# TS8013: Non-null assertions can only be used in TypeScript files.

> A non-null assertion `x!` appears in a JavaScript file. Remove the `!` or check for null.

## What it means

The postfix `!` non-null assertion is TypeScript syntax and can't appear in a `.js` file.

ts-r's parser reports this code when it reads `.js` files with tsc's rules for TypeScript syntax in JavaScript. `ts-r-check` doesn't report it yet: today it parses `.js` files with the plain JavaScript grammar and reports a syntax error at the same place instead.

## Example

```js
const el = document.getElementById("app")!;
```

## How to fix

- Remove the `!` and handle the `null` case with a check.
- Rename the file to `.ts` if it's meant to be TypeScript.

Source: https://ts-r.dev/errors/TS8013/