diff --git a/src/harness/fourslash.ts b/src/harness/fourslash.ts
index 2e98a95831fca..4c0c72a200f3f 100644
--- a/src/harness/fourslash.ts
+++ b/src/harness/fourslash.ts
@@ -258,13 +258,13 @@ namespace FourSlash {
             let startResolveFileRef: FourSlashFile;
 
             let configFileName: string;
-            ts.forEach(testData.files, file => {
+            for (const file of testData.files) {
                 // Create map between fileName and its content for easily looking up when resolveReference flag is specified
                 this.inputFiles.set(file.fileName, file.content);
-                if (isTsconfig(file)) {
+                if (isConfig(file)) {
                     const configJson = ts.parseConfigFileTextToJson(file.fileName, file.content);
                     if (configJson.config === undefined) {
-                        throw new Error(`Failed to parse test tsconfig.json: ${configJson.error.messageText}`);
+                        throw new Error(`Failed to parse test ${file.fileName}: ${configJson.error.messageText}`);
                     }
 
                     // Extend our existing compiler options so that we can also support tsconfig only options
@@ -286,7 +286,7 @@ namespace FourSlash {
                     // If entry point for resolving file references is already specified, report duplication error
                     throw new Error("There exists a Fourslash file which has resolveReference flag specified; remove duplicated resolveReference flag");
                 }
-            });
+            }
 
             if (configFileName) {
                 const baseDir = ts.normalizePath(ts.getDirectoryPath(configFileName));
@@ -295,12 +295,7 @@ namespace FourSlash {
                 const configJsonObj = ts.parseConfigFileTextToJson(configFileName, this.inputFiles.get(configFileName));
                 assert.isTrue(configJsonObj.config !== undefined);
 
-                const { options, errors } = ts.parseJsonConfigFileContent(configJsonObj.config, host, baseDir);
-
-                // Extend our existing compiler options so that we can also support tsconfig only options
-                if (!errors || errors.length === 0) {
-                    compilationOptions = ts.extend(compilationOptions, options);
-                }
+                compilationOptions = ts.parseJsonConfigFileContent(configJsonObj.config, host, baseDir, compilationOptions, configFileName).options;
             }
 
 
@@ -3401,13 +3396,14 @@ ${code}
         }
 
         // @Filename is the only directive that can be used in a test that contains tsconfig.json file.
-        if (files.some(isTsconfig)) {
+        const config = ts.find(files, isConfig);
+        if (config) {
             let directive = getNonFileNameOptionInFileList(files);
             if (!directive) {
                 directive = getNonFileNameOptionInObject(globalOptions);
             }
             if (directive) {
-                throw Error("It is not allowed to use tsconfig.json along with directive '" + directive + "'");
+                throw Error(`It is not allowed to use ${config.fileName} along with directive '${directive}'`);
             }
         }
 
@@ -3420,8 +3416,8 @@ ${code}
         };
     }
 
-    function isTsconfig(file: FourSlashFile): boolean {
-        return ts.getBaseFileName(file.fileName).toLowerCase() === "tsconfig.json";
+    function isConfig(file: FourSlashFile): boolean {
+        return Harness.getConfigNameFromFileName(file.fileName) !== undefined;
     }
 
     function getNonFileNameOptionInFileList(files: FourSlashFile[]): string {
diff --git a/src/harness/harness.ts b/src/harness/harness.ts
index 612224cb312a2..f2f8587d83412 100644
--- a/src/harness/harness.ts
+++ b/src/harness/harness.ts
@@ -1961,7 +1961,7 @@ namespace Harness {
             let tsConfigFileUnitData: TestUnitData;
             for (let i = 0; i < testUnitData.length; i++) {
                 const data = testUnitData[i];
-                if (ts.getBaseFileName(data.name).toLowerCase() === "tsconfig.json") {
+                if (getConfigNameFromFileName(data.name)) {
                     const configJson = ts.parseJsonText(data.name, data.content);
                     assert.isTrue(configJson.endOfFileToken !== undefined);
                     let baseDir = ts.normalizePath(ts.getDirectoryPath(data.name));
@@ -2172,5 +2172,10 @@ namespace Harness {
         return { unitName: libFile, content: io.readFile(libFile) };
     }
 
+    export function getConfigNameFromFileName(filename: string): "tsconfig.json" | "jsconfig.json" | undefined {
+        const flc = ts.getBaseFileName(filename).toLowerCase();
+        return ts.find(["tsconfig.json" as "tsconfig.json", "jsconfig.json" as "jsconfig.json"], x => x === flc);
+    }
+
     if (Error) (<any>Error).stackTraceLimit = 100;
 }
diff --git a/tests/cases/fourslash/jsconfig.ts b/tests/cases/fourslash/jsconfig.ts
new file mode 100644
index 0000000000000..ce09c46b8574f
--- /dev/null
+++ b/tests/cases/fourslash/jsconfig.ts
@@ -0,0 +1,16 @@
+/// <reference path="fourslash.ts" />
+
+// @Filename: /a.js
+////function f(/**/x) {
+////}
+
+// @Filename: /jsconfig.json
+////{
+////    "compilerOptions": {
+////        "checkJs": true,
+////        "noImplicitAny": true
+////    }
+////}
+
+goTo.file("/a.js");
+verify.errorExistsAfterMarker("");