Some issues with types

[adschm]

I'm currently using billboard.js with plain JS files, where ESlint is employed to have soft type-checking in my IDE. The billboard.js types are provided via npm.

This has identified a few issues of different (though generally low) severity, which I'd like to present/report and discuss here, hoping this is considered valuable enough.

Note that all of this is just about the types reported, the operation of the code itself is fine.

1. "this" for load.done:

var bbchart = bb.generate(...) # generate some chart
bbchart.load({
   json: somedata,
   done: function() {
      # in here 'this' has wrong type
      dosomething();
   }
});

Inside the done() function, ESlint won't properly detect the type "Chart" for this, but thinks the type is actually the args array from the definition here:
https://github.com/naver/billboard.js/blob/master/types/chart.d.ts#L347

I'm not sure whether this is an ESlint/TS parsing bug or whether one could specify this for the done?: statement in the definition:
https://github.com/naver/billboard.js/blob/master/types/chart.d.ts#L366

2. zoom() only accepts numbers

zoom([start, end]) only has type number[], although string is accepted for timeseries data:
https://github.com/naver/billboard.js/blob/master/types/chart.d.ts#L226

3. export.preserveaspectratio should be optional

export() works perfectly fine without setting preserveAspectRatio, so the type should be optional

https://github.com/naver/billboard.js/blob/master/types/chart.d.ts#L513

4. export() callback might return void

The callback function used with export() may return nothing (void) and will still work; however, the type requires string return value.

https://github.com/naver/billboard.js/blob/master/types/chart.d.ts#L514

[netil]

@adschm, thanks for looking into this!

[adschm]

I've tested with 3.2.0-next.5, and all issues are resolved there except the zoom for string dates.

I will provide a PR for this in a minute, and also add one for another type issue I found recently.