• geojson.io 位置情報の編集に
• epsg.io 好きな空間参照系の変換のときにproj4.jsと組み合わせて使う

import Map from 'ol/Map.js';
import View from 'ol/View.js';
import Tile from 'ol/source/Tile.js'
import OSM from 'ol/source/OSM.js'
import proj from 'ol/proj.js'

let feature = new Feature({
  geometry: new Point()
})
let map = new ol.Map({
  target: 'map',
  layers: [
    new Tile({
        source: new OSM()
    })
  ],
  view: new View({
      center: proj.fromLonLat([139.745433,35.658581]),
      zoom: 16
  })
})

import format from 'ol/format'

const features = (new format.GeoJSON()).readFeatures(loadGeoJSONFileForTest())
for (const f of features) {
  const geom = f.getGeometry()
  if (!geom) continue
  geom.transform(/* from */ 'EPSG:4326', /* to */'EPSG:3857')
}

ちなみにある座標値を変換したいときはol.proj.transform(coordinate, from, to)を使うといい

import proj from 'ol/proj'
const coordinate = [123.123, 123.123]
proj.transform(coordinate, /* from */ 'EPSG:4326', /* to */'EPSG:3857')

import DragAndDrop from 'ol/interaction/DragAndDrop'
import source from 'ol/source'
import layer from 'ol/layer'
import format from 'ol/format'

function initDragAndDrop(map) {
  const source = new source.Vector()
  const layer = new layer.Vector({
    source: source
  })
  map.addLayer(layer)
  map.addInteraction(new DragAndDrop({
    source: source,
    formatConstructors: [format.GeoJSON]
  }))
}