Bad comment location

[mamaniv]

I'm trying to parse and generate code using acorn and astring and I get the following issue:

I parse this code using acorn:

// comment before
const line = 1; // comment after

and then generate this code using astring:

// comment before
// comment after
const line = 1;

as you can see, "comment after" should be after the line and not before the line.

a working sample app to easily reproduce the issue:

'use strict';
const acorn = require('acorn');
const astring = require('astring');
const astravel = require('astravel');

const sourceCode = `// comment before
const line = 1; // comment after
`;
const comments = [];
const tree = acorn.parse(sourceCode, {
  ecmaVersion: 2020,
  locations: true,
  onComment: comments,
});
astravel.attachComments(tree, comments);
const generatedCode = astring.generate(tree, {comments: true});

console.log(generatedCode);

Versions:

node 14.9
astring 1.7.5
acorn 8.4.1
atravel 0.5.0

Reviewing the code, looks like you write the comments before the statement, even though the location index may be after the statement:

{
  "type": "VariableDeclaration",
  "start": 18,
  "end": 33,
...
  "comments": [
    {
      "type": "Line",
      "value": " comment before",
      "start": 0,
      "end": 17,
      ...
    },
    {
      "type": "Line",
      "value": " comment after",
      "start": 34,
      "end": 50,
      ...
    }
  ]
}

I think you should also consider the locations (start/end) before writing the comments.

Thanks.

[davidbonnet]

Hi @mamaniv, thanks for reporting this. The formatting of comments intentionally puts trailing comments on top due to a personal preference. However, I can see how this is confusing and the default behavior could indeed be to follow the current placement of comments as closely as possible.