diff --git a/tests/baselines/reference/narrowingByDiscriminantInLoop.js b/tests/baselines/reference/narrowingByDiscriminantInLoop.js
new file mode 100644
index 00000000000..b58357cff5b
--- /dev/null
+++ b/tests/baselines/reference/narrowingByDiscriminantInLoop.js
@@ -0,0 +1,82 @@
+//// [narrowingByDiscriminantInLoop.ts]
+// Repro from #9977
+
+type IDLMemberTypes = OperationMemberType | ConstantMemberType;
+
+interface IDLTypeDescription {
+    origin: string;
+}
+
+interface InterfaceType {
+    members: IDLMemberTypes[];
+}
+
+interface OperationMemberType {
+    type: "operation";
+    idlType: IDLTypeDescription | null;
+}
+
+interface ConstantMemberType {
+    type: "const";
+    idlType: string;
+}
+
+function insertInterface(callbackType: InterfaceType) {
+    for (const memberType of callbackType.members) {
+        if (memberType.type === "const") {
+            memberType.idlType;  // string
+        }
+        else if (memberType.type === "operation") {
+            memberType.idlType.origin;  // string
+            (memberType.idlType as IDLTypeDescription);
+        }
+    }
+}
+
+function insertInterface2(callbackType: InterfaceType) {
+    for (const memberType of callbackType.members) {
+        if (memberType.type === "operation") {
+            memberType.idlType.origin;  // string
+        }
+    }
+}
+
+function foo(memberType: IDLMemberTypes) {
+    if (memberType.type === "const") {
+        memberType.idlType;  // string
+    }
+    else if (memberType.type === "operation") {
+        memberType.idlType.origin;  // string
+    }
+}
+
+//// [narrowingByDiscriminantInLoop.js]
+// Repro from #9977
+function insertInterface(callbackType) {
+    for (var _i = 0, _a = callbackType.members; _i < _a.length; _i++) {
+        var memberType = _a[_i];
+        if (memberType.type === "const") {
+            memberType.idlType; // string
+        }
+        else if (memberType.type === "operation") {
+            memberType.idlType.origin; // string
+            memberType.idlType;
+        }
+    }
+}
+function insertInterface2(callbackType) {
+    for (var _i = 0, _a = callbackType.members; _i < _a.length; _i++) {
+        var memberType = _a[_i];
+        if (memberType.type === "operation") {
+            memberType.idlType.origin; // string
+        }
+    }
+}
+function foo(memberType) {
+    if (memberType.type === "const") {
+        memberType.idlType; // string
+    }
+    else if (memberType.type === "operation") {
+        memberType.idlType.origin; // string
+    }
+}
diff --git a/tests/baselines/reference/narrowingByDiscriminantInLoop.symbols b/tests/baselines/reference/narrowingByDiscriminantInLoop.symbols
new file mode 100644
index 00000000000..30cd87f3c2b
--- /dev/null
+++ b/tests/baselines/reference/narrowingByDiscriminantInLoop.symbols
@@ -0,0 +1,140 @@
+=== tests/cases/compiler/narrowingByDiscriminantInLoop.ts ===
+// Repro from #9977
+
+type IDLMemberTypes = OperationMemberType | ConstantMemberType;
+>IDLMemberTypes : Symbol(IDLMemberTypes, Decl(narrowingByDiscriminantInLoop.ts, 0, 0))
+>OperationMemberType : Symbol(OperationMemberType, Decl(narrowingByDiscriminantInLoop.ts, 10, 1))
+>ConstantMemberType : Symbol(ConstantMemberType, Decl(narrowingByDiscriminantInLoop.ts, 15, 1))
+
+interface IDLTypeDescription {
+>IDLTypeDescription : Symbol(IDLTypeDescription, Decl(narrowingByDiscriminantInLoop.ts, 2, 63))
+
+    origin: string;
+>origin : Symbol(IDLTypeDescription.origin, Decl(narrowingByDiscriminantInLoop.ts, 4, 30))
+}
+
+interface InterfaceType {
+>InterfaceType : Symbol(InterfaceType, Decl(narrowingByDiscriminantInLoop.ts, 6, 1))
+
+    members: IDLMemberTypes[];
+>members : Symbol(InterfaceType.members, Decl(narrowingByDiscriminantInLoop.ts, 8, 25))
+>IDLMemberTypes : Symbol(IDLMemberTypes, Decl(narrowingByDiscriminantInLoop.ts, 0, 0))
+}
+
+interface OperationMemberType {
+>OperationMemberType : Symbol(OperationMemberType, Decl(narrowingByDiscriminantInLoop.ts, 10, 1))
+
+    type: "operation";
+>type : Symbol(OperationMemberType.type, Decl(narrowingByDiscriminantInLoop.ts, 12, 31))
+
+    idlType: IDLTypeDescription | null;
+>idlType : Symbol(OperationMemberType.idlType, Decl(narrowingByDiscriminantInLoop.ts, 13, 22))
+>IDLTypeDescription : Symbol(IDLTypeDescription, Decl(narrowingByDiscriminantInLoop.ts, 2, 63))
+}
+
+interface ConstantMemberType {
+>ConstantMemberType : Symbol(ConstantMemberType, Decl(narrowingByDiscriminantInLoop.ts, 15, 1))
+
+    type: "const";
+>type : Symbol(ConstantMemberType.type, Decl(narrowingByDiscriminantInLoop.ts, 17, 30))
+
+    idlType: string;
+>idlType : Symbol(ConstantMemberType.idlType, Decl(narrowingByDiscriminantInLoop.ts, 18, 18))
+}
+
+function insertInterface(callbackType: InterfaceType) {
+>insertInterface : Symbol(insertInterface, Decl(narrowingByDiscriminantInLoop.ts, 20, 1))
+>callbackType : Symbol(callbackType, Decl(narrowingByDiscriminantInLoop.ts, 22, 25))
+>InterfaceType : Symbol(InterfaceType, Decl(narrowingByDiscriminantInLoop.ts, 6, 1))
+
+    for (const memberType of callbackType.members) {
+>memberType : Symbol(memberType, Decl(narrowingByDiscriminantInLoop.ts, 23, 14))
+>callbackType.members : Symbol(InterfaceType.members, Decl(narrowingByDiscriminantInLoop.ts, 8, 25))
+>callbackType : Symbol(callbackType, Decl(narrowingByDiscriminantInLoop.ts, 22, 25))
+>members : Symbol(InterfaceType.members, Decl(narrowingByDiscriminantInLoop.ts, 8, 25))
+
+        if (memberType.type === "const") {
+>memberType.type : Symbol(type, Decl(narrowingByDiscriminantInLoop.ts, 12, 31), Decl(narrowingByDiscriminantInLoop.ts, 17, 30))
+>memberType : Symbol(memberType, Decl(narrowingByDiscriminantInLoop.ts, 23, 14))
+>type : Symbol(type, Decl(narrowingByDiscriminantInLoop.ts, 12, 31), Decl(narrowingByDiscriminantInLoop.ts, 17, 30))
+
+            memberType.idlType;  // string
+>memberType.idlType : Symbol(ConstantMemberType.idlType, Decl(narrowingByDiscriminantInLoop.ts, 18, 18))
+>memberType : Symbol(memberType, Decl(narrowingByDiscriminantInLoop.ts, 23, 14))
+>idlType : Symbol(ConstantMemberType.idlType, Decl(narrowingByDiscriminantInLoop.ts, 18, 18))
+        }
+        else if (memberType.type === "operation") {
+>memberType.type : Symbol(OperationMemberType.type, Decl(narrowingByDiscriminantInLoop.ts, 12, 31))
+>memberType : Symbol(memberType, Decl(narrowingByDiscriminantInLoop.ts, 23, 14))
+>type : Symbol(OperationMemberType.type, Decl(narrowingByDiscriminantInLoop.ts, 12, 31))
+
+            memberType.idlType.origin;  // string
+>memberType.idlType.origin : Symbol(IDLTypeDescription.origin, Decl(narrowingByDiscriminantInLoop.ts, 4, 30))
+>memberType.idlType : Symbol(OperationMemberType.idlType, Decl(narrowingByDiscriminantInLoop.ts, 13, 22))
+>memberType : Symbol(memberType, Decl(narrowingByDiscriminantInLoop.ts, 23, 14))
+>idlType : Symbol(OperationMemberType.idlType, Decl(narrowingByDiscriminantInLoop.ts, 13, 22))
+>origin : Symbol(IDLTypeDescription.origin, Decl(narrowingByDiscriminantInLoop.ts, 4, 30))
+
+            (memberType.idlType as IDLTypeDescription);
+>memberType.idlType : Symbol(OperationMemberType.idlType, Decl(narrowingByDiscriminantInLoop.ts, 13, 22))
+>memberType : Symbol(memberType, Decl(narrowingByDiscriminantInLoop.ts, 23, 14))
+>idlType : Symbol(OperationMemberType.idlType, Decl(narrowingByDiscriminantInLoop.ts, 13, 22))
+>IDLTypeDescription : Symbol(IDLTypeDescription, Decl(narrowingByDiscriminantInLoop.ts, 2, 63))
+        }
+    }
+}
+
+function insertInterface2(callbackType: InterfaceType) {
+>insertInterface2 : Symbol(insertInterface2, Decl(narrowingByDiscriminantInLoop.ts, 32, 1))
+>callbackType : Symbol(callbackType, Decl(narrowingByDiscriminantInLoop.ts, 34, 26))
+>InterfaceType : Symbol(InterfaceType, Decl(narrowingByDiscriminantInLoop.ts, 6, 1))
+
+    for (const memberType of callbackType.members) {
+>memberType : Symbol(memberType, Decl(narrowingByDiscriminantInLoop.ts, 35, 14))
+>callbackType.members : Symbol(InterfaceType.members, Decl(narrowingByDiscriminantInLoop.ts, 8, 25))
+>callbackType : Symbol(callbackType, Decl(narrowingByDiscriminantInLoop.ts, 34, 26))
+>members : Symbol(InterfaceType.members, Decl(narrowingByDiscriminantInLoop.ts, 8, 25))
+
+        if (memberType.type === "operation") {
+>memberType.type : Symbol(type, Decl(narrowingByDiscriminantInLoop.ts, 12, 31), Decl(narrowingByDiscriminantInLoop.ts, 17, 30))
+>memberType : Symbol(memberType, Decl(narrowingByDiscriminantInLoop.ts, 35, 14))
+>type : Symbol(type, Decl(narrowingByDiscriminantInLoop.ts, 12, 31), Decl(narrowingByDiscriminantInLoop.ts, 17, 30))
+
+            memberType.idlType.origin;  // string
+>memberType.idlType.origin : Symbol(IDLTypeDescription.origin, Decl(narrowingByDiscriminantInLoop.ts, 4, 30))
+>memberType.idlType : Symbol(OperationMemberType.idlType, Decl(narrowingByDiscriminantInLoop.ts, 13, 22))
+>memberType : Symbol(memberType, Decl(narrowingByDiscriminantInLoop.ts, 35, 14))
+>idlType : Symbol(OperationMemberType.idlType, Decl(narrowingByDiscriminantInLoop.ts, 13, 22))
+>origin : Symbol(IDLTypeDescription.origin, Decl(narrowingByDiscriminantInLoop.ts, 4, 30))
+        }
+    }
+}
+
+function foo(memberType: IDLMemberTypes) {
+>foo : Symbol(foo, Decl(narrowingByDiscriminantInLoop.ts, 40, 1))
+>memberType : Symbol(memberType, Decl(narrowingByDiscriminantInLoop.ts, 42, 13))
+>IDLMemberTypes : Symbol(IDLMemberTypes, Decl(narrowingByDiscriminantInLoop.ts, 0, 0))
+
+    if (memberType.type === "const") {
+>memberType.type : Symbol(type, Decl(narrowingByDiscriminantInLoop.ts, 12, 31), Decl(narrowingByDiscriminantInLoop.ts, 17, 30))
+>memberType : Symbol(memberType, Decl(narrowingByDiscriminantInLoop.ts, 42, 13))
+>type : Symbol(type, Decl(narrowingByDiscriminantInLoop.ts, 12, 31), Decl(narrowingByDiscriminantInLoop.ts, 17, 30))
+
+        memberType.idlType;  // string
+>memberType.idlType : Symbol(ConstantMemberType.idlType, Decl(narrowingByDiscriminantInLoop.ts, 18, 18))
+>memberType : Symbol(memberType, Decl(narrowingByDiscriminantInLoop.ts, 42, 13))
+>idlType : Symbol(ConstantMemberType.idlType, Decl(narrowingByDiscriminantInLoop.ts, 18, 18))
+    }
+    else if (memberType.type === "operation") {
+>memberType.type : Symbol(OperationMemberType.type, Decl(narrowingByDiscriminantInLoop.ts, 12, 31))
+>memberType : Symbol(memberType, Decl(narrowingByDiscriminantInLoop.ts, 42, 13))
+>type : Symbol(OperationMemberType.type, Decl(narrowingByDiscriminantInLoop.ts, 12, 31))
+
+        memberType.idlType.origin;  // string
+>memberType.idlType.origin : Symbol(IDLTypeDescription.origin, Decl(narrowingByDiscriminantInLoop.ts, 4, 30))
+>memberType.idlType : Symbol(OperationMemberType.idlType, Decl(narrowingByDiscriminantInLoop.ts, 13, 22))
+>memberType : Symbol(memberType, Decl(narrowingByDiscriminantInLoop.ts, 42, 13))
+>idlType : Symbol(OperationMemberType.idlType, Decl(narrowingByDiscriminantInLoop.ts, 13, 22))
+>origin : Symbol(IDLTypeDescription.origin, Decl(narrowingByDiscriminantInLoop.ts, 4, 30))
+    }
+}
diff --git a/tests/baselines/reference/narrowingByDiscriminantInLoop.types b/tests/baselines/reference/narrowingByDiscriminantInLoop.types
new file mode 100644
index 00000000000..db8cb502a13
--- /dev/null
+++ b/tests/baselines/reference/narrowingByDiscriminantInLoop.types
@@ -0,0 +1,153 @@
+=== tests/cases/compiler/narrowingByDiscriminantInLoop.ts ===
+// Repro from #9977
+
+type IDLMemberTypes = OperationMemberType | ConstantMemberType;
+>IDLMemberTypes : IDLMemberTypes
+>OperationMemberType : OperationMemberType
+>ConstantMemberType : ConstantMemberType
+
+interface IDLTypeDescription {
+>IDLTypeDescription : IDLTypeDescription
+
+    origin: string;
+>origin : string
+}
+
+interface InterfaceType {
+>InterfaceType : InterfaceType
+
+    members: IDLMemberTypes[];
+>members : IDLMemberTypes[]
+>IDLMemberTypes : IDLMemberTypes
+}
+
+interface OperationMemberType {
+>OperationMemberType : OperationMemberType
+
+    type: "operation";
+>type : "operation"
+
+    idlType: IDLTypeDescription | null;
+>idlType : IDLTypeDescription
+>IDLTypeDescription : IDLTypeDescription
+>null : null
+}
+
+interface ConstantMemberType {
+>ConstantMemberType : ConstantMemberType
+
+    type: "const";
+>type : "const"
+
+    idlType: string;
+>idlType : string
+}
+
+function insertInterface(callbackType: InterfaceType) {
+>insertInterface : (callbackType: InterfaceType) => void
+>callbackType : InterfaceType
+>InterfaceType : InterfaceType
+
+    for (const memberType of callbackType.members) {
+>memberType : IDLMemberTypes
+>callbackType.members : IDLMemberTypes[]
+>callbackType : InterfaceType
+>members : IDLMemberTypes[]
+
+        if (memberType.type === "const") {
+>memberType.type === "const" : boolean
+>memberType.type : "operation" | "const"
+>memberType : IDLMemberTypes
+>type : "operation" | "const"
+>"const" : "const"
+
+            memberType.idlType;  // string
+>memberType.idlType : string
+>memberType : ConstantMemberType
+>idlType : string
+        }
+        else if (memberType.type === "operation") {
+>memberType.type === "operation" : boolean
+>memberType.type : "operation"
+>memberType : OperationMemberType
+>type : "operation"
+>"operation" : "operation"
+
+            memberType.idlType.origin;  // string
+>memberType.idlType.origin : string
+>memberType.idlType : IDLTypeDescription
+>memberType : OperationMemberType
+>idlType : IDLTypeDescription
+>origin : string
+
+            (memberType.idlType as IDLTypeDescription);
+>(memberType.idlType as IDLTypeDescription) : IDLTypeDescription
+>memberType.idlType as IDLTypeDescription : IDLTypeDescription
+>memberType.idlType : IDLTypeDescription
+>memberType : OperationMemberType
+>idlType : IDLTypeDescription
+>IDLTypeDescription : IDLTypeDescription
+        }
+    }
+}
+
+function insertInterface2(callbackType: InterfaceType) {
+>insertInterface2 : (callbackType: InterfaceType) => void
+>callbackType : InterfaceType
+>InterfaceType : InterfaceType
+
+    for (const memberType of callbackType.members) {
+>memberType : IDLMemberTypes
+>callbackType.members : IDLMemberTypes[]
+>callbackType : InterfaceType
+>members : IDLMemberTypes[]
+
+        if (memberType.type === "operation") {
+>memberType.type === "operation" : boolean
+>memberType.type : "operation" | "const"
+>memberType : IDLMemberTypes
+>type : "operation" | "const"
+>"operation" : "operation"
+
+            memberType.idlType.origin;  // string
+>memberType.idlType.origin : string
+>memberType.idlType : IDLTypeDescription
+>memberType : OperationMemberType
+>idlType : IDLTypeDescription
+>origin : string
+        }
+    }
+}
+
+function foo(memberType: IDLMemberTypes) {
+>foo : (memberType: IDLMemberTypes) => void
+>memberType : IDLMemberTypes
+>IDLMemberTypes : IDLMemberTypes
+
+    if (memberType.type === "const") {
+>memberType.type === "const" : boolean
+>memberType.type : "operation" | "const"
+>memberType : IDLMemberTypes
+>type : "operation" | "const"
+>"const" : "const"
+
+        memberType.idlType;  // string
+>memberType.idlType : string
+>memberType : ConstantMemberType
+>idlType : string
+    }
+    else if (memberType.type === "operation") {
+>memberType.type === "operation" : boolean
+>memberType.type : "operation"
+>memberType : OperationMemberType
+>type : "operation"
+>"operation" : "operation"
+
+        memberType.idlType.origin;  // string
+>memberType.idlType.origin : string
+>memberType.idlType : IDLTypeDescription
+>memberType : OperationMemberType
+>idlType : IDLTypeDescription
+>origin : string
+    }
+}
diff --git a/tests/cases/compiler/narrowingByDiscriminantInLoop.ts b/tests/cases/compiler/narrowingByDiscriminantInLoop.ts
new file mode 100644
index 00000000000..d9519c57a62
--- /dev/null
+++ b/tests/cases/compiler/narrowingByDiscriminantInLoop.ts
@@ -0,0 +1,50 @@
+// Repro from #9977
+
+type IDLMemberTypes = OperationMemberType | ConstantMemberType;
+
+interface IDLTypeDescription {
+    origin: string;
+}
+
+interface InterfaceType {
+    members: IDLMemberTypes[];
+}
+
+interface OperationMemberType {
+    type: "operation";
+    idlType: IDLTypeDescription | null;
+}
+
+interface ConstantMemberType {
+    type: "const";
+    idlType: string;
+}
+
+function insertInterface(callbackType: InterfaceType) {
+    for (const memberType of callbackType.members) {
+        if (memberType.type === "const") {
+            memberType.idlType;  // string
+        }
+        else if (memberType.type === "operation") {
+            memberType.idlType.origin;  // string
+            (memberType.idlType as IDLTypeDescription);
+        }
+    }
+}
+
+function insertInterface2(callbackType: InterfaceType) {
+    for (const memberType of callbackType.members) {
+        if (memberType.type === "operation") {
+            memberType.idlType.origin;  // string
+        }
+    }
+}
+
+function foo(memberType: IDLMemberTypes) {
+    if (memberType.type === "const") {
+        memberType.idlType;  // string
+    }
+    else if (memberType.type === "operation") {
+        memberType.idlType.origin;  // string
+    }
+}
\ No newline at end of file