Count ocurrences using Mapbox GL JS (geojson properties)

Question

I need a way to count the same property for each feature of a geojson file and obtain an array like this: array=["type 1", "type 2","type 3","type 2","type 1","type 2","type 1","type 3","type 1","type 1",....]

I'm loading a large geojson feature collection from a file.

Answer 1

This is not really a mapbox-gl problem. Your GeoJson is just standard JavaScript object, the features is standard Array:

const counts = new Map();

for (const feature of geojson.feature) {
  const alert = feature.properties.alert;

  if (!alert) {
    continue;
  }

  if (!counts.has(alert)) {
    counts.set(alert, 0);
  }

  const currentCount = counts.get(alert);
  counts.set(alert, currentCount + 1);
}

// counts will look like this
Map(
  "type1" -> 10,
  "type2" -> 8,
  "type3" -> ...
)

Or even more concise:

const counts = geojson.features.reduce((accumulatedCounts, feature) => {
  const alert = feature.properties.alert;

  if (!alert) return accumulatedCounts;
  if (!accumulatedCounts[alert]) accumulatedCounts[alert] = 0;

  accumulatedCounts[alert]++;

  return accumulatedCounts
}, {});