title: How to extract values from geojson efficiently?
body: <p>I have a <code>geojson</code> file of the following structure:</p>
  
  <pre><code>var areas = {
  "type": "FeatureCollection",                                                                                
  "features": [
  { "type": "Feature", "id": 0, "properties": { "name": "a", "count": "854" }, "geometry": { "type": "Polygon", "coordinates": [ [ [ 13.271687552328165, 29.359549285088008 ], [ 13.272222904657671, 29.357697459147403 ], [ 13.272586765837973, 29.35566049412985 ], [ 13.273062097784726, 29.354438105832578 ], [ 13.272652418199639, 29.360795108476978 ], [ 13.271320041078822, 29.360700647951568 ], [ 13.271687552328165, 29.359549285088008 ] ] ] } }
  ,
  { "type": "Feature", "id": 1, "properties": { "name": "b", "count": "254"}, "geometry": { "type": "Polygon", "coordinates": [ [ [ 13.277038163109875, 29.358442424220023 ], [ 13.276782949188294, 29.358122923383512 ], [ 13.275290999273452, 29.358508600578681 ], [ 13.274634727185679, 29.358485484466968 ], [ 13.282581930993208, 29.358635779719847 ], [ 13.278334024184868, 29.359814295404375 ], [ 13.277038163109875, 29.358442424220023 ] ] ] } }
  ,
  { "type": "Feature", "id": 2, "properties": {"name": "c", "count": "385"}, "geometry": { "type": "Polygon", "coordinates": [ [ [ 13.279484097462499, 29.349831113628788 ], [ 13.27792879702741, 29.349728966688758 ], [ 13.276089401418951, 29.349901260351807 ], [ 13.275677565886326, 29.349951087700632 ], [ 13.279484097462499, 29.349831113628788 ] ] ] } }
  ,
  { "type": "Feature", "id": 3, "properties": { "name": "d", "count": "243"}, "geometry": { "type": "Polygon", "coordinates": [ [ [ 13.290520299215724, 29.352306689134622 ], [ 13.289722088408338, 29.351802774685929 ], [ 13.289065241087885, 29.352101541350635 ], [ 13.28785814146197, 29.351114667998772 ],  [ 13.290520299215724, 29.352306689134622 ] ] ] } }
  
  ]
  }
  </code></pre>
  
  <p>Normally, to convert this data into an array I would create a loop to run through the appropriate variable storing this value in an array. However, I don't know if this is the best approach. Is there a way of pulling out all of the values for <code>id</code> into an array so that I would end up with:</p>
  
  <pre><code>[0,1,2,3]
  </code></pre>
  
  <p>I'm happy to use any external library to do this either - I'm mainly curious if there is a more efficient way than my current approach.</p>