diff --git a/.gitignore b/.gitignore
index 3598db9..636f55f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,2 @@
-test/**/*.js
+/node_modules
+test/test-outdir/**/*.js
diff --git a/guess.js b/guess.js
index d626a76..e6bea39 100644
--- a/guess.js
+++ b/guess.js
@@ -1,21 +1,42 @@
 'use strict';
 
 const path = require('path');
+const ts = require('typescript');
 
-let pattern = 'test/**/*.@(ts|tsx)';
 const cwd = process.cwd();
-const packageData = require(path.join(cwd, 'package.json'));
+const compilerOptions = loadCompilerOptions(cwd) || {};
+const extensions = ['ts', 'tsx'];
+if (compilerOptions.allowJs) {
+  extensions.push('js');
+  extensions.push('jsx');
+}
 
+let testDir = 'test';
+const packageData = require(path.join(cwd, 'package.json'));
 if (
   packageData &&
   typeof packageData.directories === 'object' &&
   typeof packageData.directories.test === 'string'
 ) {
-  const testDir = packageData.directories.test;
-  pattern = `${testDir + (testDir.lastIndexOf('/', 0) === 0 ? '' : '/')}**/*.@(ts|tsx)`;
+  testDir = packageData.directories.test;
 }
+const pattern = path.join(testDir, `**/*.@(${extensions.join('|')})`);
 
-require('./index')({
-  cwd: cwd,
-  pattern: pattern,
-});
+require('./index')({cwd, pattern, extensions});
+
+function loadCompilerOptions(cwd) {
+  const tsconfigPath = ts.findConfigFile(cwd, ts.sys.fileExists);
+  if (!tsconfigPath) {
+    return null;
+  }
+  const result = ts.readConfigFile(tsconfigPath, ts.sys.readFile);
+  if (result.error) {
+    throw new Error(result.error.messageText);
+  }
+  if (result.config && result.config.compilerOptions) {
+    const basepath = path.dirname(tsconfigPath);
+    const {options} = ts.parseJsonConfigFileContent(result.config, ts.sys, basepath);
+    return options;
+  }
+  return null;
+}
diff --git a/index.js b/index.js
index fe16a98..2bd5c46 100644
--- a/index.js
+++ b/index.js
@@ -2,20 +2,22 @@
 
 'use strict';
 
+const path = require('path');
 const espowerSource = require('espower-source');
 const minimatch = require('minimatch');
 const tsNodeRegister = require('ts-node').register;
 
-function espowerTypeScript(options) {
-  tsNodeRegister(options.tsNode);
-  espowerTsRegister('.ts', options);
-  espowerTsRegister('.tsx', options);
+function espowerTypeScript(options, tsNodeOptions) {
+  tsNodeRegister(tsNodeOptions);
+  const {extensions = ['ts', 'tsx']} = options;
+  extensions.forEach(ext => {
+    espowerTsRegister(`.${ext}`, options);
+  });
 }
 
 function espowerTsRegister(ext, options) {
   const cwd = options.cwd || process.cwd();
-  const separator = options.pattern.lastIndexOf('/', 0) === 0 ? '' : '/';
-  const pattern = cwd + separator + options.pattern;
+  const pattern = path.join(cwd, options.pattern);
 
   const originalExtension = require.extensions[ext];
   require.extensions[ext] = (module, filepath) => {
diff --git a/package-lock.json b/package-lock.json
index 26d0695..876c85f 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -2208,7 +2208,8 @@
     "typescript": {
       "version": "2.9.2",
       "resolved": "https://registry.npmjs.org/typescript/-/typescript-2.9.2.tgz",
-      "integrity": "sha512-Gr4p6nFNaoufRIY4NMdpQRNmgxVIGMs4Fcu/ujdYk3nAZqk7supzBE9idmvfZIlH/Cuj//dvi+019qEue9lV0w=="
+      "integrity": "sha512-Gr4p6nFNaoufRIY4NMdpQRNmgxVIGMs4Fcu/ujdYk3nAZqk7supzBE9idmvfZIlH/Cuj//dvi+019qEue9lV0w==",
+      "dev": true
     },
     "ua-parser-js": {
       "version": "0.7.18",
diff --git a/package.json b/package.json
index 51a5ff9..4b59809 100644
--- a/package.json
+++ b/package.json
@@ -14,6 +14,7 @@
     "demo": "mocha --require './guess' test/demo.ts",
     "lint": "eslint *.js",
     "test": "run-s lint test:*",
+    "test:allow-js": "cd test/test-allow-js && mocha --require ../../guess test/*_test.js",
     "test:outdir": "cd test/test-outdir && mocha --require ../../guess test/*_test.ts",
     "test:ts": "mocha --require './guess' test/*_test.ts",
     "test:tsx": "mocha --require './guess' test/*_test.tsx"
diff --git a/test/lib/expectPowerAssertMessage.ts b/test/lib/expectPowerAssertMessage.ts
index b064317..c467057 100644
--- a/test/lib/expectPowerAssertMessage.ts
+++ b/test/lib/expectPowerAssertMessage.ts
@@ -1,10 +1,11 @@
-import assert = require('assert');
+// don't use `assert` not to instrument
+import ass = require('assert');
 
 export default function expectPowerAssertMessage(body: () => void, expectedLines: string) {
   try {
     body();
-    assert.fail('AssertionError should be thrown');
+    ass.fail('AssertionError should be thrown');
   } catch(e) {
-    assert.equal(e.message.split('\n').slice(2, -1).join('\n'), expectedLines);
+    ass.equal(e.message.split('\n').slice(2, -1).join('\n'), expectedLines);
   }
 };
diff --git a/test/test-allow-js/package.json b/test/test-allow-js/package.json
new file mode 100644
index 0000000..4f1f787
--- /dev/null
+++ b/test/test-allow-js/package.json
@@ -0,0 +1,9 @@
+{
+  "name": "test-allow-js",
+  "version": "1.0.0",
+  "description": "This is a dummy file. See ../../package.json",
+  "devDependencies": {
+  },
+  "dependencies": {
+  }
+}
diff --git a/test/test-allow-js/test/to_be_instrumented_test.js b/test/test-allow-js/test/to_be_instrumented_test.js
new file mode 100644
index 0000000..5054126
--- /dev/null
+++ b/test/test-allow-js/test/to_be_instrumented_test.js
@@ -0,0 +1,34 @@
+'use strict';
+
+const assert = require('assert');
+
+describe('test for allowJs option', () => {
+  beforeEach(function() {
+    // don't use `assert` not to instrument
+    const ass = assert;
+    this.expectPowerAssertMessage = (body, expectedLines) => {
+      try {
+        body();
+        ass.fail('AssertionError should be thrown');
+      } catch (e) {
+        ass.equal(
+          e.message
+            .split('\n')
+            .slice(2, -1)
+            .join('\n'),
+          expectedLines
+        );
+      }
+    };
+  });
+
+  it('equal with Literal and Identifier: assert.equal(1, minusOne)', function() {
+    const minusOne = -1;
+    const expected = `  assert.equal(1, minusOne)
+                  |        
+                  -1       `;
+    this.expectPowerAssertMessage(() => {
+      assert.equal(1, minusOne);
+    }, expected);
+  });
+});
diff --git a/test/test-allow-js/tsconfig.json b/test/test-allow-js/tsconfig.json
new file mode 100644
index 0000000..b92d6a6
--- /dev/null
+++ b/test/test-allow-js/tsconfig.json
@@ -0,0 +1,13 @@
+{
+  "compilerOptions": {
+    "allowJs": true,
+    "outDir": "build",
+    "module": "commonjs",
+    "target": "ES5",
+    "noImplicitAny": true,
+    "jsx": "react"
+  },
+  "exclude": [
+    "node_modules"
+  ]
+}