How to change default option for sortBy widget?

To change the default selected option for your sortBy widget, you can set the default index using the configure widget, rather than inside the sortBy widget itself. 

For example:

  instantsearch.widgets.configure({
    hitsPerPage: 8,
    index: 'instant_search_price_desc',
  })

This sets instant_search_price_desc as the default index, which means the results will be sorted by price in descending order when the page loads.

You can find a working example in this CodeSandbox.