diff --git a/README.md b/README.md
index ccfbf3e..7ec4f81 100644
--- a/README.md
+++ b/README.md
@@ -74,9 +74,10 @@ export default [
 
 ### Options
 
-| Option | Type   | Description     |
-| ------ | ------ | --------------- |
-| output | string | App output file |
+| Option       | Type       | Description              | Default                  |
+| ------------ | ---------- | ------------------------ | ------------------------ |
+| `output`     | `string`   | Declarations output file | The value of `pkg.types` |
+| `extensions` | `string[]` | Valid Extensions         | .svelte, .ts, .js        |
 
 ## Using with cli
 
@@ -86,10 +87,11 @@ svelte-dts -i src/index.ts -o dist/index.d.ts
 
 ### Options
 
-| Option                         | Alias           | Description     |
-| ------------------------------ | --------------- | --------------- |
-| <code>--input [input]</code>   | <code>-i</code> | App input file  |
-| <code>--output [output]</code> | <code>-o</code> | App output file |
+| Option                                 | Alias           | Description              |
+| -------------------------------------- | --------------- | ------------------------ |
+| <code>--input [input]</code>           | <code>-i</code> | App input file           |
+| <code>--output [output]</code>         | <code>-o</code> | Declarations output file |
+| <code>--extensions [extensions]</code> | <code>-e</code> | Valid Extensions         |
 
 ## NPM Statistics
 
diff --git a/src/cli.ts b/src/cli.ts
index 1386a3c..2eca6f6 100644
--- a/src/cli.ts
+++ b/src/cli.ts
@@ -11,13 +11,14 @@ const exec = async (): Promise<void> => {
     .name(`svelte-dts`)
     .version(packageJson.version, '-v --version', 'Version number')
     .helpOption('-h --help', 'For more information')
-    .requiredOption('-i, --input <input>', 'dts input')
-    .requiredOption('-o, --output <output>', 'dts output')
+    .requiredOption('-i, --input <input>', 'input of application')
+    .requiredOption('-o, --output <output>', 'output of declarations')
+    .option('-e, --extensions <extensions...>', 'valid extensions')
     .parse(process.argv);
 
   const options = program.opts();
 
-  const generator = new Generator(options.input, { output: options.output });
+  const generator = new Generator(options.input, { output: options.output, extensions: options.extensions });
   await generator.read();
   await generator.write();
 };
diff --git a/src/generator.ts b/src/generator.ts
index 2f03652..106a03a 100644
--- a/src/generator.ts
+++ b/src/generator.ts
@@ -15,85 +15,88 @@ class Generator {
   private dir: string;
   private output: string;
   private input: string;
-  private options: Options;
+  private extensions: string[];
 
   constructor(input: string, options: Options) {
-    this.options = options;
-
     this.packageJson = require(path.join(process.cwd(), 'package.json'));
     this.input = path.join(process.cwd(), input);
     this.dir = path.dirname(this.input);
-    this.output = this.options.output || this.packageJson.types;
+    this.output = options.output || this.packageJson.types;
+    this.extensions = options.extensions || ['.svelte', '.ts', '.js'];
   }
 
   async read(): Promise<void> {
     const files: string[] = await readdir(this.dir, ['node_modules']);
+    await Promise.all(files.map((item) => this.readFile(item)));
+  }
 
-    for (let i = 0; i < files.length; i++) {
-      const filename = files[i];
-      const pathParser = path.parse(filename);
-      const extension = path.extname(filename);
+  async readFile(filename: string): Promise<void> {
+    const pathParser = path.parse(filename);
+    const extension = path.extname(filename);
 
-      if (pathParser.base.includes('.test') || pathParser.base.includes('.spec')) {
-        continue;
-      }
+    if (pathParser.base.includes('.test') || pathParser.base.includes('.spec')) {
+      return;
+    }
+
+    if (!this.extensions.includes(pathParser.ext)) {
+      return;
+    }
 
-      if (extension === '.svelte') {
-        const fileContent = await fs.readFile(filename, { encoding: 'utf-8' });
-
-        let scriptTsContent: string = '';
-        const resultPreprocess = await preprocess(
-          fileContent,
-          [
-            {
-              script: ({ content, attributes }) => {
-                if (attributes.lang === 'ts') {
-                  scriptTsContent = content;
-
-                  const resultTranspile = ts.transpileModule(content, {
-                    compilerOptions: {
-                      module: ts.ModuleKind.ESNext,
-                      target: ts.ScriptTarget.ESNext,
-                      moduleResolution: ts.ModuleResolutionKind.NodeJs,
-                      strict: true,
-                    },
-                  });
-
-                  return { code: resultTranspile.outputText };
-                }
-
-                return { code: content };
-              },
+    if (extension === '.svelte') {
+      const fileContent = await fs.readFile(filename, { encoding: 'utf-8' });
+
+      let scriptTsContent: string = '';
+      const resultPreprocess = await preprocess(
+        fileContent,
+        [
+          {
+            script: ({ content, attributes }) => {
+              if (attributes.lang === 'ts') {
+                scriptTsContent = content;
+
+                const resultTranspile = ts.transpileModule(content, {
+                  compilerOptions: {
+                    module: ts.ModuleKind.ESNext,
+                    target: ts.ScriptTarget.ESNext,
+                    moduleResolution: ts.ModuleResolutionKind.NodeJs,
+                    strict: true,
+                  },
+                });
+
+                return { code: resultTranspile.outputText };
+              }
+
+              return { code: content };
             },
-          ],
-          { filename }
-        );
+          },
+        ],
+        { filename }
+      );
+
+      if (scriptTsContent) {
+        const compiled = svelteCompile(resultPreprocess.code, {
+          filename,
+        });
 
-        if (scriptTsContent) {
-          const compiled = svelteCompile(resultPreprocess.code, {
-            filename,
-          });
-
-          this.transformers.push(
-            new SvelteTransformer(
-              scriptTsContent,
-              filename,
-              compiled.ast,
-              this.dir,
-              this.packageJson.name,
-              this.input === filename
-            )
-          );
-        }
-      } else if (extension === '.ts') {
-        this.transformers.push(
-          new TypescriptTransformer(filename, this.dir, this.packageJson.name, this.input === filename)
-        );
-      } else if (extension === '.js') {
         this.transformers.push(
-          new JavascriptTransformer(filename, this.dir, this.packageJson.name, this.input === filename)
+          new SvelteTransformer(
+            scriptTsContent,
+            filename,
+            compiled.ast,
+            this.dir,
+            this.packageJson.name,
+            this.input === filename
+          )
         );
       }
+    } else if (extension === '.ts') {
+      this.transformers.push(
+        new TypescriptTransformer(filename, this.dir, this.packageJson.name, this.input === filename)
+      );
+    } else if (extension === '.js') {
+      this.transformers.push(
+        new JavascriptTransformer(filename, this.dir, this.packageJson.name, this.input === filename)
+      );
     }
   }
 
diff --git a/src/types.ts b/src/types.ts
index f3b0599..a140a1e 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -1,5 +1,6 @@
 export type Options = {
   output: string;
+  extensions?: string[];
 };
 
 export type Prop = {