diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index ddd20c3b2c39d..d2810a857ad69 100644
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -2846,7 +2846,7 @@ namespace ts {
                         && !options.noResolve
                         && i < file.imports.length
                         && !elideImport
-                        && !(isJsFile && !options.allowJs)
+                        && !(isJsFile && !getAllowJSCompilerOption(options))
                         && (isInJSFile(file.imports[i]) || !(file.imports[i].flags & NodeFlags.JSDoc));
 
                     if (elideImport) {
@@ -3160,7 +3160,7 @@ namespace ts {
                 createDiagnosticForOptionName(Diagnostics.Option_0_cannot_be_specified_when_option_target_is_ES3, "useDefineForClassFields");
             }
 
-            if (options.checkJs && !options.allowJs) {
+            if (options.checkJs && !getAllowJSCompilerOption(options)) {
                 programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Option_0_cannot_be_specified_without_specifying_option_1, "checkJs", "allowJs"));
             }
 
@@ -3774,7 +3774,7 @@ namespace ts {
             return options.jsx ? undefined : Diagnostics.Module_0_was_resolved_to_1_but_jsx_is_not_set;
         }
         function needAllowJs() {
-            return options.allowJs || !getStrictOptionValue(options, "noImplicitAny") ? undefined : Diagnostics.Could_not_find_a_declaration_file_for_module_0_1_implicitly_has_an_any_type;
+            return getAllowJSCompilerOption(options) || !getStrictOptionValue(options, "noImplicitAny") ? undefined : Diagnostics.Could_not_find_a_declaration_file_for_module_0_1_implicitly_has_an_any_type;
         }
         function needResolveJsonModule() {
             return options.resolveJsonModule ? undefined : Diagnostics.Module_0_was_resolved_to_1_but_resolveJsonModule_is_not_used;
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index 6b58a9aafd6d3..00564bddb016a 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -5922,6 +5922,10 @@ namespace ts {
         return compilerOptions[flag] === undefined ? !!compilerOptions.strict : !!compilerOptions[flag];
     }
 
+    export function getAllowJSCompilerOption(compilerOptions: CompilerOptions): boolean {
+        return compilerOptions.allowJs === undefined ? !!compilerOptions.checkJs : compilerOptions.allowJs;
+    }
+
     export function compilerOptionsAffectSemanticDiagnostics(newOptions: CompilerOptions, oldOptions: CompilerOptions): boolean {
         return oldOptions !== newOptions &&
             semanticDiagnosticsOptionDeclarations.some(option => !isJsonEqual(getCompilerOptionValue(oldOptions, option), getCompilerOptionValue(newOptions, option)));
@@ -6375,7 +6379,7 @@ namespace ts {
     export function getSupportedExtensions(options?: CompilerOptions): readonly Extension[];
     export function getSupportedExtensions(options?: CompilerOptions, extraFileExtensions?: readonly FileExtensionInfo[]): readonly string[];
     export function getSupportedExtensions(options?: CompilerOptions, extraFileExtensions?: readonly FileExtensionInfo[]): readonly string[] {
-        const needJsExtensions = options && options.allowJs;
+        const needJsExtensions = options && getAllowJSCompilerOption(options);
 
         if (!extraFileExtensions || extraFileExtensions.length === 0) {
             return needJsExtensions ? allSupportedExtensions : supportedTSExtensions;
diff --git a/src/harness/fourslashImpl.ts b/src/harness/fourslashImpl.ts
index 61a35dc1857ad..590913fd34c5d 100644
--- a/src/harness/fourslashImpl.ts
+++ b/src/harness/fourslashImpl.ts
@@ -623,7 +623,7 @@ namespace FourSlash {
             ts.forEachKey(this.inputFiles, fileName => {
                 if (!ts.isAnySupportedFileExtension(fileName)
                     || Harness.getConfigNameFromFileName(fileName)
-                    || !this.getProgram().getCompilerOptions().allowJs && !ts.resolutionExtensionIsTSOrJson(ts.extensionFromPath(fileName))) return;
+                    || !ts.getAllowJSCompilerOption(this.getProgram().getCompilerOptions()) && !ts.resolutionExtensionIsTSOrJson(ts.extensionFromPath(fileName))) return;
                 const errors = this.getDiagnostics(fileName).filter(e => e.category !== ts.DiagnosticCategory.Suggestion);
                 if (errors.length) {
                     this.printErrorLog(/*expectErrors*/ false, errors);
diff --git a/src/harness/harnessIO.ts b/src/harness/harnessIO.ts
index c7f04cf277d92..0c62fc78ec88d 100644
--- a/src/harness/harnessIO.ts
+++ b/src/harness/harnessIO.ts
@@ -469,7 +469,7 @@ namespace Harness {
                 if (vpath.isDeclaration(file.unitName) || vpath.isJson(file.unitName)) {
                     dtsFiles.push(file);
                 }
-                else if (vpath.isTypeScript(file.unitName) || (vpath.isJavaScript(file.unitName) && options.allowJs)) {
+                else if (vpath.isTypeScript(file.unitName) || (vpath.isJavaScript(file.unitName) && ts.getAllowJSCompilerOption(options))) {
                     const declFile = findResultCodeFile(file.unitName);
                     if (declFile && !findUnit(declFile.file, declInputFiles) && !findUnit(declFile.file, declOtherFiles)) {
                         dtsFiles.push({ unitName: declFile.file, content: Utils.removeByteOrderMark(declFile.text) });
diff --git a/src/server/project.ts b/src/server/project.ts
index 0e4f937552ea8..510f1291d4375 100644
--- a/src/server/project.ts
+++ b/src/server/project.ts
@@ -274,7 +274,7 @@ namespace ts.server {
                 this.compilerOptions.allowNonTsExtensions = true;
                 this.compilerOptions.allowJs = true;
             }
-            else if (hasExplicitListOfFiles || this.compilerOptions.allowJs || this.projectService.hasDeferredExtension()) {
+            else if (hasExplicitListOfFiles || getAllowJSCompilerOption(this.compilerOptions) || this.projectService.hasDeferredExtension()) {
                 // If files are listed explicitly or allowJs is specified, allow all extensions
                 this.compilerOptions.allowNonTsExtensions = true;
             }
diff --git a/src/server/typingsCache.ts b/src/server/typingsCache.ts
index 1d63a6df6ae6f..a20346c2b2e87 100644
--- a/src/server/typingsCache.ts
+++ b/src/server/typingsCache.ts
@@ -71,7 +71,7 @@ namespace ts.server {
 
     function compilerOptionsChanged(opt1: CompilerOptions, opt2: CompilerOptions): boolean {
         // TODO: add more relevant properties
-        return opt1.allowJs !== opt2.allowJs;
+        return getAllowJSCompilerOption(opt1) !== getAllowJSCompilerOption(opt2);
     }
 
     function unresolvedImportsChanged(imports1: SortedReadonlyArray<string> | undefined, imports2: SortedReadonlyArray<string> | undefined): boolean {
diff --git a/tests/cases/conformance/salsa/inferringClassMembersFromAssignments2.ts b/tests/cases/conformance/salsa/inferringClassMembersFromAssignments2.ts
index ba7ea55b2a36f..29eee549fa023 100644
--- a/tests/cases/conformance/salsa/inferringClassMembersFromAssignments2.ts
+++ b/tests/cases/conformance/salsa/inferringClassMembersFromAssignments2.ts
@@ -1,4 +1,3 @@
-// @allowJs: true
 // @checkJs: true
 // @noEmit: true
 // @filename: a.js
diff --git a/tests/cases/conformance/salsa/moduleExportAlias4.ts b/tests/cases/conformance/salsa/moduleExportAlias4.ts
index 0572401b80209..5d4d7d780a97a 100644
--- a/tests/cases/conformance/salsa/moduleExportAlias4.ts
+++ b/tests/cases/conformance/salsa/moduleExportAlias4.ts
@@ -1,5 +1,4 @@
 // @checkJs: true
-// @allowJS: true
 // @noEmit: true
 // @Filename: bug24024.js
 // #24024