Fix eslint errors

You are using eslint for code style checking but it turns out that you have a lot of errors.

Just checkout out the project and tried `npm run eslint`:
```
✖ 124 problems (123 errors, 1 warning)
  71 errors and 0 warnings potentially fixable with the `--fix` option.
```

I think code style check should report no errors and should be also executed in CI