Fit the map to the boundaries of all features in the feature layer by querying the feature service. To learn more about feature layers, go to the Data hosting guide.
<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <title>Zoom to all features</title>
    <meta name="viewport" content="initial-scale=1, maximum-scale=1, user-scalable=no">
    <link rel="stylesheet" href="https://unpkg.com/leaflet@1.9.4/dist/leaflet.css" crossorigin="">
    <script src="https://unpkg.com/leaflet@1.9.4/dist/leaflet.js" crossorigin=""></script>
    <!-- Load Esri Leaflet from CDN -->
    <script src="https://unpkg.com/esri-leaflet@3.0.19/dist/esri-leaflet.js"></script>
    <!-- Load Esri Leaflet Vector from CDN -->
    <script src="https://unpkg.com/esri-leaflet-vector@4.3.2/dist/esri-leaflet-vector.js" crossorigin=""></script>
    <style>
      html,
      body,
      #map {
        padding: 0;
        margin: 0;
        height: 100%;
        width: 100%;
        font-family: Arial, Helvetica, sans-serif;
        font-size: 14px;
        color: #323232;
      }
    </style>
  </head>
  <body>
    <div id="map"></div>
    <script>
      const accessToken = "YOUR_ACCESS_TOKEN";
      const map = L.map("map").setView([40.34, -76.34], 5);
      L.esri.Vector.vectorBasemapLayer("arcgis/dark-gray/base", {
        token: accessToken
      }).addTo(map);
      const trails = L.esri
        .featureLayer({
          url: "https://services3.arcgis.com/GVgbJbqm8hXASVYi/arcgis/rest/services/Trailheads_Styled/FeatureServer/0"
        })
        .addTo(map);
      trails.query().bounds(function (error, latlngbounds) {
        if (error) {
          console.log("Error running \"Query\" operation: " + error);
        }
        map.fitBounds(latlngbounds);
      });
    </script>
  </body>
</html>