldHandling, TripledotHandling tripledotHandling,
                     TokenKind tt, bool allowCallSyntax = true,
                     PossibleError* possibleError = nullptr,
@@ -1532,6 +1537,18 @@ class Parser final : public ParserBase, private JS::AutoGCRooter
     JSAtom* prefixAccessorName(PropertyType propType, HandleAtom propAtom);
 
     bool asmJS(Node list);
+
+    enum class OptionalKind {
+      NonOptional = 0,
+      Optional,
+    };
+    Node memberPropertyAccess(
+        Node lhs, OptionalKind optionalKind = OptionalKind::NonOptional);
+    Node memberElemAccess(Node lhs, YieldHandling yieldHandling,
+                          OptionalKind optionalKind = OptionalKind::NonOptional);
+    Node memberCall(TokenKind tt, Node lhs, YieldHandling yieldHandling,
+                    PossibleError* possibleError,
+                    OptionalKind optionalKind = OptionalKind::NonOptional);
 };
 
 template <typename ParseHandler>
diff --git a/js/src/frontend/SyntaxParseHandler.h b/js/src/frontend/SyntaxParseHandler.h
index 4fef3584c8..c55db41086 100644
--- a/js/src/frontend/SyntaxParseHandler.h
+++ b/js/src/frontend/SyntaxParseHandler.h
@@ -58,6 +58,7 @@ class SyntaxParseHandler
         // in code not actually executed (or at least not executed enough to be
         // noticed).
         NodeFunctionCall,
+        NodeOptionalFunctionCall,
 
         // Nodes representing *parenthesized* IsValidSimpleAssignmentTarget
         // nodes.  We can't simply treat all such parenthesized nodes
@@ -78,7 +79,9 @@ class SyntaxParseHandler
         NodeParenthesizedName,
 
         NodeDottedProperty,
+        NodeOptionalDottedProperty,
         NodeElement,
+        NodeOptionalElement,
 
         // Destructuring target patterns can't be parenthesized: |([a]) = [3];|
         // must be a syntax error.  (We can't use NodeGeneric instead of these
@@ -283,6 +286,7 @@ class SyntaxParseHandler
     void addArrayElement(Node literal, Node element) { }
 
     Node newCall() { return NodeFunctionCall; }
+    Node newOptionalCall() { return NodeOptionalFunctionCall; }
     Node newTaggedTemplate() { return NodeGeneric; }
 
     Node newObjectLiteral(uint32_t begin) { return NodeUnparenthesizedObject; }
@@ -303,6 +307,7 @@ class SyntaxParseHandler
     Node newYieldExpression(uint32_t begin, Node value) { return NodeGeneric; }
     Node newYieldStarExpression(uint32_t begin, Node value) { return NodeGeneric; }
     Node newAwaitExpression(uint32_t begin, Node value) { return NodeGeneric; }
+    Node newOptionalChain(uint32_t begin, Node value) { return NodeGeneric; }
 
     // Statements
 
@@ -353,8 +358,15 @@ class SyntaxParseHandler
         return NodeDottedProperty;
     }
 
+    Node newOptionalPropertyAccess(Node pn, PropertyName* name, uint32_t end) {
+        lastAtom = name;
+        return NodeOptionalDottedProperty;
+    }
+
     Node newPropertyByValue(Node pn, Node kid, uint32_t end) { return NodeElement; }
 
+    Node newOptionalPropertyByValue(Node pn, Node kid, uint32_t end) { return NodeOptionalElement; }
+
     MOZ_MUST_USE bool addCatchBlock(Node catchList, Node letBlock, Node catchName,
                                     Node catchGuard, Node catchBody) { return true; }
 
@@ -471,7 +483,8 @@ class SyntaxParseHandler
                    list == NodeUnparenthesizedCommaExpr ||
                    list == NodeVarDeclaration ||
                    list == NodeLexicalDeclaration ||
-                   list == NodeFunctionCall);
+                   list == NodeFunctionCall ||
+                   list == NodeOptionalFunctionCall);
     }
 
     Node newAssignment(ParseNodeKind kind, Node lhs, Node rhs, JSOp op) {
diff --git a/js/src/frontend/TokenKind.h b/js/src/frontend/TokenKind.h
index f4e1afa328..27da8ecf3e 100644
--- a/js/src/frontend/TokenKind.h
+++ b/js/src/frontend/TokenKind.h
@@ -63,6 +63,7 @@
     macro(DEC,          "'--'")   /* decrement */ \
     macro(DOT,          "'.'")    /* member operator */ \
     macro(TRIPLEDOT,    "'...'")  /* rest arguments and spread operator */ \
+    macro(OPTCHAIN,     "'?.'") \
     macro(LB,           "'['") \
     macro(RB,           "']'") \
     macro(LC,           "'{'") \
diff --git a/js/src/frontend/TokenStream.cpp b/js/src/frontend/TokenStream.cpp
index 083bcd5045..8f9e206d9f 100644
--- a/js/src/frontend/TokenStream.cpp
+++ b/js/src/frontend/TokenStream.cpp
@@ -1285,7 +1285,7 @@ static const uint8_t firstCharKinds[] = {
 /*  30+ */ _______, _______,   Space, _______,  String, _______,   Ident, _______, _______,  String,
 /*  40+ */  TOK_LP,  TOK_RP, _______, _______, T_COMMA,_______,  _______, _______,BasePrefix,  Dec,
 /*  50+ */     Dec,     Dec,     Dec,     Dec,     Dec,     Dec,     Dec,    Dec,  T_COLON,TOK_SEMI,
-/*  60+ */ _______, _______, _______,TOK_HOOK, _______,   Ident,   Ident,   Ident,   Ident,   Ident,
+/*  60+ */ _______, _______, _______, _______, _______,   Ident,   Ident,   Ident,   Ident,   Ident,
 /*  70+ */   Ident,   Ident,   Ident,   Ident,   Ident,   Ident,   Ident,   Ident,   Ident,   Ident,
 /*  80+ */   Ident,   Ident,   Ident,   Ident,   Ident,   Ident,   Ident,   Ident,   Ident,   Ident,
 /*  90+ */   Ident,  TOK_LB, _______,  TOK_RB, _______,   Ident, Templat,   Ident,   Ident,   Ident,
@@ -1796,6 +1796,25 @@ TokenStream::getTokenInternal(TokenKind* ttp, Modifier modifier)
             tp->type = matchChar('=') ? TOK_BITANDASSIGN : TOK_BITAND;
         goto out;
 
+      case '?':
+        if (matchChar('.')) {
+            c = getCharIgnoreEOL();
+            if (JS7_ISDEC(c)) {
+                // if the code unit is followed by a number, for example it has
+                // the following form `<...> ?.5 <..> then it should be treated
+                // as a ternary rather than as an optional chain
+                tp->type = TOK_HOOK;
+                ungetCharIgnoreEOL(c);
+                ungetChar('.');
+            } else {
+                ungetCharIgnoreEOL(c);
+                tp->type = TOK_OPTCHAIN;
+            }
+        } else {
+            tp->type = TOK_HOOK;
+        }
+        goto out;
+
       case '!':
         if (matchChar('='))
             tp->type = matchChar('=') ? TOK_STRICTNE : TOK_NE;
diff --git a/js/src/js.msg b/js/src/js.msg
index b03da4153d..566b55479b 100644
--- a/js/src/js.msg
+++ b/js/src/js.msg
@@ -354,6 +354,8 @@ MSG_DEF(JSMSG_BAD_COLUMN_NUMBER,       0, JSEXN_RANGEERR, "column number out of
 MSG_DEF(JSMSG_COMPUTED_NAME_IN_PATTERN,0, JSEXN_SYNTAXERR, "computed property names aren't supported in this destructuring declaration")
 MSG_DEF(JSMSG_DEFAULT_IN_PATTERN,      0, JSEXN_SYNTAXERR, "destructuring defaults aren't supported in this destructuring declaration")
 MSG_DEF(JSMSG_BAD_NEWTARGET,           0, JSEXN_SYNTAXERR, "new.target only allowed within functions")
+MSG_DEF(JSMSG_BAD_NEW_OPTIONAL,        0, JSEXN_SYNTAXERR, "new keyword cannot be used with an optional chain")
+MSG_DEF(JSMSG_BAD_OPTIONAL_TEMPLATE,   0, JSEXN_SYNTAXERR, "tagged template cannot be used with optional chain")
 MSG_DEF(JSMSG_ESCAPED_KEYWORD,         0, JSEXN_SYNTAXERR, "keywords must be written literally, without embedded escapes")
 
 // asm.js