Selecting all objects with specific value from GeoJSON in new variable

Question

So I have a GeoJSON that I want to pull/filer/select all kitespots with a wind direction that includes "S" in Leaflet. (..and yes, am a beginner at coding..)
var kitespots = {
  "type": "FeatureCollection",
  "features":[    
    {
     "type": "Feature",
     "properties": {"name": "STUBBSAND", "windDirection": "S/SW"},
     "geometry": {
       "type": "Point",
       "coordinates": [18.959655761718746, 63.220636696106915]}
    },
    {
     "type": "Feature",
     "properties": {"name": "GULLVIK", "windDirection": "SW/S/SE"},
     "geometry": {
       "type": "Point",
       "coordinates": [ 18.82786273956299, 63.214932082292805]}
    },
    {
     "type": "Feature",
     "properties": {"name": "VANNVIKEN", "windDirection": "SE/S"},
     "geometry": {
       "type": "Point",
       "coordinates": [18.815631866455078, 63.20794965080512]}
    },
    {
     "type": "Feature",
     "properties": {"name": "STORSANDEN", "windDirection": "N/NW"},
     "geometry": {
       "type": "Point",
       "coordinates": [18.528785705566406, 62.99013053076868]}
    }
  ]
}

So far I have tried;
// - Select all kitespots facing south - //
var southSpotCluster = new L.markerClusterGroup();
var windDirectionS = L.geoJson(kitespots,{windDirection: "S"});
southSpotCluster.addLayer(windDirectionS);

// - ...and.. - //

var southSpotCluster = L.geoJson(kitespots)
var windDirection = ["S"]
for(var windDirection in southSpotCluster){
    windDirection.push(southSpotCluster["S"]);
}
southSpotCluster.addLayer(windDirection);

var overLays = {
    'All Kitespots': allSpotsCluster,
    'Kitespots S': southSpotCluster
};

L.control.layers(baseLayers, overLays).addTo(map);

Timothy Dalton · Answer

This is probably more of a question for stackoverflow directly, however if you are able to use ES6, you could use something like this:
const filteredFeatures = kitespots.features.filter(item => { 
    return item.properties.windDirection.split("/").includes("S");
});

The filter function will return merely those items which include S. Splitting of the string beforehand will make sure it won't match something like SW in case S alone doesn't exist.

Deniz · Answer

There is a practical library you can use to filter Geojson data, https://github.com/digidem/feature-filter-geojson.
With this library you can create mapbox-style-definition like filters.
import createFilter from "feature-filter-geojson";

function geojson_filter(geojson, filter_exp) {
  let features = geojson.features;
  geojson.features = [];
  let geojson_clon = Object.assign({}, geojson);
  geojson.features = features;

let filter = createFilter(filter_exp);
  geojson_clon .features = filter(geojson.features, filter);    
  return geojson_clon ;
}

var ffilter = ["all",["==", "windDirection", "S"],];

var filtered_geojson = geojson_filter(kitespots , ffilter);

Selecting all objects with specific value from GeoJSON in new variable

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