Don’t think that option is exposed in the sketch JS API just yet. You can do something like this:
let sketch = require('sketch');
let Slice = sketch.Slice
let Rectangle = sketch.Rectangle
let document = sketch.getSelectedDocument()
let parentGroup = document.pages[0].layers[0].layers[0] //position your slice inside some group
let exportSlice = new Slice({
frame: new Rectangle(10,10,40,40),
parent: parentGroup
})
exportSlice.sketchObject.exportOptions().layerOptions = 2