> For AI agents: the complete documentation index is available at /llms.txt, the full documentation bundle is available at /llms-full.txt.

# security.checkSyntax

- **Type:**

```ts
type CheckSyntax =
  | boolean
  | {
      targets?: string[];
      exclude?: RegExp | RegExp[];
      ecmaVersion?: EcmaVersion;
    };
```

- **Default:** `false`

Used to analyze whether there is incompatible advanced syntax in the build artifacts under the current browser scope. If any incompatible syntax is found, detailed information will be printed to the terminal.

### Enable Detection

You can set `checkSyntax` to `true` to enable syntax checking.

```ts
export default {
  security: {
    checkSyntax: true,
  },
};
```

When you enable `checkSyntax`, Modern.js will perform the detection during production builds. If any incompatible advanced syntax is detected in the build artifacts, error logs will be printed to the terminal, and the current build process will be terminated.

### Error Logs

The format of the error logs is as follows, including the source file, artifact location, error reason, and source code:

```bash
error   [Syntax Checker] Find some syntax errors after production build:

  Error 1
  source:  /node_modules/foo/index.js:1:0
  output:  /dist/static/js/main.3f7a4d7e.js:2:39400
  reason:  Unexpected token (1:178)
  code:
     9 |
    10 | var b = 2;
    11 |
  > 12 | console.log(() => {
    13 |   return a + b;
    14 | });
    15 |
```

:::tip
Currently, syntax checking is implemented based on AST parser. Each time it performs a check, it can only identify the first incompatible syntax found in the file. If there are multiple incompatible syntaxes in the file, you need to fix the detected syntax and re-run the check.
If the corresponding source location is not shown in the log, try setting **output.minify** to false and rebuild again.
:::

### Solutions

If a syntax error is detected, you can handle it in the following ways:

- If you want to downgrade this syntax to ensure good code compatibility, you can compile the corresponding module through the `source.include` config.
- If you don't want to downgrade the syntax, you can adjust the project's browserslist to match the syntax.
- If you do not want to check the syntax of certain products, you can use the `checkSyntax.exclude` configuration to exclude the files to be checked.

### Options

`security.checkSyntax` is implemented based on `@rsbuild/plugin-check-syntax`. For specific options, please refer to [@rsbuild/plugin-check-syntax](https://github.com/rstackjs/rsbuild-plugin-check-syntax).