https://github.com/masayuki0812/c3/blob/master/src/data.convert.js#L37-L41
These lines of code are causing me headache because they are mutating my settings array that holds my keys by pushing the x key onto the end of the array. I use this array for filtering my collection in my subscription model, e.g., {key:{$exists:true}} in Mongo. When x is pushed onto this array, my subscription expands to include all documents that have x. (It also seems to push x repeatedely whenever I have a refresh. So I get multiple instances on x on the array.)

To make this snippet of code more immutable, I've changed it to the following (avoiding the .push() part of it.)

if (keys.x) {
  targetKeys = keys.value.concat(keys.x);
  $$.config.data_x = keys.x;
} else {
  targetKeys = keys.value;
}

I'm not exactly sure why or how it's mutating my array, but it sure has been a pain. Please consider committing this change in a future update.

Thanks!