diff --git a/src/compiler/commandLineParser.ts b/src/compiler/commandLineParser.ts
index d1a6e56d818..fb58384c469 100644
--- a/src/compiler/commandLineParser.ts
+++ b/src/compiler/commandLineParser.ts
@@ -515,7 +515,7 @@ namespace ts {
                         if (!hasConflictingExtension) {
                             // Add the file only if there is no higher priority extension file already included
                             // eg. when a.d.ts and a.js are present in the folder, include only a.d.ts not a.js
-                            const baseName = fileName.substr(0, fileName.length - currentExtension.length - 1);
+                            const baseName = fileName.substr(0, fileName.length - currentExtension.length);
                             if (!hasProperty(filesSeen, baseName)) {
                                 filesSeen[baseName] = true;
                                 fileNames.push(fileName);
diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index 26e53aebab1..018f3022191 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -721,15 +721,15 @@ namespace ts {
 
     export function fileExtensionIs(path: string, extension: string): boolean {
         let pathLen = path.length;
-        let extLen = extension.length + 1;
-        return pathLen > extLen && path.substr(pathLen - extLen, extLen) === "." + extension;
+        let extLen = extension.length;
+        return pathLen > extLen && path.substr(pathLen - extLen, extLen) === extension;
     }
 
     /**
      *  List of supported extensions in order of file resolution precedence.
      */
-    export const supportedTypeScriptExtensions = ["ts", "tsx", "d.ts"];
-    export const supportedJavascriptExtensions = ["js", "jsx"];
+    export const supportedTypeScriptExtensions = [".ts", ".tsx", ".d.ts"];
+    export const supportedJavascriptExtensions = [".js", ".jsx"];
     export const supportedExtensionsWhenAllowedJs = supportedTypeScriptExtensions.concat(supportedJavascriptExtensions);
 
     export function getSupportedExtensions(options?: CompilerOptions): string[] {
@@ -747,11 +747,11 @@ namespace ts {
         return false;
     }
 
-    export const extensionsToRemove = ["d.ts", "ts", "js", "tsx", "jsx"]; 
+    export const extensionsToRemove = [".d.ts", ".ts", ".js", ".tsx", ".jsx"];
     export function removeFileExtension(path: string): string {
         for (let ext of extensionsToRemove) {
             if (fileExtensionIs(path, ext)) {
-                return path.substr(0, path.length - ext.length - 1);
+                return path.substr(0, path.length - ext.length);
             }
         }
         return path;
diff --git a/src/compiler/parser.ts b/src/compiler/parser.ts
index d8d2b2fdb26..a1076464bbd 100644
--- a/src/compiler/parser.ts
+++ b/src/compiler/parser.ts
@@ -669,7 +669,7 @@ namespace ts {
             sourceFile.bindDiagnostics = [];
             sourceFile.languageVersion = languageVersion;
             sourceFile.fileName = normalizePath(fileName);
-            sourceFile.flags = fileExtensionIs(sourceFile.fileName, "d.ts") ? NodeFlags.DeclarationFile : 0;
+            sourceFile.flags = fileExtensionIs(sourceFile.fileName, ".d.ts") ? NodeFlags.DeclarationFile : 0;
             sourceFile.languageVariant = getLanguageVariant(sourceFile.fileName);
 
             return sourceFile;
diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index ccb2d2c346e..db56f86e93b 100644
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -73,7 +73,7 @@ namespace ts {
         return forEach(supportedExtensions, tryLoad);
 
         function tryLoad(ext: string): string {
-            let fileName = fileExtensionIs(candidate, ext) ? candidate : candidate + "." + ext;
+            let fileName = fileExtensionIs(candidate, ext) ? candidate : candidate + ext;
             if (host.fileExists(fileName)) {
                 return fileName;
             }
@@ -165,13 +165,13 @@ namespace ts {
         while (true) {
             searchName = normalizePath(combinePaths(searchPath, moduleName));
             referencedSourceFile = forEach(getSupportedExtensions(compilerOptions), extension => {
-                if (extension === "tsx" && !compilerOptions.jsx) {
+                if (extension === ".tsx" && !compilerOptions.jsx) {
                     // resolve .tsx files only if jsx support is enabled 
                     // 'logical not' handles both undefined and None cases
                     return undefined;
                 }
 
-                let candidate = searchName + "." + extension;
+                let candidate = searchName + extension;
                 if (host.fileExists(candidate)) {
                     return candidate;
                 }
@@ -964,7 +964,7 @@ namespace ts {
                         diagnostic = Diagnostics.File_0_not_found;
                         diagnosticArgument = [fileName];
                     }
-                    else if (!forEach(getSupportedExtensions(options), extension => findSourceFile(fileName + "." + extension, isDefaultLib, supportedExtensions, refFile, refPos, refEnd))) {
+                    else if (!forEach(getSupportedExtensions(options), extension => findSourceFile(fileName + extension, isDefaultLib, supportedExtensions, refFile, refPos, refEnd))) {
                         // (TODO: shkamat) Should this message be different given we support multiple extensions
                         diagnostic = Diagnostics.File_0_not_found;
                         fileName += ".ts";
@@ -1077,7 +1077,7 @@ namespace ts {
                                 let start = getTokenPosOfNode(file.imports[i], file);
                                 fileProcessingDiagnostics.add(createFileDiagnostic(file, start, file.imports[i].end - start, Diagnostics.Exported_external_package_typings_file_0_is_not_a_module_Please_contact_the_package_author_to_update_the_package_definition, importedFile.fileName));
                             }
-                            else if (!fileExtensionIs(importedFile.fileName, "d.ts")) {
+                            else if (!fileExtensionIs(importedFile.fileName, ".d.ts")) {
                                 let start = getTokenPosOfNode(file.imports[i], file);
                                 fileProcessingDiagnostics.add(createFileDiagnostic(file, start, file.imports[i].end - start, Diagnostics.Exported_external_package_typings_can_only_be_in_d_ts_files_Please_contact_the_package_author_to_update_the_package_definition));
                             }
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index 7c88e1a9417..885d529f69e 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -2143,7 +2143,7 @@ namespace ts {
     }
 
     export function isTsx(fileName: string) {
-        return fileExtensionIs(fileName, "tsx");
+        return fileExtensionIs(fileName, ".tsx");
     }
 
     /**
diff --git a/src/harness/compilerRunner.ts b/src/harness/compilerRunner.ts
index e3c48e33e5c..11c1b9a8029 100644
--- a/src/harness/compilerRunner.ts
+++ b/src/harness/compilerRunner.ts
@@ -151,7 +151,7 @@ class CompilerBaselineRunner extends RunnerBase {
             });
 
             it("Correct JS output for " + fileName, () => {
-                if (!(units.length === 1 && ts.fileExtensionIs(lastUnit.name, "d.ts")) && this.emit) {
+                if (!(units.length === 1 && ts.fileExtensionIs(lastUnit.name, ".d.ts")) && this.emit) {
                     if (result.files.length === 0 && result.errors.length === 0) {
                         throw new Error("Expected at least one js file to be emitted or at least one error to be created.");
                     }
diff --git a/src/services/services.ts b/src/services/services.ts
index dce341cc035..d80e60d171b 100644
--- a/src/services/services.ts
+++ b/src/services/services.ts
@@ -1875,7 +1875,7 @@ namespace ts {
         let compilerHost: CompilerHost = {
             getSourceFile: (fileName, target) => fileName === normalizeSlashes(inputFileName) ? sourceFile : undefined,
             writeFile: (name, text, writeByteOrderMark) => {
-                if (fileExtensionIs(name, "map")) {
+                if (fileExtensionIs(name, ".map")) {
                     Debug.assert(sourceMapText === undefined, `Unexpected multiple source map outputs for the file '${name}'`);
                     sourceMapText = text;
                 }
diff --git a/tests/baselines/reference/jsFileCompilationWithMapFileAsJs.errors.txt b/tests/baselines/reference/jsFileCompilationWithMapFileAsJs.errors.txt
index aaeb023dbdb..a97dc2c3cd6 100644
--- a/tests/baselines/reference/jsFileCompilationWithMapFileAsJs.errors.txt
+++ b/tests/baselines/reference/jsFileCompilationWithMapFileAsJs.errors.txt
@@ -1,9 +1,9 @@
 error TS5055: Cannot write file 'tests/cases/compiler/b.js' which is one of the input files.
-error TS6054: File 'tests/cases/compiler/b.js.map' has unsupported extension. The only supported extensions are 'ts', 'tsx', 'd.ts', 'js', 'jsx'.
+error TS6054: File 'tests/cases/compiler/b.js.map' has unsupported extension. The only supported extensions are '.ts', '.tsx', '.d.ts', '.js', '.jsx'.
 
 
 !!! error TS5055: Cannot write file 'tests/cases/compiler/b.js' which is one of the input files.
-!!! error TS6054: File 'tests/cases/compiler/b.js.map' has unsupported extension. The only supported extensions are 'ts', 'tsx', 'd.ts', 'js', 'jsx'.
+!!! error TS6054: File 'tests/cases/compiler/b.js.map' has unsupported extension. The only supported extensions are '.ts', '.tsx', '.d.ts', '.js', '.jsx'.
 ==== tests/cases/compiler/a.ts (0 errors) ====
     
     class c {
diff --git a/tests/baselines/reference/jsFileCompilationWithMapFileAsJsWithInlineSourceMap.errors.txt b/tests/baselines/reference/jsFileCompilationWithMapFileAsJsWithInlineSourceMap.errors.txt
index aaeb023dbdb..a97dc2c3cd6 100644
--- a/tests/baselines/reference/jsFileCompilationWithMapFileAsJsWithInlineSourceMap.errors.txt
+++ b/tests/baselines/reference/jsFileCompilationWithMapFileAsJsWithInlineSourceMap.errors.txt
@@ -1,9 +1,9 @@
 error TS5055: Cannot write file 'tests/cases/compiler/b.js' which is one of the input files.
-error TS6054: File 'tests/cases/compiler/b.js.map' has unsupported extension. The only supported extensions are 'ts', 'tsx', 'd.ts', 'js', 'jsx'.
+error TS6054: File 'tests/cases/compiler/b.js.map' has unsupported extension. The only supported extensions are '.ts', '.tsx', '.d.ts', '.js', '.jsx'.
 
 
 !!! error TS5055: Cannot write file 'tests/cases/compiler/b.js' which is one of the input files.
-!!! error TS6054: File 'tests/cases/compiler/b.js.map' has unsupported extension. The only supported extensions are 'ts', 'tsx', 'd.ts', 'js', 'jsx'.
+!!! error TS6054: File 'tests/cases/compiler/b.js.map' has unsupported extension. The only supported extensions are '.ts', '.tsx', '.d.ts', '.js', '.jsx'.
 ==== tests/cases/compiler/a.ts (0 errors) ====
     
     class c {
diff --git a/tests/baselines/reference/jsFileCompilationWithMapFileAsJsWithOutDir.errors.txt b/tests/baselines/reference/jsFileCompilationWithMapFileAsJsWithOutDir.errors.txt
index 81d9b67d334..c9a4408f423 100644
--- a/tests/baselines/reference/jsFileCompilationWithMapFileAsJsWithOutDir.errors.txt
+++ b/tests/baselines/reference/jsFileCompilationWithMapFileAsJsWithOutDir.errors.txt
@@ -1,9 +1,9 @@
-error TS6054: File 'tests/cases/compiler/b.js' has unsupported extension. The only supported extensions are 'ts', 'tsx', 'd.ts'.
-error TS6054: File 'tests/cases/compiler/b.js.map' has unsupported extension. The only supported extensions are 'ts', 'tsx', 'd.ts'.
+error TS6054: File 'tests/cases/compiler/b.js' has unsupported extension. The only supported extensions are '.ts', '.tsx', '.d.ts'.
+error TS6054: File 'tests/cases/compiler/b.js.map' has unsupported extension. The only supported extensions are '.ts', '.tsx', '.d.ts'.
 
 
-!!! error TS6054: File 'tests/cases/compiler/b.js' has unsupported extension. The only supported extensions are 'ts', 'tsx', 'd.ts'.
-!!! error TS6054: File 'tests/cases/compiler/b.js.map' has unsupported extension. The only supported extensions are 'ts', 'tsx', 'd.ts'.
+!!! error TS6054: File 'tests/cases/compiler/b.js' has unsupported extension. The only supported extensions are '.ts', '.tsx', '.d.ts'.
+!!! error TS6054: File 'tests/cases/compiler/b.js.map' has unsupported extension. The only supported extensions are '.ts', '.tsx', '.d.ts'.
 ==== tests/cases/compiler/a.ts (0 errors) ====
     
     class c {
diff --git a/tests/baselines/reference/jsFileCompilationWithoutJsExtensions.errors.txt b/tests/baselines/reference/jsFileCompilationWithoutJsExtensions.errors.txt
index 4276669ff76..0b1a757d4fd 100644
--- a/tests/baselines/reference/jsFileCompilationWithoutJsExtensions.errors.txt
+++ b/tests/baselines/reference/jsFileCompilationWithoutJsExtensions.errors.txt
@@ -1,6 +1,6 @@
-error TS6054: File 'tests/cases/compiler/a.js' has unsupported extension. The only supported extensions are 'ts', 'tsx', 'd.ts'.
+error TS6054: File 'tests/cases/compiler/a.js' has unsupported extension. The only supported extensions are '.ts', '.tsx', '.d.ts'.
 
 
-!!! error TS6054: File 'tests/cases/compiler/a.js' has unsupported extension. The only supported extensions are 'ts', 'tsx', 'd.ts'.
+!!! error TS6054: File 'tests/cases/compiler/a.js' has unsupported extension. The only supported extensions are '.ts', '.tsx', '.d.ts'.
 ==== tests/cases/compiler/a.js (0 errors) ====
     declare var v;
\ No newline at end of file
diff --git a/tests/baselines/reference/project/invalidRootFile/amd/invalidRootFile.errors.txt b/tests/baselines/reference/project/invalidRootFile/amd/invalidRootFile.errors.txt
index 76d5def7b5c..9cd0dd7b0cf 100644
--- a/tests/baselines/reference/project/invalidRootFile/amd/invalidRootFile.errors.txt
+++ b/tests/baselines/reference/project/invalidRootFile/amd/invalidRootFile.errors.txt
@@ -1,6 +1,6 @@
 error TS6053: File 'a.ts' not found.
-error TS6054: File 'a.t' has unsupported extension. The only supported extensions are 'ts', 'tsx', 'd.ts'.
+error TS6054: File 'a.t' has unsupported extension. The only supported extensions are '.ts', '.tsx', '.d.ts'.
 
 
 !!! error TS6053: File 'a.ts' not found.
-!!! error TS6054: File 'a.t' has unsupported extension. The only supported extensions are 'ts', 'tsx', 'd.ts'.
\ No newline at end of file
+!!! error TS6054: File 'a.t' has unsupported extension. The only supported extensions are '.ts', '.tsx', '.d.ts'.
\ No newline at end of file
diff --git a/tests/baselines/reference/project/invalidRootFile/node/invalidRootFile.errors.txt b/tests/baselines/reference/project/invalidRootFile/node/invalidRootFile.errors.txt
index 76d5def7b5c..9cd0dd7b0cf 100644
--- a/tests/baselines/reference/project/invalidRootFile/node/invalidRootFile.errors.txt
+++ b/tests/baselines/reference/project/invalidRootFile/node/invalidRootFile.errors.txt
@@ -1,6 +1,6 @@
 error TS6053: File 'a.ts' not found.
-error TS6054: File 'a.t' has unsupported extension. The only supported extensions are 'ts', 'tsx', 'd.ts'.
+error TS6054: File 'a.t' has unsupported extension. The only supported extensions are '.ts', '.tsx', '.d.ts'.
 
 
 !!! error TS6053: File 'a.ts' not found.
-!!! error TS6054: File 'a.t' has unsupported extension. The only supported extensions are 'ts', 'tsx', 'd.ts'.
\ No newline at end of file
+!!! error TS6054: File 'a.t' has unsupported extension. The only supported extensions are '.ts', '.tsx', '.d.ts'.
\ No newline at end of file
diff --git a/tests/baselines/reference/project/jsFileCompilationDifferentNamesSpecified/amd/jsFileCompilationDifferentNamesSpecified.errors.txt b/tests/baselines/reference/project/jsFileCompilationDifferentNamesSpecified/amd/jsFileCompilationDifferentNamesSpecified.errors.txt
index a7fd60e03d5..5fa60d18af9 100644
--- a/tests/baselines/reference/project/jsFileCompilationDifferentNamesSpecified/amd/jsFileCompilationDifferentNamesSpecified.errors.txt
+++ b/tests/baselines/reference/project/jsFileCompilationDifferentNamesSpecified/amd/jsFileCompilationDifferentNamesSpecified.errors.txt
@@ -1,6 +1,6 @@
-error TS6054: File 'DifferentNamesSpecified/b.js' has unsupported extension. The only supported extensions are 'ts', 'tsx', 'd.ts'.
+error TS6054: File 'DifferentNamesSpecified/b.js' has unsupported extension. The only supported extensions are '.ts', '.tsx', '.d.ts'.
 
 
-!!! error TS6054: File 'DifferentNamesSpecified/b.js' has unsupported extension. The only supported extensions are 'ts', 'tsx', 'd.ts'.
+!!! error TS6054: File 'DifferentNamesSpecified/b.js' has unsupported extension. The only supported extensions are '.ts', '.tsx', '.d.ts'.
 ==== DifferentNamesSpecified/a.ts (0 errors) ====
     var test = 10;
\ No newline at end of file
diff --git a/tests/baselines/reference/project/jsFileCompilationDifferentNamesSpecified/node/jsFileCompilationDifferentNamesSpecified.errors.txt b/tests/baselines/reference/project/jsFileCompilationDifferentNamesSpecified/node/jsFileCompilationDifferentNamesSpecified.errors.txt
index a7fd60e03d5..5fa60d18af9 100644
--- a/tests/baselines/reference/project/jsFileCompilationDifferentNamesSpecified/node/jsFileCompilationDifferentNamesSpecified.errors.txt
+++ b/tests/baselines/reference/project/jsFileCompilationDifferentNamesSpecified/node/jsFileCompilationDifferentNamesSpecified.errors.txt
@@ -1,6 +1,6 @@
-error TS6054: File 'DifferentNamesSpecified/b.js' has unsupported extension. The only supported extensions are 'ts', 'tsx', 'd.ts'.
+error TS6054: File 'DifferentNamesSpecified/b.js' has unsupported extension. The only supported extensions are '.ts', '.tsx', '.d.ts'.
 
 
-!!! error TS6054: File 'DifferentNamesSpecified/b.js' has unsupported extension. The only supported extensions are 'ts', 'tsx', 'd.ts'.
+!!! error TS6054: File 'DifferentNamesSpecified/b.js' has unsupported extension. The only supported extensions are '.ts', '.tsx', '.d.ts'.
 ==== DifferentNamesSpecified/a.ts (0 errors) ====
     var test = 10;
\ No newline at end of file
diff --git a/tests/cases/unittests/moduleResolution.ts b/tests/cases/unittests/moduleResolution.ts
index 6f9497fde1d..a1767f58026 100644
--- a/tests/cases/unittests/moduleResolution.ts
+++ b/tests/cases/unittests/moduleResolution.ts
@@ -38,7 +38,7 @@ module ts {
         function testLoadAsFile(containingFileName: string, moduleFileNameNoExt: string, moduleName: string): void {
             for (let ext of supportedTypeScriptExtensions) {
                 let containingFile = { name: containingFileName }
-                let moduleFile = { name: moduleFileNameNoExt + "." + ext }
+                let moduleFile = { name: moduleFileNameNoExt + ext }
                 let resolution = nodeModuleNameResolver(moduleName, containingFile.name, supportedTypeScriptExtensions, createModuleResolutionHost(containingFile, moduleFile));                
                 assert.equal(resolution.resolvedModule.resolvedFileName, moduleFile.name);
                 assert.equal(!!resolution.resolvedModule.isExternalLibraryImport, false);
@@ -50,7 +50,7 @@ module ts {
                         break;
                     }
                     else {
-                        failedLookupLocations.push(normalizePath(getRootLength(moduleName) === 0 ? combinePaths(dir, moduleName) : moduleName) + "." + e);
+                        failedLookupLocations.push(normalizePath(getRootLength(moduleName) === 0 ? combinePaths(dir, moduleName) : moduleName) + e);
                     }
                 }