Option to Enable or Disable Trailing or Leading Comments

I am experiencing duplicate comments when transforming ast to code.

I want to propose a way to enable or disable trailing and leading comments. Rather than doing attachComment: true, maybe we can do something like

{
    attachComment: {
        leading: true,
        trailing: false
    }
}

but the old way of attaching comment should still be supported so as not to break things. If you do

{
    attachComment: true
}

then it will still attach leading and trailing comments.

this is espree 2.2.5, by the way.

Here are some issues I found related to this that might also help other users if implemented

https://github.com/eslint/espree/issues/184 https://github.com/estools/escodegen/issues/152

[platinumazure]

@kaicataldo We (you) created a new comment attachment strategy in ESLint some time ago. Were any corresponding changes made to espree that would fix the issue described here?

[kaicataldo]

@platinumazure No changes were made to Espree. We just don’t use the comment attachment option when parsing.

As @nzakas has said, this isn’t a bug, but a feature request. When Espree parses, there is no way to know whether a comment is a trailing comment for the previous node or a leading comment for the next node. Given a node, you should be able to check if it has a comment before it or after it. It’s up to the consumer to decide what they do with this information.

It feels to me like thinking of these as duplicated comments is wrong. The intended use case here is to be able to easily access comments around nodes. Attached comments are not nodes in the tree (this wouldn’t make sense) - they are just references to the comment tokens that exist in the comments array.

I don’t think adding an option to only include leading or trailing comments makes sense either, because some comments would be missed (at the beginning or end of the file, for instance).

As @nzakas mentioned, it’s on the consumer to use this information correctly. My naive take is that it would either involve using attached comments and deduping them or using the comments array generated by Espree using the comment: true option and inserting comments in the correct place (this is more work, but I think it would be more accurate).

For reference, Babel does the first of the two options above. Babylon has a very similar comment attachment strategy and Babel does the work to dedupe comments when it generates code: https://github.com/babel/babel/blob/master/packages/babel-generator/src/printer.js#L528