A significant portion of the code-base has now been converted to use `let`/`const`, rather than `var`, hence it should be possible to simply enable the ESLint `no-var` rule globally.
This way we can ensure that new code won't accidentally use `var`, and it also removes the need to manually enable the rule in various folders.
Obviously it makes sense to continue the efforts to replace `var`, but that should probably happen on a file and/or folder basis.
Please note that this patch excludes the following code:
- The `extensions/` folder, since that seemed easiest for now (and I don't know exactly what the support situation is for the Chromium-extension).
- The entire `external/` folder is ignored, since most of it's currently excluded from linting.
For the code that isn't imported from elsewhere (and should be ignored), we should probably (at some point) bring the code up to the same linting/formatting standard as the rest of the code-base.
- Various files in the `test/` folder are ignored, as necessary, since the way that a lot of this code is loaded will require some care (or perhaps larger re-factoring) when removing `var` usage.
The `debugger`-statement would only, potentially, make sense during development and we thus want to prevent it from being accidentally included when landing code.
The `alert`, `confirm`, and `prompt` functions should generally be avoided, with the few intended cases manually allowed.
Please find additional details about the ESLint rules at:
- https://eslint.org/docs/rules/no-debugger
- https://eslint.org/docs/rules/no-alert
This major version bump required two changes:
- The global line in the mobile viewer example should be removed because
the `.eslintrc` file already defines these globals and with the new
`eslint` version we otherwise get an error saying "'pdfjsLib' is already
defined as a built-in global variable".
- The ECMA version for the examples must be set to 6 since we're using
modules, otherwise we get an error saying "sourceType 'module' is not
supported when ecmaVersion < 2015". It turns out that the previous
version of `eslint` already used ECMA version 6 silently even though
we set 5, see https://github.com/eslint/eslint/issues/9687#issuecomment-432413384,
so in terms of our code nothing really changes.
