Thanks for the bug report @strech345

If this is still an open issue, I would like to tackle it as it seems like a good first contribution to this project.

I investigated this issue, and it's solved in 6.2.0-alpha.1.

To test, I added an id of 0 to the linestring used in the unit tests. Here's the created feature object with the id attached:

{
  type: 'Feature',
  id: 0,
  properties: { name: 'test line' },
  geometry: { type: 'LineString', coordinates: [ [Array], [Array] ] }
}

If this needs more work, I can be involved :)

This looks good. Just for reference, here is the section of the GeoJSON spec that backs this up:

https://tools.ietf.org/html/rfc7946#section-3.2

If a Feature has a commonly used identifier, that identifier SHOULD be included as a member of the Feature object with the name "id", and the value of this member is either a JSON string or number.

@morganherlocker If the bug exists in the latest shipped released but is fixed in the current master, should the issue be closed? Or should it be fixed in a patch version? If there needs to be a fix in a patch, I would be able to do so and update the tests for it, just would appreciate some direction :)