microsoft · RyanCavanaugh · Feb 4, 2026 · Feb 3, 2026 · Feb 3, 2026 · Feb 3, 2026
diff --git a/src/compiler/commandLineParser.ts b/src/compiler/commandLineParser.ts
@@ -899,7 +899,7 @@ const commandOptionsWithoutBuild: CommandLineOption[] = [
         showInSimplifiedHelpView: true,
         category: Diagnostics.Type_Checking,
         description: Diagnostics.Enable_all_strict_type_checking_options,
-        defaultValueDescription: false,
+        defaultValueDescription: true,
     },
     {
         name: "noImplicitAny",

diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
@@ -9310,7 +9310,7 @@ export type StrictOptionName =
 
 /** @internal */
 export function getStrictOptionValue(compilerOptions: CompilerOptions, flag: StrictOptionName): boolean {
-    return compilerOptions[flag] === undefined ? !!compilerOptions.strict : !!compilerOptions[flag];
+    return compilerOptions[flag] === undefined ? (compilerOptions.strict !== false) : !!compilerOptions[flag];
 }
 
 /** @internal */

diff --git a/tests/baselines/reference/2dArrays.js b/tests/baselines/reference/2dArrays.js
@@ -18,6 +18,7 @@ class Board {
 }
 
 //// [2dArrays.js]
+"use strict";
 class Cell {
 }
 class Ship {

diff --git a/tests/baselines/reference/2dArrays.types b/tests/baselines/reference/2dArrays.types
@@ -24,14 +24,14 @@ class Board {
     ships: Ship[] = [];
 >ships : Ship[]
 >      : ^^^^^^
->[] : undefined[]
->   : ^^^^^^^^^^^
+>[] : never[]
+>   : ^^^^^^^
 
     cells: Cell[] = [];
 >cells : Cell[]
 >      : ^^^^^^
->[] : undefined[]
->   : ^^^^^^^^^^^
+>[] : never[]
+>   : ^^^^^^^
 
     private allShipsSunk() {
 >allShipsSunk : () => boolean

diff --git a/tests/baselines/reference/AmbientModuleAndAmbientFunctionWithTheSameNameAndCommonRoot.js b/tests/baselines/reference/AmbientModuleAndAmbientFunctionWithTheSameNameAndCommonRoot.js
@@ -14,6 +14,7 @@ var cl = Point();
 var cl = Point.Origin;
 
 //// [test.js]
+"use strict";
 var cl;
 var cl = Point();
 var cl = Point.Origin;
diff --git a/tests/baselines/reference/AmbientModuleAndAmbientWithSameNameAndCommonRoot.js b/tests/baselines/reference/AmbientModuleAndAmbientWithSameNameAndCommonRoot.js
@@ -26,6 +26,7 @@ var p = new A.Point(0, 0); // unexpected error here, bug 840000
 
 
 //// [test.js]
+"use strict";
 var p;
 var p = A.Point.Origin;
 var p = new A.Point(0, 0); // unexpected error here, bug 840000
diff --git a/tests/baselines/reference/AmbientModuleAndNonAmbientClassWithSameNameAndCommonRoot.js b/tests/baselines/reference/AmbientModuleAndNonAmbientClassWithSameNameAndCommonRoot.js
@@ -23,6 +23,7 @@ var p = A.Point.Origin;
 var p = new A.Point(0, 0); // unexpected error here, bug 840000
 
 //// [classPoint.js]
+"use strict";
 var A;
 (function (A) {
     class Point {
@@ -34,6 +35,7 @@ var A;
     A.Point = Point;
 })(A || (A = {}));
 //// [test.js]
+"use strict";
 var p;
 var p = A.Point.Origin;
 var p = new A.Point(0, 0); // unexpected error here, bug 840000
diff --git a/tests/baselines/reference/AmbientModuleAndNonAmbientFunctionWithTheSameNameAndCommonRoot.js b/tests/baselines/reference/AmbientModuleAndNonAmbientFunctionWithTheSameNameAndCommonRoot.js
@@ -16,10 +16,12 @@ var cl = Point();
 var cl = Point.Origin;
 
 //// [function.js]
+"use strict";
 function Point() {
     return { x: 0, y: 0 };
 }
 //// [test.js]
+"use strict";
 var cl;
 var cl = Point();
 var cl = Point.Origin;
diff --git a/tests/baselines/reference/ArrowFunction1.js b/tests/baselines/reference/ArrowFunction1.js
@@ -6,5 +6,6 @@ var v = (a: ) => {
 };
 
 //// [ArrowFunction1.js]
+"use strict";
 var v = (a) => {
 };
diff --git a/tests/baselines/reference/ArrowFunction3.js b/tests/baselines/reference/ArrowFunction3.js
@@ -6,6 +6,7 @@ var v = (a): => {
 };
 
 //// [ArrowFunction3.js]
+"use strict";
 var v = (a);
 {
 }

diff --git a/tests/baselines/reference/ArrowFunctionExpression1.js b/tests/baselines/reference/ArrowFunctionExpression1.js
@@ -4,4 +4,5 @@
 var v = (public x: string) => { };
 
 //// [ArrowFunctionExpression1.js]
+"use strict";
 var v = (x) => { };
diff --git a/...es/reference/ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.errors.txt b/...es/reference/ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.errors.txt
@@ -1,17 +1,31 @@
+ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.ts(5,5): error TS2564: Property 'id' has no initializer and is not definitely assigned in the constructor.
+ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.ts(6,5): error TS2564: Property 'value' has no initializer and is not definitely assigned in the constructor.
 ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.ts(10,19): error TS2304: Cannot find name 'T'.
+ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.ts(15,5): error TS2564: Property 'id' has no initializer and is not definitely assigned in the constructor.
+ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.ts(16,5): error TS2564: Property 'value' has no initializer and is not definitely assigned in the constructor.
 ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.ts(20,12): error TS2304: Cannot find name 'T'.
 ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.ts(22,23): error TS2304: Cannot find name 'T'.
+ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.ts(23,9): error TS2564: Property 'id' has no initializer and is not definitely assigned in the constructor.
+ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.ts(24,9): error TS2564: Property 'value' has no initializer and is not definitely assigned in the constructor.
+ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.ts(30,5): error TS2564: Property 'id' has no initializer and is not definitely assigned in the constructor.
+ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.ts(31,5): error TS2564: Property 'value' has no initializer and is not definitely assigned in the constructor.
 ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.ts(35,26): error TS2304: Cannot find name 'T'.
+ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.ts(40,5): error TS2564: Property 'id' has no initializer and is not definitely assigned in the constructor.
+ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.ts(41,5): error TS2564: Property 'value' has no initializer and is not definitely assigned in the constructor.
 ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.ts(46,15): error TS2304: Cannot find name 'T'.
 
 
-==== ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.ts (5 errors) ====
+==== ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.ts (15 errors) ====
     // all expected to be errors
 
     class clodule1<T>{
 
         id: string;
+        ~~
+!!! error TS2564: Property 'id' has no initializer and is not definitely assigned in the constructor.
         value: T;
+        ~~~~~
+!!! error TS2564: Property 'value' has no initializer and is not definitely assigned in the constructor.
     }
 
     namespace clodule1 {
@@ -23,7 +37,11 @@ ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.ts(46,15): err
     class clodule2<T>{
 
         id: string;
+        ~~
+!!! error TS2564: Property 'id' has no initializer and is not definitely assigned in the constructor.
         value: T;
+        ~~~~~
+!!! error TS2564: Property 'value' has no initializer and is not definitely assigned in the constructor.
     }
 
     namespace clodule2 {
@@ -35,14 +53,22 @@ ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.ts(46,15): err
                           ~
 !!! error TS2304: Cannot find name 'T'.
             id: string;
+            ~~
+!!! error TS2564: Property 'id' has no initializer and is not definitely assigned in the constructor.
             value: U;
+            ~~~~~
+!!! error TS2564: Property 'value' has no initializer and is not definitely assigned in the constructor.
         }
     }
 
     class clodule3<T>{
 
         id: string;
+        ~~
+!!! error TS2564: Property 'id' has no initializer and is not definitely assigned in the constructor.
         value: T;
+        ~~~~~
+!!! error TS2564: Property 'value' has no initializer and is not definitely assigned in the constructor.
     }
 
     namespace clodule3 {
@@ -54,7 +80,11 @@ ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.ts(46,15): err
     class clodule4<T>{
 
         id: string;
+        ~~
+!!! error TS2564: Property 'id' has no initializer and is not definitely assigned in the constructor.
         value: T;
+        ~~~~~
+!!! error TS2564: Property 'value' has no initializer and is not definitely assigned in the constructor.
     }
 
     namespace clodule4 {

diff --git a/.../baselines/reference/ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.js b/.../baselines/reference/ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.js
@@ -52,6 +52,7 @@ namespace clodule4 {
 
 
 //// [ClassAndModuleThatMergeWithModuleMemberThatUsesClassTypeParameter.js]
+"use strict";
 // all expected to be errors
 class clodule1 {
 }

diff --git a/...geWithModulesExportedGenericFunctionAndGenericClassStaticFunctionOfTheSameName.errors.txt b/...geWithModulesExportedGenericFunctionAndGenericClassStaticFunctionOfTheSameName.errors.txt
@@ -1,11 +1,17 @@
+ClassAndModuleThatMergeWithModulesExportedGenericFunctionAndGenericClassStaticFunctionOfTheSameName.ts(2,5): error TS2564: Property 'id' has no initializer and is not definitely assigned in the constructor.
+ClassAndModuleThatMergeWithModulesExportedGenericFunctionAndGenericClassStaticFunctionOfTheSameName.ts(3,5): error TS2564: Property 'value' has no initializer and is not definitely assigned in the constructor.
 ClassAndModuleThatMergeWithModulesExportedGenericFunctionAndGenericClassStaticFunctionOfTheSameName.ts(5,12): error TS2300: Duplicate identifier 'fn'.
 ClassAndModuleThatMergeWithModulesExportedGenericFunctionAndGenericClassStaticFunctionOfTheSameName.ts(10,21): error TS2300: Duplicate identifier 'fn'.
 
 
-==== ClassAndModuleThatMergeWithModulesExportedGenericFunctionAndGenericClassStaticFunctionOfTheSameName.ts (2 errors) ====
+==== ClassAndModuleThatMergeWithModulesExportedGenericFunctionAndGenericClassStaticFunctionOfTheSameName.ts (4 errors) ====
     class clodule<T> {
         id: string;
+        ~~
+!!! error TS2564: Property 'id' has no initializer and is not definitely assigned in the constructor.
         value: T;
+        ~~~~~
+!!! error TS2564: Property 'value' has no initializer and is not definitely assigned in the constructor.
 
         static fn<U>(id: U) { }
                ~~

diff --git a/...eThatMergeWithModulesExportedGenericFunctionAndGenericClassStaticFunctionOfTheSameName.js b/...eThatMergeWithModulesExportedGenericFunctionAndGenericClassStaticFunctionOfTheSameName.js
@@ -18,6 +18,7 @@ namespace clodule {
 
 
 //// [ClassAndModuleThatMergeWithModulesExportedGenericFunctionAndGenericClassStaticFunctionOfTheSameName.js]
+"use strict";
 class clodule {
     static fn(id) { }
 }

diff --git a/...ithModulesExportedGenericFunctionAndNonGenericClassStaticFunctionOfTheSameName.errors.txt b/...ithModulesExportedGenericFunctionAndNonGenericClassStaticFunctionOfTheSameName.errors.txt
@@ -1,11 +1,17 @@
+ClassAndModuleThatMergeWithModulesExportedGenericFunctionAndNonGenericClassStaticFunctionOfTheSameName.ts(2,5): error TS2564: Property 'id' has no initializer and is not definitely assigned in the constructor.
+ClassAndModuleThatMergeWithModulesExportedGenericFunctionAndNonGenericClassStaticFunctionOfTheSameName.ts(3,5): error TS2564: Property 'value' has no initializer and is not definitely assigned in the constructor.
 ClassAndModuleThatMergeWithModulesExportedGenericFunctionAndNonGenericClassStaticFunctionOfTheSameName.ts(5,12): error TS2300: Duplicate identifier 'fn'.
 ClassAndModuleThatMergeWithModulesExportedGenericFunctionAndNonGenericClassStaticFunctionOfTheSameName.ts(10,21): error TS2300: Duplicate identifier 'fn'.
 
 
-==== ClassAndModuleThatMergeWithModulesExportedGenericFunctionAndNonGenericClassStaticFunctionOfTheSameName.ts (2 errors) ====
+==== ClassAndModuleThatMergeWithModulesExportedGenericFunctionAndNonGenericClassStaticFunctionOfTheSameName.ts (4 errors) ====
     class clodule<T> {
         id: string;
+        ~~
+!!! error TS2564: Property 'id' has no initializer and is not definitely assigned in the constructor.
         value: T;
+        ~~~~~
+!!! error TS2564: Property 'value' has no initializer and is not definitely assigned in the constructor.
 
         static fn(id: string) { }
                ~~

diff --git a/...atMergeWithModulesExportedGenericFunctionAndNonGenericClassStaticFunctionOfTheSameName.js b/...atMergeWithModulesExportedGenericFunctionAndNonGenericClassStaticFunctionOfTheSameName.js
@@ -18,6 +18,7 @@ namespace clodule {
 
 
 //// [ClassAndModuleThatMergeWithModulesExportedGenericFunctionAndNonGenericClassStaticFunctionOfTheSameName.js]
+"use strict";
 class clodule {
     static fn(id) { }
 }

diff --git a/...assAndModuleThatMergeWithModulesExportedStaticFunctionUsingClassPrivateStatics.errors.txt b/...assAndModuleThatMergeWithModulesExportedStaticFunctionUsingClassPrivateStatics.errors.txt
@@ -1,10 +1,16 @@
+ClassAndModuleThatMergeWithModulesExportedStaticFunctionUsingClassPrivateStatics.ts(2,5): error TS2564: Property 'id' has no initializer and is not definitely assigned in the constructor.
+ClassAndModuleThatMergeWithModulesExportedStaticFunctionUsingClassPrivateStatics.ts(3,5): error TS2564: Property 'value' has no initializer and is not definitely assigned in the constructor.
 ClassAndModuleThatMergeWithModulesExportedStaticFunctionUsingClassPrivateStatics.ts(11,24): error TS2341: Property 'sfn' is private and only accessible within class 'clodule<T>'.
 
 
-==== ClassAndModuleThatMergeWithModulesExportedStaticFunctionUsingClassPrivateStatics.ts (1 errors) ====
+==== ClassAndModuleThatMergeWithModulesExportedStaticFunctionUsingClassPrivateStatics.ts (3 errors) ====
     class clodule<T> {
         id: string;
+        ~~
+!!! error TS2564: Property 'id' has no initializer and is not definitely assigned in the constructor.
         value: T;
+        ~~~~~
+!!! error TS2564: Property 'value' has no initializer and is not definitely assigned in the constructor.
 
         private static sfn(id: string) { return 42; }
     }

diff --git a/...rence/ClassAndModuleThatMergeWithModulesExportedStaticFunctionUsingClassPrivateStatics.js b/...rence/ClassAndModuleThatMergeWithModulesExportedStaticFunctionUsingClassPrivateStatics.js
@@ -18,6 +18,7 @@ namespace clodule {
 
 
 //// [ClassAndModuleThatMergeWithModulesExportedStaticFunctionUsingClassPrivateStatics.js]
+"use strict";
 class clodule {
     static sfn(id) { return 42; }
 }

diff --git a/...s/reference/ClassAndModuleThatMergeWithStaticFunctionAndExportedFunctionThatShareAName.js b/...s/reference/ClassAndModuleThatMergeWithStaticFunctionAndExportedFunctionThatShareAName.js
@@ -25,6 +25,7 @@ namespace A {
 }
 
 //// [ClassAndModuleThatMergeWithStaticFunctionAndExportedFunctionThatShareAName.js]
+"use strict";
 class Point {
     constructor(x, y) {
         this.x = x;

diff --git a/...eference/ClassAndModuleThatMergeWithStaticFunctionAndExportedFunctionThatShareAName.types b/...eference/ClassAndModuleThatMergeWithStaticFunctionAndExportedFunctionThatShareAName.types
@@ -31,8 +31,8 @@ namespace Point {
 >      : ^^^^^^^^^^^^
 
     export function Origin() { return null; } //expected duplicate identifier error
->Origin : () => any
->       : ^^^^^^^^^
+>Origin : () => null
+>       : ^^^^^^^^^^
 }
 
 

diff --git a/...eference/ClassAndModuleThatMergeWithStaticFunctionAndNonExportedFunctionThatShareAName.js b/...eference/ClassAndModuleThatMergeWithStaticFunctionAndNonExportedFunctionThatShareAName.js
@@ -25,6 +25,7 @@ namespace A {
 }
 
 //// [ClassAndModuleThatMergeWithStaticFunctionAndNonExportedFunctionThatShareAName.js]
+"use strict";
 class Point {
     constructor(x, y) {
         this.x = x;

diff --git a/...elines/reference/ClassAndModuleThatMergeWithStaticVariableAndExportedVarThatShareAName.js b/...elines/reference/ClassAndModuleThatMergeWithStaticVariableAndExportedVarThatShareAName.js
@@ -25,6 +25,7 @@ namespace A {
 }
 
 //// [ClassAndModuleThatMergeWithStaticVariableAndExportedVarThatShareAName.js]
+"use strict";
 class Point {
     constructor(x, y) {
         this.x = x;

diff --git a/...nes/reference/ClassAndModuleThatMergeWithStaticVariableAndNonExportedVarThatShareAName.js b/...nes/reference/ClassAndModuleThatMergeWithStaticVariableAndNonExportedVarThatShareAName.js
@@ -25,6 +25,7 @@ namespace A {
 }
 
 //// [ClassAndModuleThatMergeWithStaticVariableAndNonExportedVarThatShareAName.js]
+"use strict";
 class Point {
     constructor(x, y) {
         this.x = x;

diff --git a/...sAndModuleThatMergeWithStringIndexerAndExportedFunctionWithTypeIncompatibleWithIndexer.js b/...sAndModuleThatMergeWithStringIndexerAndExportedFunctionWithTypeIncompatibleWithIndexer.js
@@ -4,3 +4,4 @@
 
 
 //// [ClassAndModuleThatMergeWithStringIndexerAndExportedFunctionWithTypeIncompatibleWithIndexer.js]
+"use strict";
diff --git a/tests/baselines/reference/ClassAndModuleWithSameNameAndCommonRoot.errors.txt b/tests/baselines/reference/ClassAndModuleWithSameNameAndCommonRoot.errors.txt
@@ -1,4 +1,5 @@
 module.ts(2,22): error TS2433: A namespace declaration cannot be in a different file from a class or function with which it is merged.
+simple.ts(2,5): error TS2564: Property 'id' has no initializer and is not definitely assigned in the constructor.
 
 
 ==== class.ts (0 errors) ====
@@ -28,9 +29,11 @@ module.ts(2,22): error TS2433: A namespace declaration cannot be in a different
     var cl = X.Y.Point.Origin; // error not expected here same as bug 83996 ?
 
 
-==== simple.ts (0 errors) ====
+==== simple.ts (1 errors) ====
     class A {
         id: string;
+        ~~
+!!! error TS2564: Property 'id' has no initializer and is not definitely assigned in the constructor.
     }
 
     namespace A {

diff --git a/tests/baselines/reference/ClassAndModuleWithSameNameAndCommonRoot.js b/tests/baselines/reference/ClassAndModuleWithSameNameAndCommonRoot.js
@@ -41,6 +41,7 @@ var a: { id: string };
 
 
 //// [class.js]
+"use strict";
 var X;
 (function (X) {
     var Y;
@@ -55,6 +56,7 @@ var X;
     })(Y = X.Y || (X.Y = {}));
 })(X || (X = {}));
 //// [module.js]
+"use strict";
 var X;
 (function (X) {
     var Y;
@@ -66,10 +68,12 @@ var X;
     })(Y = X.Y || (X.Y = {}));
 })(X || (X = {}));
 //// [test.js]
+"use strict";
 //var cl: { x: number; y: number; }
 var cl = new X.Y.Point(1, 1);
 var cl = X.Y.Point.Origin; // error not expected here same as bug 83996 ?
 //// [simple.js]
+"use strict";
 class A {
 }
 (function (A) {

diff --git a/tests/baselines/reference/ClassAndModuleWithSameNameAndCommonRootES6.errors.txt b/tests/baselines/reference/ClassAndModuleWithSameNameAndCommonRootES6.errors.txt
@@ -1,4 +1,5 @@
 module.ts(2,22): error TS2433: A namespace declaration cannot be in a different file from a class or function with which it is merged.
+simple.ts(2,5): error TS2564: Property 'id' has no initializer and is not definitely assigned in the constructor.
 
 
 ==== class.ts (0 errors) ====
@@ -28,9 +29,11 @@ module.ts(2,22): error TS2433: A namespace declaration cannot be in a different
     var cl = X.Y.Point.Origin; // error not expected here same as bug 83996 ?
 
 
-==== simple.ts (0 errors) ====
+==== simple.ts (1 errors) ====
     class A {
         id: string;
+        ~~
+!!! error TS2564: Property 'id' has no initializer and is not definitely assigned in the constructor.
     }
 
     namespace A {
-Original file line number
+Diff line change
@@ Expand Up / @@ -6,6 +6,7 @@ var v = (a): => { @@
     };
     //// [ArrowFunction3.js]
+    "use strict";
     var v = (a);
     {
     }
@@ Expand Down @@
Original file line number	Diff line number	Diff line change
Expand Up		@@ -4,3 +4,4 @@


		//// [ClassAndModuleThatMergeWithStringIndexerAndExportedFunctionWithTypeIncompatibleWithIndexer.js]
		"use strict";