diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 6ae5fdbf92741..656830d132d92 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -14960,30 +14960,34 @@ namespace ts {
                 if (type.flags & TypeFlags.Any && literal.text === "function") {
                     return type;
                 }
-                if (assumeTrue && !(type.flags & TypeFlags.Union)) {
-                    if (type.flags & TypeFlags.Unknown && literal.text === "object") {
-                        return getUnionType([nonPrimitiveType, nullType]);
-                    }
-                    // We narrow a non-union type to an exact primitive type if the non-union type
-                    // is a supertype of that primitive type. For example, type 'any' can be narrowed
-                    // to one of the primitive types.
-                    const targetType = literal.text === "function" ? globalFunctionType : typeofTypesByName.get(literal.text);
-                    if (targetType) {
-                        if (isTypeSubtypeOf(targetType, type)) {
-                            return targetType;
-                        }
-                        if (type.flags & TypeFlags.Instantiable) {
-                            const constraint = getBaseConstraintOfType(type) || anyType;
-                            if (isTypeSubtypeOf(targetType, constraint)) {
-                                return getIntersectionType([type, targetType]);
+                const facts = assumeTrue ?
+                    typeofEQFacts.get(literal.text) || TypeFacts.TypeofEQHostObject :
+                    typeofNEFacts.get(literal.text) || TypeFacts.TypeofNEHostObject;
+                return getTypeWithFacts(assumeTrue ? mapType(type, narrowTypeForTypeof) : type, facts);
+
+                function narrowTypeForTypeof(type: Type) {
+                    if (assumeTrue && !(type.flags & TypeFlags.Union)) {
+                        if (type.flags & TypeFlags.Unknown && literal.text === "object") {
+                            return getUnionType([nonPrimitiveType, nullType]);
+                        }
+                        // We narrow a non-union type to an exact primitive type if the non-union type
+                        // is a supertype of that primitive type. For example, type 'any' can be narrowed
+                        // to one of the primitive types.
+                        const targetType = literal.text === "function" ? globalFunctionType : typeofTypesByName.get(literal.text);
+                        if (targetType) {
+                            if (isTypeSubtypeOf(targetType, type)) {
+                                return isTypeAny(type) ? targetType : getIntersectionType([type, targetType]); // Intersection to handle `string` being a subtype of `keyof T`
+                            }
+                            if (type.flags & TypeFlags.Instantiable) {
+                                const constraint = getBaseConstraintOfType(type) || anyType;
+                                if (isTypeSubtypeOf(targetType, constraint)) {
+                                    return getIntersectionType([type, targetType]);
+                                }
                             }
                         }
                     }
+                    return type;
                 }
-                const facts = assumeTrue ?
-                    typeofEQFacts.get(literal.text) || TypeFacts.TypeofEQHostObject :
-                    typeofNEFacts.get(literal.text) || TypeFacts.TypeofNEHostObject;
-                return getTypeWithFacts(type, facts);
             }
 
             function narrowTypeBySwitchOnDiscriminant(type: Type, switchStatement: SwitchStatement, clauseStart: number, clauseEnd: number) {
diff --git a/tests/baselines/reference/controlFlowIfStatement.types b/tests/baselines/reference/controlFlowIfStatement.types
index 39083598ee24a..03141e261853b 100644
--- a/tests/baselines/reference/controlFlowIfStatement.types
+++ b/tests/baselines/reference/controlFlowIfStatement.types
@@ -104,7 +104,7 @@ function c<T>(data: string | T): T {
 >JSON.parse : (text: string, reviver?: (key: any, value: any) => any) => any
 >JSON : JSON
 >parse : (text: string, reviver?: (key: any, value: any) => any) => any
->data : string
+>data : string | (T & string)
     }
     else {
         return data;
diff --git a/tests/baselines/reference/recursiveTypeRelations.types b/tests/baselines/reference/recursiveTypeRelations.types
index 893986d2dab59..3ca21ccd8e57e 100644
--- a/tests/baselines/reference/recursiveTypeRelations.types
+++ b/tests/baselines/reference/recursiveTypeRelations.types
@@ -58,9 +58,9 @@ export function css<S extends { [K in keyof S]: string }>(styles: S, ...classNam
 >"string" : "string"
 
       return styles[arg];
->styles[arg] : S[keyof S]
+>styles[arg] : S[keyof S & string]
 >styles : S
->arg : keyof S
+>arg : keyof S & string
     }
     if (typeof arg == "object") {
 >typeof arg == "object" : boolean
diff --git a/tests/baselines/reference/strictTypeofUnionNarrowing.js b/tests/baselines/reference/strictTypeofUnionNarrowing.js
new file mode 100644
index 0000000000000..93b9c7d433013
--- /dev/null
+++ b/tests/baselines/reference/strictTypeofUnionNarrowing.js
@@ -0,0 +1,32 @@
+//// [strictTypeofUnionNarrowing.ts]
+function stringify1(anything: { toString(): string } | undefined): string {
+    return typeof anything === "string" ? anything.toUpperCase() : "";
+}
+
+function stringify2(anything: {} | undefined): string {
+    return typeof anything === "string" ? anything.toUpperCase() : "";
+}
+
+function stringify3(anything: unknown | undefined): string { // should simplify to just `unknown` which should narrow fine
+    return typeof anything === "string" ? anything.toUpperCase() : "";
+}
+
+function stringify4(anything: { toString?(): string } | undefined): string {
+    return typeof anything === "string" ? anything.toUpperCase() : "";
+}
+
+
+//// [strictTypeofUnionNarrowing.js]
+"use strict";
+function stringify1(anything) {
+    return typeof anything === "string" ? anything.toUpperCase() : "";
+}
+function stringify2(anything) {
+    return typeof anything === "string" ? anything.toUpperCase() : "";
+}
+function stringify3(anything) {
+    return typeof anything === "string" ? anything.toUpperCase() : "";
+}
+function stringify4(anything) {
+    return typeof anything === "string" ? anything.toUpperCase() : "";
+}
diff --git a/tests/baselines/reference/strictTypeofUnionNarrowing.symbols b/tests/baselines/reference/strictTypeofUnionNarrowing.symbols
new file mode 100644
index 0000000000000..83a602eeaf1d4
--- /dev/null
+++ b/tests/baselines/reference/strictTypeofUnionNarrowing.symbols
@@ -0,0 +1,47 @@
+=== tests/cases/compiler/strictTypeofUnionNarrowing.ts ===
+function stringify1(anything: { toString(): string } | undefined): string {
+>stringify1 : Symbol(stringify1, Decl(strictTypeofUnionNarrowing.ts, 0, 0))
+>anything : Symbol(anything, Decl(strictTypeofUnionNarrowing.ts, 0, 20))
+>toString : Symbol(toString, Decl(strictTypeofUnionNarrowing.ts, 0, 31))
+
+    return typeof anything === "string" ? anything.toUpperCase() : "";
+>anything : Symbol(anything, Decl(strictTypeofUnionNarrowing.ts, 0, 20))
+>anything.toUpperCase : Symbol(String.toUpperCase, Decl(lib.es5.d.ts, --, --))
+>anything : Symbol(anything, Decl(strictTypeofUnionNarrowing.ts, 0, 20))
+>toUpperCase : Symbol(String.toUpperCase, Decl(lib.es5.d.ts, --, --))
+}
+
+function stringify2(anything: {} | undefined): string {
+>stringify2 : Symbol(stringify2, Decl(strictTypeofUnionNarrowing.ts, 2, 1))
+>anything : Symbol(anything, Decl(strictTypeofUnionNarrowing.ts, 4, 20))
+
+    return typeof anything === "string" ? anything.toUpperCase() : "";
+>anything : Symbol(anything, Decl(strictTypeofUnionNarrowing.ts, 4, 20))
+>anything.toUpperCase : Symbol(String.toUpperCase, Decl(lib.es5.d.ts, --, --))
+>anything : Symbol(anything, Decl(strictTypeofUnionNarrowing.ts, 4, 20))
+>toUpperCase : Symbol(String.toUpperCase, Decl(lib.es5.d.ts, --, --))
+}
+
+function stringify3(anything: unknown | undefined): string { // should simplify to just `unknown` which should narrow fine
+>stringify3 : Symbol(stringify3, Decl(strictTypeofUnionNarrowing.ts, 6, 1))
+>anything : Symbol(anything, Decl(strictTypeofUnionNarrowing.ts, 8, 20))
+
+    return typeof anything === "string" ? anything.toUpperCase() : "";
+>anything : Symbol(anything, Decl(strictTypeofUnionNarrowing.ts, 8, 20))
+>anything.toUpperCase : Symbol(String.toUpperCase, Decl(lib.es5.d.ts, --, --))
+>anything : Symbol(anything, Decl(strictTypeofUnionNarrowing.ts, 8, 20))
+>toUpperCase : Symbol(String.toUpperCase, Decl(lib.es5.d.ts, --, --))
+}
+
+function stringify4(anything: { toString?(): string } | undefined): string {
+>stringify4 : Symbol(stringify4, Decl(strictTypeofUnionNarrowing.ts, 10, 1))
+>anything : Symbol(anything, Decl(strictTypeofUnionNarrowing.ts, 12, 20))
+>toString : Symbol(toString, Decl(strictTypeofUnionNarrowing.ts, 12, 31))
+
+    return typeof anything === "string" ? anything.toUpperCase() : "";
+>anything : Symbol(anything, Decl(strictTypeofUnionNarrowing.ts, 12, 20))
+>anything.toUpperCase : Symbol(String.toUpperCase, Decl(lib.es5.d.ts, --, --))
+>anything : Symbol(anything, Decl(strictTypeofUnionNarrowing.ts, 12, 20))
+>toUpperCase : Symbol(String.toUpperCase, Decl(lib.es5.d.ts, --, --))
+}
+
diff --git a/tests/baselines/reference/strictTypeofUnionNarrowing.types b/tests/baselines/reference/strictTypeofUnionNarrowing.types
new file mode 100644
index 0000000000000..3039ece15d68b
--- /dev/null
+++ b/tests/baselines/reference/strictTypeofUnionNarrowing.types
@@ -0,0 +1,71 @@
+=== tests/cases/compiler/strictTypeofUnionNarrowing.ts ===
+function stringify1(anything: { toString(): string } | undefined): string {
+>stringify1 : (anything: { toString(): string; } | undefined) => string
+>anything : { toString(): string; } | undefined
+>toString : () => string
+
+    return typeof anything === "string" ? anything.toUpperCase() : "";
+>typeof anything === "string" ? anything.toUpperCase() : "" : string
+>typeof anything === "string" : boolean
+>typeof anything : "string" | "number" | "boolean" | "symbol" | "undefined" | "object" | "function"
+>anything : { toString(): string; } | undefined
+>"string" : "string"
+>anything.toUpperCase() : string
+>anything.toUpperCase : () => string
+>anything : { toString(): string; } & string
+>toUpperCase : () => string
+>"" : ""
+}
+
+function stringify2(anything: {} | undefined): string {
+>stringify2 : (anything: {} | undefined) => string
+>anything : {} | undefined
+
+    return typeof anything === "string" ? anything.toUpperCase() : "";
+>typeof anything === "string" ? anything.toUpperCase() : "" : string
+>typeof anything === "string" : boolean
+>typeof anything : "string" | "number" | "boolean" | "symbol" | "undefined" | "object" | "function"
+>anything : {} | undefined
+>"string" : "string"
+>anything.toUpperCase() : string
+>anything.toUpperCase : () => string
+>anything : string & {}
+>toUpperCase : () => string
+>"" : ""
+}
+
+function stringify3(anything: unknown | undefined): string { // should simplify to just `unknown` which should narrow fine
+>stringify3 : (anything: unknown) => string
+>anything : unknown
+
+    return typeof anything === "string" ? anything.toUpperCase() : "";
+>typeof anything === "string" ? anything.toUpperCase() : "" : string
+>typeof anything === "string" : boolean
+>typeof anything : "string" | "number" | "boolean" | "symbol" | "undefined" | "object" | "function"
+>anything : unknown
+>"string" : "string"
+>anything.toUpperCase() : string
+>anything.toUpperCase : () => string
+>anything : string
+>toUpperCase : () => string
+>"" : ""
+}
+
+function stringify4(anything: { toString?(): string } | undefined): string {
+>stringify4 : (anything: {} | undefined) => string
+>anything : {} | undefined
+>toString : (() => string) | undefined
+
+    return typeof anything === "string" ? anything.toUpperCase() : "";
+>typeof anything === "string" ? anything.toUpperCase() : "" : string
+>typeof anything === "string" : boolean
+>typeof anything : "string" | "number" | "boolean" | "symbol" | "undefined" | "object" | "function"
+>anything : {} | undefined
+>"string" : "string"
+>anything.toUpperCase() : string
+>anything.toUpperCase : () => string
+>anything : {} & string
+>toUpperCase : () => string
+>"" : ""
+}
+
diff --git a/tests/baselines/reference/typeGuardOfFormTypeOfPrimitiveSubtype.types b/tests/baselines/reference/typeGuardOfFormTypeOfPrimitiveSubtype.types
index 4787c07d75807..6510b73d44d53 100644
--- a/tests/baselines/reference/typeGuardOfFormTypeOfPrimitiveSubtype.types
+++ b/tests/baselines/reference/typeGuardOfFormTypeOfPrimitiveSubtype.types
@@ -14,7 +14,7 @@ if (typeof a === "number") {
 
     let c: number = a;
 >c : number
->a : number
+>a : number & {}
 }
 if (typeof a === "string") {
 >typeof a === "string" : boolean
@@ -24,7 +24,7 @@ if (typeof a === "string") {
 
     let c: string = a;
 >c : string
->a : string
+>a : string & {}
 }
 if (typeof a === "boolean") {
 >typeof a === "boolean" : boolean
@@ -34,7 +34,7 @@ if (typeof a === "boolean") {
 
     let c: boolean = a;
 >c : boolean
->a : boolean
+>a : (false & {}) | (true & {})
 }
 
 if (typeof b === "number") {
@@ -45,7 +45,7 @@ if (typeof b === "number") {
 
     let c: number = b;
 >c : number
->b : number
+>b : { toString(): string; } & number
 }
 if (typeof b === "string") {
 >typeof b === "string" : boolean
@@ -55,7 +55,7 @@ if (typeof b === "string") {
 
     let c: string = b;
 >c : string
->b : string
+>b : { toString(): string; } & string
 }
 if (typeof b === "boolean") {
 >typeof b === "boolean" : boolean
@@ -65,6 +65,6 @@ if (typeof b === "boolean") {
 
     let c: boolean = b;
 >c : boolean
->b : boolean
+>b : ({ toString(): string; } & false) | ({ toString(): string; } & true)
 }
 
diff --git a/tests/baselines/reference/typeGuardTypeOfUndefined.types b/tests/baselines/reference/typeGuardTypeOfUndefined.types
index a28dab044907a..00777ac0eae0e 100644
--- a/tests/baselines/reference/typeGuardTypeOfUndefined.types
+++ b/tests/baselines/reference/typeGuardTypeOfUndefined.types
@@ -134,7 +134,7 @@ function test5(a: boolean | void) {
     }
     else {
         a;
->a : boolean | void
+>a : undefined
     }
 }
 
@@ -151,15 +151,15 @@ function test6(a: boolean | void) {
         if (typeof a === "boolean") {
 >typeof a === "boolean" : boolean
 >typeof a : "string" | "number" | "boolean" | "symbol" | "undefined" | "object" | "function"
->a : boolean | void
+>a : undefined
 >"boolean" : "boolean"
 
             a;
->a : boolean
+>a : never
         }
         else {
             a;
->a : void
+>a : undefined
         }
     }
     else {
@@ -184,7 +184,7 @@ function test7(a: boolean | void) {
 >"boolean" : "boolean"
 
 		a;
->a : boolean | void
+>a : boolean
     }
     else {
         a;
@@ -212,7 +212,7 @@ function test8(a: boolean | void) {
     }
     else {
         a;
->a : boolean | void
+>a : undefined
     }
 }
 
@@ -242,7 +242,7 @@ function test9(a: boolean | number) {
     }
     else {
         a;
->a : number | boolean
+>a : undefined
     }
 }
 
@@ -259,15 +259,15 @@ function test10(a: boolean | number) {
         if (typeof a === "boolean") {
 >typeof a === "boolean" : boolean
 >typeof a : "string" | "number" | "boolean" | "symbol" | "undefined" | "object" | "function"
->a : number | boolean
+>a : undefined
 >"boolean" : "boolean"
 
             a;
->a : boolean
+>a : never
         }
         else {
             a;
->a : number
+>a : undefined
         }
     }
     else {
@@ -292,7 +292,7 @@ function test11(a: boolean | number) {
 >"boolean" : "boolean"
 
 		a;
->a : number | boolean
+>a : boolean
     }
     else {
         a;
@@ -320,7 +320,7 @@ function test12(a: boolean | number) {
     }
     else {
         a;
->a : number | boolean
+>a : number
     }
 }
 
@@ -350,7 +350,7 @@ function test13(a: boolean | number | void) {
     }
     else {
         a;
->a : number | boolean | void
+>a : undefined
     }
 }
 
@@ -367,15 +367,15 @@ function test14(a: boolean | number | void) {
         if (typeof a === "boolean") {
 >typeof a === "boolean" : boolean
 >typeof a : "string" | "number" | "boolean" | "symbol" | "undefined" | "object" | "function"
->a : number | boolean | void
+>a : undefined
 >"boolean" : "boolean"
 
             a;
->a : boolean
+>a : never
         }
         else {
             a;
->a : number | void
+>a : undefined
         }
     }
     else {
@@ -400,7 +400,7 @@ function test15(a: boolean | number | void) {
 >"boolean" : "boolean"
 
 		a;
->a : number | boolean | void
+>a : boolean
     }
     else {
         a;
@@ -428,7 +428,7 @@ function test16(a: boolean | number | void) {
     }
     else {
         a;
->a : number | boolean | void
+>a : number
     }
 }
 
diff --git a/tests/cases/compiler/strictTypeofUnionNarrowing.ts b/tests/cases/compiler/strictTypeofUnionNarrowing.ts
new file mode 100644
index 0000000000000..f2bddf3f939e1
--- /dev/null
+++ b/tests/cases/compiler/strictTypeofUnionNarrowing.ts
@@ -0,0 +1,16 @@
+// @strict: true
+function stringify1(anything: { toString(): string } | undefined): string {
+    return typeof anything === "string" ? anything.toUpperCase() : "";
+}
+
+function stringify2(anything: {} | undefined): string {
+    return typeof anything === "string" ? anything.toUpperCase() : "";
+}
+
+function stringify3(anything: unknown | undefined): string { // should simplify to just `unknown` which should narrow fine
+    return typeof anything === "string" ? anything.toUpperCase() : "";
+}
+
+function stringify4(anything: { toString?(): string } | undefined): string {
+    return typeof anything === "string" ? anything.toUpperCase() : "";
+}