--- a/js/src/frontend/BinSource-auto.cpp
+++ b/js/src/frontend/BinSource-auto.cpp
@@ -7127,18 +7127,21 @@ BinASTParser<Tok>::parseInterfaceThisExp
 
 MOZ_TRY(tokenizer_->checkFields0(kind, fields));
 
     if (parseContext_->isFunctionBox())
         parseContext_->functionBox()->usesThis = true;
 
     TokenPos pos = tokenizer_->pos(start);
     ParseNode* thisName(nullptr);
-    if (parseContext_->sc()->thisBinding() == ThisBinding::Function)
-        BINJS_TRY_VAR(thisName, factory_.newName(cx_->names().dotThis, pos, cx_));
+    if (parseContext_->sc()->thisBinding() == ThisBinding::Function) {
+        HandlePropertyName dotThis = cx_->names().dotThis;
+        BINJS_TRY(usedNames_.noteUse(cx_, dotThis, parseContext_->scriptId(), parseContext_->innermostScope()->id()));
+        BINJS_TRY_VAR(thisName, factory_.newName(dotThis, pos, cx_));
+    }
 
     BINJS_TRY_DECL(result, factory_.newThisLiteral(pos, thisName));
     return result;
 }
 
 
 /*
  interface ThrowStatement : Node {

--- a/js/src/frontend/BinSource.cpp
+++ b/js/src/frontend/BinSource.cpp
@@ -149,21 +149,20 @@ BinASTParser<Tok>::buildFunctionBox(Gene
     funbox->initWithEnclosingParseContext(parseContext_, syntax);
     return funbox;
 }
 
 template<typename Tok> JS::Result<ParseNode*>
 BinASTParser<Tok>::buildFunction(const size_t start, const BinKind kind, ParseNode* name,
                                  ParseNode* params, ParseNode* body, FunctionBox* funbox)
 {
-    // Check all our bindings before doing anything else.
-    MOZ_TRY(checkFunctionClosedVars());
-
     TokenPos pos = tokenizer_->pos(start);
 
+    // Set the argument count for building argument packets. Function.length is handled
+    // by setting the appropriate funbox field during argument parsing.
     funbox->function()->setArgCount(params ? uint16_t(params->pn_count) : 0);
 
     // ParseNode represents the body as concatenated after the params.
     params->appendWithoutOrderAssumption(body);
 
     bool isStatement = kind == BinKind::EagerFunctionDeclaration ||
                        kind == BinKind::SkippableFunctionDeclaration;
 
@@ -181,18 +180,23 @@ BinASTParser<Tok>::buildFunction(const s
 
     if (declareThis) {
         ParseContext::Scope& funScope = parseContext_->functionScope();
         ParseContext::Scope::AddDeclaredNamePtr p = funScope.lookupDeclaredNameForAdd(dotThis);
         MOZ_ASSERT(!p);
         BINJS_TRY(funScope.addDeclaredName(parseContext_, p, dotThis, DeclarationKind::Var,
                                      DeclaredNameInfo::npos));
         funbox->setHasThisBinding();
+
+        // TODO (efaust): This capture will have to come from encoder side for arrow functions.
     }
 
+    // Check all our bindings after maybe adding function This.
+    MOZ_TRY(checkFunctionClosedVars());
+
     BINJS_TRY_DECL(bindings,
              NewFunctionScopeData(cx_, parseContext_->functionScope(),
                                   /* hasParameterExprs = */ false, alloc_, parseContext_));
 
     funbox->functionScopeBindings().set(*bindings);
 
     if (funbox->function()->isNamedLambda()) {
         BINJS_TRY_DECL(recursiveBinding,

--- a/js/src/frontend/BinSource.yaml
+++ b/js/src/frontend/BinSource.yaml
@@ -913,18 +913,21 @@ StaticMemberExpression:
 
 ThisExpression:
     build: |
         if (parseContext_->isFunctionBox())
             parseContext_->functionBox()->usesThis = true;
 
         TokenPos pos = tokenizer_->pos(start);
         ParseNode* thisName(nullptr);
-        if (parseContext_->sc()->thisBinding() == ThisBinding::Function)
-            BINJS_TRY_VAR(thisName, factory_.newName(cx_->names().dotThis, pos, cx_));
+        if (parseContext_->sc()->thisBinding() == ThisBinding::Function) {
+            HandlePropertyName dotThis = cx_->names().dotThis;
+            BINJS_TRY(usedNames_.noteUse(cx_, dotThis, parseContext_->scriptId(), parseContext_->innermostScope()->id()));
+            BINJS_TRY_VAR(thisName, factory_.newName(dotThis, pos, cx_));
+        }
 
         BINJS_TRY_DECL(result, factory_.newThisLiteral(pos, thisName));
 
 ThrowStatement:
     build:
         BINJS_TRY_DECL(result, factory_.newThrowStatement(expression, tokenizer_->pos(start)));
 
 TryCatchStatement: