--- name: web-maps-leaflet description: Leaflet interactive maps - map setup, tile layers, markers, popups, GeoJSON, custom controls, plugins, clustering, events --- # Leaflet Interactive Map Patterns > **Quick Guide:** Use Leaflet (v1.9.4) for lightweight interactive maps. `L.map` for initialization, `L.tileLayer` for base maps, `L.marker`/`L.popup` for points of interest, `L.geoJSON` for vector data with `onEachFeature`/`pointToLayer`/`style`/`filter` callbacks. Always include tile layer attribution. Always clean up maps with `map.remove()` on teardown. Use `L.markerClusterGroup` for 100+ markers. Use `@types/leaflet` for TypeScript support. --- ## CRITICAL: Before Using This Skill > **All code must follow project conventions in CLAUDE.md** (kebab-case, named exports, import ordering, `import type`, named constants) **(You MUST call `map.remove()` when tearing down a map instance -- prevents memory leaks and orphaned event listeners)** **(You MUST include attribution on tile layers -- most tile providers require it legally)** **(You MUST use `L.markerClusterGroup` or canvas rendering for 100+ markers -- DOM markers do not scale)** **(You MUST use named constants for coordinates, zoom levels, and style values -- NO magic numbers)** --- **Auto-detection:** Leaflet, L.map, L.tileLayer, L.marker, L.popup, L.geoJSON, L.control, L.layerGroup, L.featureGroup, L.icon, L.divIcon, L.circleMarker, L.polyline, L.polygon, L.circle, markerClusterGroup, leaflet.markercluster, @types/leaflet, leaflet.css, addTo(map), bindPopup, onEachFeature, pointToLayer, flyTo, fitBounds **When to use:** - Rendering interactive maps with markers, popups, and overlays - Displaying GeoJSON data (points, lines, polygons) on a map - Building maps with layer switching (base layers, overlays) - Creating custom map controls and interactions - Handling large marker datasets with clustering **When NOT to use:** - 3D globe or terrain visualization (consider a WebGL-based mapping library) - Real-time collaborative map editing (consider a specialized collaborative mapping tool) - Vector tiles or client-side styling of map tiles (Leaflet renders raster tiles natively; vector tile support requires plugins) **Key patterns covered:** - Map initialization with tile layers and attribution - Markers, popups, tooltips, and custom icons - GeoJSON layers with `onEachFeature`, `pointToLayer`, `style`, `filter` - Layer groups, feature groups, and layer control - Custom controls via `L.Control.extend` - Marker clustering with `L.markerClusterGroup` - Events and interactive behavior - TypeScript setup with `@types/leaflet` - Performance strategies for large datasets --- **Detailed Resources:** - [examples/core.md](examples/core.md) - Map setup, tile layers, markers, popups, GeoJSON, layer control, events - [examples/advanced.md](examples/advanced.md) - Custom controls, clustering, performance, canvas rendering, TypeScript - [reference.md](reference.md) - Decision frameworks, API quick reference, anti-patterns --- ## Philosophy Leaflet is a lightweight (~42KB gzipped) open-source library for mobile-friendly interactive maps. It provides a small, well-designed API covering the essentials, with a rich plugin ecosystem for everything else. **Core principles:** 1. **Simplicity first** -- The core API covers 95% of map use cases. Plugins extend the rest. 2. **Layer-based architecture** -- Everything on the map is a layer (tiles, markers, GeoJSON, controls). Layers are added/removed independently. 3. **Method chaining** -- Most methods return `this`, enabling fluent builder-style setup. 4. **Event-driven interaction** -- Maps, markers, and layers emit events (`click`, `moveend`, `zoomend`). Subscribe with `.on()`. 5. **Mobile-first** -- Touch interactions, pinch zoom, and retina tile support are built in. **When to use Leaflet:** - Standard 2D web maps with markers, popups, and overlays - GeoJSON visualization and interaction - Projects needing a small bundle size - Maps with up to ~10K markers (with clustering) **When NOT to use Leaflet:** - Maps requiring WebGL rendering for 100K+ features (consider a GL-based library) - 3D visualization or globe projection - Client-side vector tile styling (requires plugins or a different library) --- ## Core Patterns ### Pattern 1: Map Initialization and Tile Layers Create a map targeting a DOM element, set the view, and add a tile layer with attribution. ```typescript import L from "leaflet"; import "leaflet/dist/leaflet.css"; const INITIAL_CENTER: L.LatLngExpression = [51.505, -0.09]; const INITIAL_ZOOM = 13; const MAX_ZOOM = 19; const map = L.map("map").setView(INITIAL_CENTER, INITIAL_ZOOM); L.tileLayer("https://tile.openstreetmap.org/{z}/{x}/{y}.png", { maxZoom: MAX_ZOOM, attribution: '© OpenStreetMap contributors', }).addTo(map); ``` **Why good:** Named constants for coordinates and zoom, attribution included (legally required by most providers), CSS import ensures controls render correctly ```typescript // Bad -- magic numbers, missing attribution, missing CSS import const map = L.map("map").setView([51.505, -0.09], 13); L.tileLayer("https://tile.openstreetmap.org/{z}/{x}/{y}.png").addTo(map); ``` **Why bad:** Magic numbers for coordinates and zoom, missing attribution violates tile provider terms, missing CSS import causes broken control rendering See [examples/core.md](examples/core.md) Pattern 1 for tile provider options and `invalidateSize` usage. --- ### Pattern 2: Markers, Popups, and Tooltips Markers pin locations on the map. Bind popups (click-to-open) or tooltips (hover) for additional information. ```typescript const MARKER_POSITION: L.LatLngExpression = [51.5, -0.09]; const marker = L.marker(MARKER_POSITION).addTo(map); marker.bindPopup("Hello
I am a popup."); marker.bindTooltip("Hover text", { permanent: false, direction: "top" }); ``` Standalone popups (not attached to a marker): ```typescript L.popup().setLatLng([51.513, -0.09]).setContent("Standalone popup").openOn(map); ``` **Gotcha:** `openOn(map)` closes any previously open popup. Use `addTo(map)` if multiple popups should be open simultaneously. See [examples/core.md](examples/core.md) Pattern 2 for custom icons, `L.divIcon`, and icon class extension. --- ### Pattern 3: GeoJSON Layers `L.geoJSON` renders GeoJSON data with powerful callbacks for styling, filtering, and interaction. ```typescript const CIRCLE_MARKER_RADIUS = 8; const CIRCLE_MARKER_FILL_COLOR = "#ff7800"; const CIRCLE_MARKER_WEIGHT = 1; const geoLayer = L.geoJSON(geojsonData, { pointToLayer: (feature, latlng) => L.circleMarker(latlng, { radius: CIRCLE_MARKER_RADIUS, fillColor: CIRCLE_MARKER_FILL_COLOR, weight: CIRCLE_MARKER_WEIGHT, fillOpacity: 0.8, }), onEachFeature: (feature, layer) => { if (feature.properties?.name) { layer.bindPopup(feature.properties.name); } }, style: (feature) => ({ color: feature?.properties?.color ?? "#3388ff", weight: 2, }), filter: (feature) => feature?.properties?.visible !== false, }).addTo(map); ``` **Why good:** `pointToLayer` customizes point rendering, `onEachFeature` binds interactivity, `style` enables data-driven visualization, `filter` excludes features declaratively See [examples/core.md](examples/core.md) Pattern 3 for dynamic style updates, `addData`, and `resetStyle`. --- ### Pattern 4: Layer Groups and Layer Control Group layers logically and provide a UI for toggling visibility. ```typescript const cities = L.layerGroup([markerA, markerB]); const parks = L.layerGroup([polygonA, polygonB]); const baseLayers = { OpenStreetMap: osmTileLayer, Satellite: satelliteTileLayer, }; const overlays = { Cities: cities, Parks: parks, }; L.control.layers(baseLayers, overlays).addTo(map); ``` **Key distinction:** Base layers are radio buttons (one active at a time). Overlays are checkboxes (multiple can be active). Use `L.featureGroup` instead of `L.layerGroup` when you need `getBounds()` or `bindPopup` on the group. See [examples/core.md](examples/core.md) Pattern 4 for programmatic layer toggling and feature group bounds. --- ### Pattern 5: Events and Interaction Leaflet uses `.on()` for event binding. Maps, markers, and layers all support events. ```typescript map.on("click", (e: L.LeafletMouseEvent) => { const { lat, lng } = e.latlng; L.popup() .setLatLng(e.latlng) .setContent(`Clicked at ${lat.toFixed(5)}, ${lng.toFixed(5)}`) .openOn(map); }); map.on("zoomend", () => { console.log("Zoom level:", map.getZoom()); }); marker.on("dragend", (e: L.DragEndEvent) => { const pos = (e.target as L.Marker).getLatLng(); console.log("Marker moved to:", pos); }); ``` Remove listeners with `.off()`: ```typescript const handler = () => { /* ... */ }; map.on("moveend", handler); map.off("moveend", handler); // cleanup ``` See [examples/core.md](examples/core.md) Pattern 5 for common event types reference. --- ### Pattern 6: Custom Controls Extend `L.Control` to build custom map controls. ```typescript const InfoControl = L.Control.extend({ options: { position: "bottomleft" as L.ControlPosition }, onAdd(_map: L.Map): HTMLElement { const container = L.DomUtil.create("div", "info-control"); container.innerHTML = "

Map Info

"; L.DomEvent.disableClickPropagation(container); return container; }, onRemove(_map: L.Map): void { // cleanup event listeners if needed }, }); new InfoControl().addTo(map); ``` **Key rule:** Call `L.DomEvent.disableClickPropagation(container)` on interactive controls to prevent map clicks from firing through the control. See [examples/advanced.md](examples/advanced.md) Pattern 1 for interactive controls with buttons and update methods. --- ### Pattern 7: Marker Clustering Use `leaflet.markercluster` plugin for large marker datasets. Without clustering, 1000+ DOM markers will degrade performance severely. ```typescript import "leaflet.markercluster"; import "leaflet.markercluster/dist/MarkerCluster.css"; import "leaflet.markercluster/dist/MarkerCluster.Default.css"; const MAX_CLUSTER_RADIUS = 50; const DISABLE_CLUSTERING_ZOOM = 18; const clusterGroup = L.markerClusterGroup({ maxClusterRadius: MAX_CLUSTER_RADIUS, disableClusteringAtZoom: DISABLE_CLUSTERING_ZOOM, chunkedLoading: true, showCoverageOnHover: false, }); markers.forEach((m) => clusterGroup.addLayer(m)); map.addLayer(clusterGroup); ``` **Why good:** `chunkedLoading` prevents UI freeze for bulk additions, `disableClusteringAtZoom` shows individual markers at close zoom, `maxClusterRadius` controls granularity See [examples/advanced.md](examples/advanced.md) Pattern 2 for custom cluster icons and `refreshClusters`. --- ### Pattern 8: TypeScript Setup Install `leaflet` and `@types/leaflet` for full type safety. Leaflet types cover all classes, options, and events. ```typescript import L, { type LatLngExpression, type MapOptions, type TileLayerOptions, } from "leaflet"; const options: MapOptions = { center: [51.505, -0.09], zoom: 13, zoomControl: true, }; const map = L.map("map", options); ``` **Install:** `npm install leaflet` + `npm install -D @types/leaflet` For markercluster types: `npm install -D @types/leaflet.markercluster` See [examples/advanced.md](examples/advanced.md) Pattern 3 for typed GeoJSON features and event handlers. --- ### Pattern 9: Map Cleanup Always remove map instances on component teardown to prevent memory leaks and orphaned event listeners. ```typescript // Vanilla JS / framework-agnostic cleanup function destroyMap(map: L.Map): void { map.off(); // remove all event listeners map.remove(); // destroy the map instance, clean up DOM } ``` **Why this matters:** Leaflet attaches resize observers, animation frames, and event listeners to the DOM. Calling `map.remove()` is the single most important cleanup action -- it handles all internal teardown. **Gotcha:** If you re-initialize a map on the same DOM element without calling `remove()` first, Leaflet throws "Map container is already initialized." --- ## Performance Optimization ### Marker Count Thresholds | Marker Count | Strategy | Notes | | ------------ | --------------------------------------------- | ---------------------------------- | | < 100 | Standard `L.marker` | DOM markers are fine | | 100 - 10K | `L.markerClusterGroup` | Clusters reduce DOM nodes | | 10K - 50K | `L.markerClusterGroup` + `chunkedLoading` | Batch additions to avoid UI freeze | | 50K+ | Canvas-based rendering plugin or vector tiles | DOM/SVG cannot handle this volume | ### GeoJSON Performance Tips - Use `filter` option to exclude features before rendering (cheaper than rendering then hiding) - Simplify geometry server-side for overview zoom levels (reduce coordinate precision) - Add GeoJSON data in chunks using `addData()` for progressive rendering - Call `clearLayers()` and re-add instead of updating individual feature styles when dataset changes completely ### General Tips - Use `map.invalidateSize()` after container resize (CSS transitions, accordion expand) - Set `preferCanvas: true` in map options for vector layers (circles, polylines) to render on canvas instead of SVG - Use `L.circleMarker` instead of `L.marker` for data points -- renders on the vector renderer and is lighter than DOM markers - Avoid attaching large HTML to popups -- use `setContent()` lazily on popup open event --- ## Decision Framework ### Choosing a Marker Strategy ``` How many markers? | +-> < 100 -> Standard L.marker with L.icon or L.divIcon | +-> 100 - 10K -> L.markerClusterGroup (leaflet.markercluster plugin) | +-> 10K - 50K -> L.markerClusterGroup with chunkedLoading + consider L.circleMarker | +-> 50K+ -> Canvas rendering plugin or switch to vector tiles ``` ### Choosing a Layer Type ``` What data are you displaying? | +-> Single coordinate point -> L.marker (with icon) or L.circleMarker (data viz) | +-> Line path -> L.polyline | +-> Area boundary -> L.polygon or L.circle | +-> GeoJSON dataset -> L.geoJSON (handles all geometry types) | +-> Grouped items needing toggle -> L.layerGroup (no shared popup) or L.featureGroup (shared popup/bounds) ``` ### Choosing an Icon ``` What does the marker represent? | +-> Location pin (default) -> L.marker() with no icon option (uses default blue pin) | +-> Custom image -> L.icon({ iconUrl, iconSize, iconAnchor }) | +-> HTML/CSS content -> L.divIcon({ html, className, iconSize }) | +-> Data point (many) -> L.circleMarker (renders on vector layer, not DOM) ``` --- ## RED FLAGS **High Priority Issues:** - Missing `map.remove()` on teardown -- causes memory leaks, orphaned DOM listeners, and "container already initialized" errors on re-mount - Missing tile layer attribution -- violates terms of service for OpenStreetMap and most tile providers - Using standard `L.marker` for 100+ points -- DOM markers cause severe performance degradation; use clustering or canvas - Magic numbers for coordinates, zoom levels, or style values -- use named constants **Medium Priority Issues:** - Not calling `map.invalidateSize()` after container resize -- map renders incorrectly (grey tiles, offset clicks) - Not importing `leaflet.css` -- controls, popups, and markers render without proper styling - Using `openOn(map)` when multiple popups should be visible simultaneously -- it auto-closes the previous popup; use `addTo(map)` instead - Not disabling click propagation on custom controls -- clicks on control elements trigger map click events **Gotchas & Edge Cases:** - **Default icon path issue:** Leaflet's default marker icon references images relative to the CSS file. With bundlers, the path breaks. Fix by importing and setting `L.Icon.Default.prototype.options` or using `L.divIcon`/`L.icon` explicitly. - `L.geoJSON` expects coordinates in `[longitude, latitude]` order (GeoJSON spec), but `L.latLng` uses `[latitude, longitude]` -- mixing them up is the most common coordinate bug - `flyTo` and `panTo` cancel each other if called in rapid succession -- debounce or guard against concurrent calls - `map.fitBounds` on an empty `FeatureGroup` throws an error -- check `getBounds().isValid()` first - `L.Control.extend` uses Leaflet's class system, not ES6 classes -- `new L.Control.extend({...})` returns a constructor, not an instance - Tile layer `maxZoom` vs map `maxZoom`: tile layer's `maxZoom` limits tile availability; map's `maxZoom` limits user zoom. If map allows zoom 20 but tiles only go to 18, you see grey tiles. - `L.markerClusterGroup.refreshClusters()` must be called after changing marker icons or data -- clusters do not auto-update --- ## CRITICAL REMINDERS > **All code must follow project conventions in CLAUDE.md** **(You MUST call `map.remove()` when tearing down a map instance -- prevents memory leaks and orphaned event listeners)** **(You MUST include attribution on tile layers -- most tile providers require it legally)** **(You MUST use `L.markerClusterGroup` or canvas rendering for 100+ markers -- DOM markers do not scale)** **(You MUST use named constants for coordinates, zoom levels, and style values -- NO magic numbers)** **Failure to follow these rules will cause memory leaks, legal violations, and performance degradation.**