Downloading features

After uploading data and editing it, we want to let our users download the result. To do this, we'll serialize our feature data as GeoJSON and create an <a> element with a download attribute that triggers the browser's file save dialog. At the same time, we'll add a button to the map that let's users clear existing features and start over.

First, we need a bit of markup to represent the buttons. Add the following elements below the map-container in your index.html:

<div id="tools">
  <a id="clear">Clear</a>
  <a id="download" download="features.json">Download</a>
</div>

Now we need some CSS to make the buttons look right. Add something like this to the <style> element in index.html:

#tools {
  position: absolute;
  top: 1rem;
  right: 1rem;
}
#tools a {
  display: inline-block;
  padding: 0.5rem;
  background: white;
  cursor: pointer;
}

Clearing features is the easier part, so we'll do that first. The vector source has a source.clear() method. We want clicks on the "Clear" button to call that method, so we'll add a listener for click in our main.js:

const clear = document.getElementById('clear');
clear.addEventListener('click', function() {
  source.clear();
});

To serialize our feature data for download, we'll use a GeoJSON format. Since we want the "Download" button to work at any time during editing, we'll serialize features on every change event from the source and construct a data URI for the anchor element's href attribute:

const format = new GeoJSON({featureProjection: 'EPSG:3857'});
const download = document.getElementById('download');
source.on('change', function() {
  const features = source.getFeatures();
  const json = format.writeFeatures(features);
  download.href = 'data:text/json;charset=utf-8,' + json;
});
Buttons to clear and download data
Buttons to clear and download data

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