The avg_temp field for the Wyoming feature is null in the shapefile. When converted to geojson via shp2json, this value becomes 0.
npm start
This will output us-states.geojson which will have a value of 0 for Wyoming's avg_temp, instead of null.
{
"type": "Feature",
"properties": {
"name": "Wyoming",
"fid": "WY",
"avg_temp": 0
}
}