ESLint error when using optional catch binding

It's an ES2019 feature, so make sure you set an ecmaVersion of 10. For example, your .eslintrc.json should have:

{
    "parserOptions": {
        "ecmaVersion": 10
    }
}

Setting parser options helps ESLint determine what is a parsing error.

Similarly, for the online demo, make sure to select 2019 from the "ECMA Version" dropdown list.