Extract coordinates from GeoJSON file in OpenLayers

Question

I'm trying to extract coordinates from a GeoJSON file . I tried this line of code but I didn't have result in the console.

<script type="text/javascript">
    var vectorLayer = new ol.layer.Vector({
        source: new ol.source.Vector({
            url: '/area.geojson',
            format: new ol.format.GeoJSON()
        })
    });

    function getCoordinatesOfCounties() {
        var features = vectorLayer.getSource().getFeatures();
        features.forEach((feature) => {
            var coordinates = feature.getGeometry().getCoordinates();
            console.log(coordinates);
        });
    }
      
    </script> 

What's wrong?

Answer 1

You have to wait till features are loaded into the source, then you can get coordinates of the features. You can use vector source event featuresloadend for that (see https://openlayers.org/en/latest/apidoc/module-ol_source_Vector-VectorSource.html and https://openlayers.org/en/latest/apidoc/module-ol_source_Vector.VectorSourceEvent.html).

Event featuresloadend is fired when all the features are loaded into the source. Property evt.features of event processing function parameter evt contains all the loaded features. For this to work you have to add vector layer to the map.

Code could the look something like this:

function getCoordinatesOfCounties(evt) {
  var features = evt.features;
  features.forEach((feature) => {
      var coordinates = feature.getGeometry().getCoordinates();
      console.log(coordinates);
  });
}

var vectorSource = new ol.source.Vector({
  url: '/area.geojson',
  format: new ol.format.GeoJSON()
});
vectorSource.on('featuresloadend', getCoordinatesOfCounties);

var vectorLayer = new ol.layer.Vector({
  source: vectorSource;
});
map.addLyer(vectorLayer);

If you don't want to add layer to the map and just read features, you can use JS fetch to read data and ol.format.GeoJSON().readFeatures to read the features:

function getCoordinatesOfCounties(features) {
  features.forEach((feature) => {
      var coordinates = feature.getGeometry().getCoordinates();
      console.log('coords', coordinates);
  });
}

function readFeatures(data) {
  var features = new ol.format.GeoJSON().readFeatures(data, {
    dataProjection:'EPSG:4326',
    featureProjection: 'EPSG:3857'
  });
  getCoordinatesOfCounties(features);
}

fetch('/area.geojson')
  .then(response => response.json())
  .then(data => {readFeatures(data)});