How to determine number of features in GeoJSON file using Python or JavaScript?

If you have GeoJson that looks like that on the wikipedia page

var json={
  "type": "FeatureCollection",
  "features": [
   {
    "type": "Feature",
    "geometry": {
      "type": "Point",
      "coordinates": [102.0, 0.6]
   },
    "properties": {
      "prop0": "value0"
    }
  },
 {
   "type": "Feature",
   "geometry": {
      "type": "LineString",
      "coordinates": [
         [102.0, 0.0], [103.0, 1.0], [104.0, 0.0], [105.0, 1.0]
      ]
  },
    "properties": {
      "prop1": 0.0,
      "prop0": "value0"
    }
  },
  {
    "type": "Feature",
    "geometry": {
      "type": "Polygon",
        "coordinates": [
          [
            [100.0, 0.0], [101.0, 0.0], [101.0, 1.0], [100.0, 1.0],
            [100.0, 0.0]
          ]
        ]
   },
    "properties": {
      "prop1": {
        "this": "that"
     },
      "prop0": "value0"
    }
   }
  ]
}

then all you need to do is:

json.features.length

as this is a native Javascript object and so the embedded feature array has a length property, which in this case is 3.

If you want to do this in Python, you can use json.loads(your_json) see https://docs.python.org/2/library/json.html, which will parse the json into dictionaries and lists, so you can again get the value of the length of the list containing the features.