In this example a custom javascript class is supplied to read features from a url. The class implements the required method async readFeatures(chr, start, end)
class CytobandReader { async readFeatures(chr, start, end) { const response = await fetch(`https://lk85l6ycte.execute-api.us-east-1.amazonaws.com/dev/testservice/bands?chr=${chr}&start=${start}&end=${end}`, {headers: {'Content-Type': 'application/json'}}); return response.json(); } } ... const config = { genome: "hg38", locus: "chr1", tracks: [ { name: "Cytobands", type: "annotation", displayMode: "collapsed", reader: new CytobandReader({}) } ] }