diff --git a/src/compiler/builderState.ts b/src/compiler/builderState.ts
index 67f3540657299..25b0db377c4df 100644
--- a/src/compiler/builderState.ts
+++ b/src/compiler/builderState.ts
@@ -434,7 +434,7 @@ namespace ts {
                 );
                 const firstDts = firstOrUndefined(emitOutput.outputFiles);
                 if (firstDts) {
-                    Debug.assert(fileExtensionIsOneOf(firstDts.name, [Extension.Dts, Extension.Dmts, Extension.Dcts]), "File extension for signature expected to be dts", () => `Found: ${getAnyExtensionFromPath(firstDts.name)} for ${firstDts.name}:: All output files: ${JSON.stringify(emitOutput.outputFiles.map(f => f.name))}`);
+                    Debug.assert(isDeclarationFileName(firstDts.name), "File extension for signature expected to be dts", () => `Found: ${getAnyExtensionFromPath(firstDts.name)} for ${firstDts.name}:: All output files: ${JSON.stringify(emitOutput.outputFiles.map(f => f.name))}`);
                     latestSignature = (computeHash || generateDjb2Hash)(firstDts.text);
                     if (exportedModulesMapCache && latestSignature !== prevSignature) {
                         updateExportedModules(sourceFile, emitOutput.exportedModulesFromDeclarationEmit, exportedModulesMapCache);
diff --git a/src/compiler/emitter.ts b/src/compiler/emitter.ts
index a0fbb74b33be1..aca15c666a6ef 100644
--- a/src/compiler/emitter.ts
+++ b/src/compiler/emitter.ts
@@ -167,7 +167,7 @@ namespace ts {
     }
 
     function getOwnOutputFileNames(configFile: ParsedCommandLine, inputFileName: string, ignoreCase: boolean, addOutput: ReturnType<typeof createAddOutput>["addOutput"], getCommonSourceDirectory?: () => string) {
-        if (fileExtensionIs(inputFileName, Extension.Dts)) return;
+        if (isDeclarationFileName(inputFileName)) return;
         const js = getOutputJSFileName(inputFileName, configFile, ignoreCase, getCommonSourceDirectory);
         addOutput(js);
         if (fileExtensionIs(inputFileName, Extension.Json)) return;
@@ -219,7 +219,7 @@ namespace ts {
     export function getCommonSourceDirectoryOfConfig({ options, fileNames }: ParsedCommandLine, ignoreCase: boolean): string {
         return getCommonSourceDirectory(
             options,
-            () => filter(fileNames, file => !(options.noEmitForJsFiles && fileExtensionIsOneOf(file, supportedJSExtensionsFlat)) && !fileExtensionIs(file, Extension.Dts)),
+            () => filter(fileNames, file => !(options.noEmitForJsFiles && fileExtensionIsOneOf(file, supportedJSExtensionsFlat)) && !isDeclarationFileName(file)),
             getDirectoryPath(normalizeSlashes(Debug.checkDefined(options.configFilePath))),
             createGetCanonicalFileName(!ignoreCase)
         );
@@ -263,7 +263,7 @@ namespace ts {
 
         const getCommonSourceDirectory = memoize(() => getCommonSourceDirectoryOfConfig(configFile, ignoreCase));
         for (const inputFileName of configFile.fileNames) {
-            if (fileExtensionIs(inputFileName, Extension.Dts)) continue;
+            if (isDeclarationFileName(inputFileName)) continue;
             const jsFilePath = getOutputJSFileName(inputFileName, configFile, ignoreCase, getCommonSourceDirectory);
             if (jsFilePath) return jsFilePath;
             if (fileExtensionIs(inputFileName, Extension.Json)) continue;
diff --git a/src/compiler/moduleNameResolver.ts b/src/compiler/moduleNameResolver.ts
index 104223d1e750d..874344ae321be 100644
--- a/src/compiler/moduleNameResolver.ts
+++ b/src/compiler/moduleNameResolver.ts
@@ -1498,9 +1498,9 @@ namespace ts {
     }
 
     function loadJSOrExactTSFileName(extensions: Extensions, candidate: string, onlyRecordFailures: boolean, state: ModuleResolutionState): PathAndExtension | undefined {
-        if ((extensions === Extensions.TypeScript || extensions === Extensions.DtsOnly) && fileExtensionIsOneOf(candidate, [Extension.Dts, Extension.Dcts, Extension.Dmts])) {
+        if ((extensions === Extensions.TypeScript || extensions === Extensions.DtsOnly) && isDeclarationFileName(candidate)) {
             const result = tryFile(candidate, onlyRecordFailures, state);
-            return result !== undefined ? { path: candidate, ext: forEach([Extension.Dts, Extension.Dcts, Extension.Dmts], e => fileExtensionIs(candidate, e) ? e : undefined)! } : undefined;
+            return result !== undefined ? { path: candidate, ext: forEach(supportedDeclarationExtensions, e => fileExtensionIs(candidate, e) ? e : undefined)! } : undefined;
         }
 
         return loadModuleFromFileNoImplicitExtensions(extensions, candidate, onlyRecordFailures, state);
diff --git a/src/compiler/parser.ts b/src/compiler/parser.ts
index 7642612e27a44..2a96a3b7cd34a 100644
--- a/src/compiler/parser.ts
+++ b/src/compiler/parser.ts
@@ -9413,7 +9413,7 @@ namespace ts {
 
     /** @internal */
     export function isDeclarationFileName(fileName: string): boolean {
-        return fileExtensionIsOneOf(fileName, [Extension.Dts, Extension.Dmts, Extension.Dcts]);
+        return fileExtensionIsOneOf(fileName, supportedDeclarationExtensions);
     }
 
     /*@internal*/
diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index 021c34ce3a7d6..615d52fb68e44 100644
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -1186,7 +1186,7 @@ namespace ts {
                             else if (getEmitModuleKind(parsedRef.commandLine.options) === ModuleKind.None) {
                                 const getCommonSourceDirectory = memoize(() => getCommonSourceDirectoryOfConfig(parsedRef.commandLine, !host.useCaseSensitiveFileNames()));
                                 for (const fileName of parsedRef.commandLine.fileNames) {
-                                    if (!fileExtensionIs(fileName, Extension.Dts) && !fileExtensionIs(fileName, Extension.Json)) {
+                                    if (!isDeclarationFileName(fileName) && !fileExtensionIs(fileName, Extension.Json)) {
                                         processProjectReferenceFile(getOutputDeclarationFileName(fileName, parsedRef.commandLine, !host.useCaseSensitiveFileNames(), getCommonSourceDirectory), { kind: FileIncludeKind.OutputFromProjectReference, index });
                                     }
                                 }
@@ -1393,7 +1393,7 @@ namespace ts {
 
         function getRedirectReferenceForResolution(file: SourceFile) {
             const redirect = getResolvedProjectReferenceToRedirect(file.originalFileName);
-            if (redirect || !fileExtensionIsOneOf(file.originalFileName, [Extension.Dts, Extension.Dcts, Extension.Dmts])) return redirect;
+            if (redirect || !isDeclarationFileName(file.originalFileName)) return redirect;
 
             // The originalFileName could not be actual source file name if file found was d.ts from referecned project
             // So in this case try to look up if this is output from referenced project, if it is use the redirected project in that case
@@ -2969,7 +2969,7 @@ namespace ts {
 
         function getProjectReferenceRedirectProject(fileName: string) {
             // Ignore dts or any json files
-            if (!resolvedProjectReferences || !resolvedProjectReferences.length || fileExtensionIs(fileName, Extension.Dts) || fileExtensionIs(fileName, Extension.Json)) {
+            if (!resolvedProjectReferences || !resolvedProjectReferences.length || isDeclarationFileName(fileName) || fileExtensionIs(fileName, Extension.Json)) {
                 return undefined;
             }
 
@@ -3025,7 +3025,7 @@ namespace ts {
                     else {
                         const getCommonSourceDirectory = memoize(() => getCommonSourceDirectoryOfConfig(resolvedRef.commandLine, !host.useCaseSensitiveFileNames()));
                         forEach(resolvedRef.commandLine.fileNames, fileName => {
-                            if (!fileExtensionIs(fileName, Extension.Dts) && !fileExtensionIs(fileName, Extension.Json)) {
+                            if (!isDeclarationFileName(fileName) && !fileExtensionIs(fileName, Extension.Json)) {
                                 const outputDts = getOutputDeclarationFileName(fileName, resolvedRef.commandLine, !host.useCaseSensitiveFileNames(), getCommonSourceDirectory);
                                 mapFromToProjectReferenceRedirectSource!.set(toPath(outputDts), fileName);
                             }
@@ -3947,7 +3947,7 @@ namespace ts {
                 return containsPath(options.outDir, filePath, currentDirectory, !host.useCaseSensitiveFileNames());
             }
 
-            if (fileExtensionIsOneOf(filePath, supportedJSExtensionsFlat) || fileExtensionIs(filePath, Extension.Dts)) {
+            if (fileExtensionIsOneOf(filePath, supportedJSExtensionsFlat) || isDeclarationFileName(filePath)) {
                 // Otherwise just check if sourceFile with the name exists
                 const filePathWithoutExtension = removeFileExtension(filePath);
                 return !!getSourceFileByPath((filePathWithoutExtension + Extension.Ts) as Path) ||
diff --git a/src/compiler/tsbuildPublic.ts b/src/compiler/tsbuildPublic.ts
index ad9f97f2dd8ab..e0825db610005 100644
--- a/src/compiler/tsbuildPublic.ts
+++ b/src/compiler/tsbuildPublic.ts
@@ -72,10 +72,6 @@ namespace ts {
         return date2 > date1 ? date2 : date1;
     }
 
-    function isDeclarationFile(fileName: string) {
-        return fileExtensionIs(fileName, Extension.Dts);
-    }
-
     export type ReportEmitErrorSummary = (errorCount: number, filesInError: (ReportFileInError | undefined)[]) => void;
 
     export interface ReportFileInError {
@@ -972,7 +968,7 @@ namespace ts {
             const emittedOutputs = new Map<Path, string>();
             outputFiles.forEach(({ name, text, writeByteOrderMark }) => {
                 let priorChangeTime: Date | undefined;
-                if (!anyDtsChanged && isDeclarationFile(name)) {
+                if (!anyDtsChanged && isDeclarationFileName(name)) {
                     // Check for unchanged .d.ts files
                     if (host.fileExists(name) && state.readFileWithCache(name) === text) {
                         priorChangeTime = host.getModifiedTime(name);
@@ -1421,7 +1417,7 @@ namespace ts {
                 // In addition to file timestamps, we also keep track of when a .d.ts file
                 // had its file touched but not had its contents changed - this allows us
                 // to skip a downstream typecheck
-                if (isDeclarationFile(output)) {
+                if (isDeclarationFileName(output)) {
                     const outputModifiedTime = getModifiedTime(host, output);
                     newestDeclarationFileContentChangedTime = newer(newestDeclarationFileContentChangedTime, outputModifiedTime);
                 }
@@ -1589,7 +1585,7 @@ namespace ts {
                     reportStatus(state, verboseMessage, proj.options.configFilePath!);
                 }
 
-                if (isDeclarationFile(file)) {
+                if (isDeclarationFileName(file)) {
                     priorNewestUpdateTime = newer(priorNewestUpdateTime, getModifiedTime(host, file));
                 }
 
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index 3454d260df225..19f3f5b36927c 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -6913,6 +6913,7 @@ namespace ts {
     export const supportedJSExtensionsFlat: readonly Extension[] = flatten(supportedJSExtensions);
     const allSupportedExtensions: readonly Extension[][] = [[Extension.Ts, Extension.Tsx, Extension.Dts, Extension.Js, Extension.Jsx], [Extension.Cts, Extension.Dcts, Extension.Cjs], [Extension.Mts, Extension.Dmts, Extension.Mjs]];
     const allSupportedExtensionsWithJson: readonly Extension[][] = [...allSupportedExtensions, [Extension.Json]];
+    export const supportedDeclarationExtensions: readonly Extension[] = [Extension.Dts, Extension.Dcts, Extension.Dmts];
 
     export function getSupportedExtensions(options?: CompilerOptions): readonly Extension[][];
     export function getSupportedExtensions(options?: CompilerOptions, extraFileExtensions?: readonly FileExtensionInfo[]): readonly string[][];
diff --git a/src/compiler/watchUtilities.ts b/src/compiler/watchUtilities.ts
index adbf9f1287b1b..e2fba1da6575d 100644
--- a/src/compiler/watchUtilities.ts
+++ b/src/compiler/watchUtilities.ts
@@ -495,7 +495,7 @@ namespace ts {
         if (outFile(options) || options.outDir) return false;
 
         // File if emitted next to input needs to be ignored
-        if (fileExtensionIs(fileOrDirectoryPath, Extension.Dts)) {
+        if (isDeclarationFileName(fileOrDirectoryPath)) {
             // If its declaration directory: its not ignored if not excluded by config
             if (options.declarationDir) return false;
         }
diff --git a/src/harness/sourceMapRecorder.ts b/src/harness/sourceMapRecorder.ts
index 0d9fd7a1aab5d..a8b0f43044808 100644
--- a/src/harness/sourceMapRecorder.ts
+++ b/src/harness/sourceMapRecorder.ts
@@ -282,7 +282,7 @@ namespace Harness.SourceMapRecorder {
             const sourceMapData = sourceMapDataList[i];
             let prevSourceFile: ts.SourceFile | undefined;
             let currentFile: documents.TextDocument;
-            if (ts.endsWith(sourceMapData.sourceMap.file, ts.Extension.Dts)) {
+            if (ts.isDeclarationFileName(sourceMapData.sourceMap.file)) {
                 if (sourceMapDataList.length > jsFiles.length) {
                     currentFile = declarationFiles[Math.floor(i / 2)]; // When both kinds of source map are present, they alternate js/dts
                 }
diff --git a/src/harness/vpathUtil.ts b/src/harness/vpathUtil.ts
index 44ef5b0f6af3f..ec573861fbb4d 100644
--- a/src/harness/vpathUtil.ts
+++ b/src/harness/vpathUtil.ts
@@ -108,7 +108,7 @@ namespace vpath {
     }
 
     export function isDeclaration(path: string) {
-        return ts.fileExtensionIsOneOf(path, [ts.Extension.Dmts, ts.Extension.Dcts, ts.Extension.Dts]);
+        return ts.isDeclarationFileName(path);
     }
 
     export function isSourceMap(path: string) {
diff --git a/src/server/project.ts b/src/server/project.ts
index cabeb6d1e62e2..715a659281788 100644
--- a/src/server/project.ts
+++ b/src/server/project.ts
@@ -32,7 +32,7 @@ namespace ts.server {
                     result.jsxSize! += fileSize;
                     break;
                 case ScriptKind.TS:
-                    if (fileExtensionIs(info.fileName, Extension.Dts)) {
+                    if (isDeclarationFileName(info.fileName)) {
                         result.dts += 1;
                         result.dtsSize! += fileSize;
                     }
@@ -71,7 +71,7 @@ namespace ts.server {
 
     /* @internal */
     export function hasNoTypeScriptSource(fileNames: string[]): boolean {
-        return !fileNames.some(fileName => (fileExtensionIs(fileName, Extension.Ts) && !fileExtensionIs(fileName, Extension.Dts)) || fileExtensionIs(fileName, Extension.Tsx));
+        return !fileNames.some(fileName => (fileExtensionIs(fileName, Extension.Ts) && !isDeclarationFileName(fileName)) || fileExtensionIs(fileName, Extension.Tsx));
     }
 
     /* @internal */
@@ -686,7 +686,7 @@ namespace ts.server {
 
                 // Update the signature
                 if (this.builderState && getEmitDeclarations(this.compilerOptions)) {
-                    const dtsFiles = outputFiles.filter(f => fileExtensionIs(f.name, Extension.Dts));
+                    const dtsFiles = outputFiles.filter(f => isDeclarationFileName(f.name));
                     if (dtsFiles.length === 1) {
                         const sourceFile = this.program!.getSourceFile(scriptInfo.fileName)!;
                         const signature = this.projectService.host.createHash ?
diff --git a/src/server/session.ts b/src/server/session.ts
index 6deac433729e9..16b04c6979220 100644
--- a/src/server/session.ts
+++ b/src/server/session.ts
@@ -1931,7 +1931,7 @@ namespace ts.server {
 
                     const compilationSettings = project.getCompilationSettings();
 
-                    if (!!compilationSettings.noEmit || fileExtensionIs(info.fileName, Extension.Dts) && !dtsChangeCanAffectEmit(compilationSettings)) {
+                    if (!!compilationSettings.noEmit || isDeclarationFileName(info.fileName) && !dtsChangeCanAffectEmit(compilationSettings)) {
                         // avoid triggering emit when a change is made in a .d.ts when declaration emit and decorator metadata emit are disabled
                         return undefined;
                     }
@@ -2479,7 +2479,7 @@ namespace ts.server {
                 else {
                     const info = this.projectService.getScriptInfo(fileNameInProject)!; // TODO: GH#18217
                     if (!info.isScriptOpen()) {
-                        if (fileExtensionIs(fileNameInProject, Extension.Dts)) {
+                        if (isDeclarationFileName(fileNameInProject)) {
                             veryLowPriorityFiles.push(fileNameInProject);
                         }
                         else {
diff --git a/src/services/documentRegistry.ts b/src/services/documentRegistry.ts
index 88d7daf468bf1..5274aed06fd00 100644
--- a/src/services/documentRegistry.ts
+++ b/src/services/documentRegistry.ts
@@ -236,7 +236,7 @@ namespace ts {
                 // It is fairly suspicious to have one path in two buckets - you'd expect dependencies to have similar configurations.
                 // If this occurs unexpectedly, the fix is likely to synchronize the project settings.
                 // Skip .d.ts files to reduce noise (should also cover most of node_modules).
-                const otherBucketKey = !fileExtensionIs(path, Extension.Dts) &&
+                const otherBucketKey = !isDeclarationFileName(path) &&
                     forEachEntry(buckets, (bucket, bucketKey) => bucketKey !== key && bucket.has(path) && bucketKey);
                 if (otherBucketKey) {
                     tracing.instant(tracing.Phase.Session, "documentRegistryBucketOverlap", { path, key1: otherBucketKey, key2: key });