用turf.js求多个点的外包多边形

// 通过geojson获取features
function getFeatures(geojson) {
    let features = (new ol.format.GeoJSON()).readFeatures(geojson);
    features.forEach(f => {
      f.setGeometry(f.getGeometry().transform('EPSG:4326', "EPSG:3857"))
    })
    return features
}
// turf.convex
 fetch('./data/college.geojson').then(res => res.json()).then(res => {
    let features = [...getFeatures(res)]
    res = turf.convex(res);
    features = [...features, ...getFeatures(res)]
    vectorSource.addFeatures(features)
  })

// turf.tin
 fetch('./data/college.geojson').then(res => res.json()).then(res => {
    let features = [...getFeatures(res)]
    res = turf.tin(res)
    res = res.features
    let _res = res.shift()
    while (res.length > 0) {
      const r = res.shift()
      _res = turf.union(_res, r)
    }
    features = [...features, ...getFeatures(_res)]
    vectorSource.addFeatures(features)
  })