Imported from agents-inc/skills (src/skills/web-maps-mapbox/SKILL.md). Install upstream with npx skills add agents-inc/skills --skill web-maps-mapbox. Copyright stays with the author.

Mapbox GL JS Patterns

Quick Guide: Use Mapbox GL JS v3 for interactive vector maps. Initialize with new mapboxgl.Map(), add data via sources (GeoJSON, vector), visualize with layers (fill, line, circle, symbol, fill-extrusion, heatmap), style dynamically with expressions. Use the Standard style as the default base with slots (bottom, middle, top) for layer placement. Enable clustering on GeoJSON sources for large point datasets. Use setTerrain + setFog for 3D terrain. Types are included in the mapbox-gl package (no @types/mapbox-gl needed).

<critical_requirements>

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 add sources before layers that reference them -- adding a layer without its source throws a runtime error)

(You MUST listen for load or style.load before calling addSource/addLayer -- the style is not ready on construction)

(You MUST clean up map instances with map.remove() on unmount -- leaks GPU memory and event listeners)

(You MUST use named constants for coordinates, zoom levels, and style values -- NO magic numbers)

(You MUST use expressions for data-driven styling instead of iterating features and setting styles individually)

</critical_requirements>

Auto-detection: Mapbox, mapbox-gl, mapboxgl, Map, Marker, Popup, NavigationControl, GeolocateControl, addSource, addLayer, GeoJSON source, vector source, expressions, flyTo, easeTo, fitBounds, setTerrain, setFog, fill-extrusion, clustering, slot, Standard style, mapbox-gl-geocoder, mapbox-gl-directions, mapbox-gl-draw

When to use:

• Rendering interactive vector tile maps with custom styling
• Displaying point/line/polygon data on a map with data-driven styling
• Building map-based UIs with markers, popups, and custom controls
• Visualizing large datasets with clustering, heatmaps, or 3D extrusions
• Adding geocoding search, routing directions, or drawing tools
• Creating 3D terrain visualizations with elevation data

When NOT to use:

• Static map images without interactivity (use Mapbox Static Images API)
• Simple embedded maps without custom data (a basic iframe embed suffices)
• Applications requiring offline-only maps without a Mapbox access token

Key patterns covered:

• Map initialization with Standard style and access token
• Markers, popups, and built-in controls
• Source/layer model (GeoJSON, vector, raster-dem)
• Expression-based data-driven styling
• Clustering with automatic expansion on click
• 3D terrain, fog, and fill-extrusion buildings
• Camera animation (flyTo, easeTo, fitBounds)
• Event handling (click, mouseenter, mouseleave on layers)
• v3 slot system and Standard style configuration

Detailed Resources:

• examples/core.md - Map setup, markers, popups, controls, events, camera animation
• examples/layers.md - Sources, layers, expressions, clustering, data-driven styling
• examples/interaction.md - 3D terrain, fog, fill-extrusion, drawing, geocoding, directions
• reference.md - Decision frameworks, layer types, expression operators, anti-patterns

Philosophy

Mapbox GL JS renders vector tiles on the GPU using WebGL 2, enabling smooth 60fps map interactions with large datasets. The core mental model is sources + layers + expressions:

1. Sources hold the data (GeoJSON, vector tiles, raster tiles, images)
2. Layers define how to visualize sources (fill, line, circle, symbol, fill-extrusion, heatmap, raster)
3. Expressions make layers data-driven (color by property, size by zoom, filter by attribute)

This separation means one source can power multiple layers (e.g., same GeoJSON rendered as both a fill layer and a line layer for borders), and layers can be styled entirely through expressions without touching the data.

v3 Standard style: The default style is mapbox://styles/mapbox/standard, which includes 3D buildings, terrain-aware rendering, and a slot system (bottom, middle, top) for inserting custom layers at predetermined positions in the visual stack. Use setConfigProperty to customize the Standard style's appearance without replacing it.

TypeScript: Types are bundled with mapbox-gl since v3 -- do not install @types/mapbox-gl.

Core Patterns

Pattern 1: Map Initialization

Initialize with container, style, center, zoom. Always wait for load event before adding sources/layers.

import mapboxgl from "mapbox-gl";
import "mapbox-gl/dist/mapbox-gl.css";

const DEFAULT_CENTER: [number, number] = [-74.006, 40.7128]; // [lng, lat]
const DEFAULT_ZOOM = 12;

mapboxgl.accessToken = process.env.MAPBOX_ACCESS_TOKEN!;

const map = new mapboxgl.Map({
  container: "map", // HTML element ID or element reference
  style: "mapbox://styles/mapbox/standard",
  center: DEFAULT_CENTER,
  zoom: DEFAULT_ZOOM,
});

map.on("load", () => {
  // Safe to add sources and layers here
});

Why good: Named constants for coordinates/zoom, waits for load before data operations, uses Standard style

See examples/core.md Pattern 1 for cleanup patterns and bad examples.

Pattern 2: Markers, Popups, and Controls

Markers are DOM elements placed at coordinates. Popups display content on click. Controls add navigation UI.

const MARKER_COLOR = "#e74c3c";

const popup = new mapboxgl.Popup({ offset: 25, maxWidth: "300px" }).setHTML(
  "<h3>Location</h3><p>Description</p>",
);

new mapboxgl.Marker({ color: MARKER_COLOR })
  .setLngLat([-74.006, 40.7128])
  .setPopup(popup)
  .addTo(map);

map.addControl(new mapboxgl.NavigationControl(), "top-right");
map.addControl(
  new mapboxgl.GeolocateControl({ trackUserLocation: true }),
  "top-right",
);
map.addControl(new mapboxgl.ScaleControl({ unit: "metric" }), "bottom-left");

Why good: Popup bound to marker (opens on click automatically), controls positioned explicitly, named color constant

See examples/core.md Pattern 2 for custom marker elements and programmatic popup examples.

Pattern 3: Source and Layer Model

Add a GeoJSON source, then one or more layers that reference it. Sources and layers are independent -- one source can feed multiple layers.

map.on("load", () => {
  map.addSource("parks", {
    type: "geojson",
    data: {
      type: "FeatureCollection",
      features: [
        {
          type: "Feature",
          geometry: {
            type: "Polygon",
            coordinates: [
              /* ... */
            ],
          },
          properties: { name: "Central Park", area: 3.41 },
        },
      ],
    },
  });

  map.addLayer({
    id: "parks-fill",
    type: "fill",
    source: "parks",
    slot: "middle", // v3 Standard style slot
    paint: {
      "fill-color": "#2ecc71",
      "fill-opacity": 0.5,
    },
  });

  map.addLayer({
    id: "parks-outline",
    type: "line",
    source: "parks",
    slot: "middle",
    paint: {
      "line-color": "#27ae60",
      "line-width": 2,
    },
  });
});

Why good: Source defined once, two layers visualize it differently, slot: "middle" places layers correctly in Standard style

See examples/layers.md Pattern 1-2 for all source types and layer configuration.

Pattern 4: Data-Driven Styling with Expressions

Expressions are JSON arrays that style features based on their properties or zoom level.

map.addLayer({
  id: "population-circles",
  type: "circle",
  source: "cities",
  paint: {
    // Size by population
    "circle-radius": [
      "interpolate",
      ["linear"],
      ["get", "population"],
      10000,
      5,
      100000,
      15,
      1000000,
      30,
    ],
    // Color by category
    "circle-color": [
      "match",
      ["get", "type"],
      "capital",
      "#e74c3c",
      "major",
      "#3498db",
      "#95a5a6", // fallback
    ],
  },
});

Why good: Expressions handle all styling on the GPU -- no JavaScript loops over features, scales with any dataset size

See examples/layers.md Pattern 3-4 for expression operators and filter expressions.

Pattern 5: Clustering

Enable clustering on a GeoJSON source for large point datasets. Use three layers: cluster circles, count labels, unclustered points.

const CLUSTER_RADIUS = 50;
const CLUSTER_MAX_ZOOM = 14;

map.addSource("earthquakes", {
  type: "geojson",
  data: "/data/earthquakes.geojson",
  cluster: true,
  clusterMaxZoom: CLUSTER_MAX_ZOOM,
  clusterRadius: CLUSTER_RADIUS,
});

Why good: Clustering is handled entirely by the source -- no external library needed, automatic point_count property on clusters

See examples/layers.md Pattern 5 for complete cluster layers and click-to-expand interaction.

Pattern 6: Camera Animation

flyTo for dramatic transitions, easeTo for smooth pans, fitBounds for fitting data in view.

const FLY_ZOOM = 15;
const FLY_SPEED = 1.2;
const BOUNDS_PADDING_PX = 50;

map.flyTo({
  center: [-122.4194, 37.7749],
  zoom: FLY_ZOOM,
  speed: FLY_SPEED,
  essential: true, // not affected by prefers-reduced-motion
});

map.fitBounds(
  [
    [-122.5, 37.7],
    [-122.3, 37.8],
  ], // [sw, ne]
  { padding: BOUNDS_PADDING_PX },
);

Why good: essential: true ensures critical navigation animations still play even with reduced-motion preferences, padding keeps data away from edges

See examples/core.md Pattern 4 for easeTo, moveend listener, and bearing/pitch animation.

Pattern 7: Layer Event Handling

Listen for events on specific layers for interactive features (click popups, hover effects).

map.on("click", "parks-fill", (e) => {
  const feature = e.features?.[0];
  if (!feature) return;

  const coordinates = e.lngLat;
  const name = feature.properties?.name ?? "Unknown";

  new mapboxgl.Popup()
    .setLngLat(coordinates)
    .setHTML(`<strong>${name}</strong>`)
    .addTo(map);
});

// Cursor feedback on hover
map.on("mouseenter", "parks-fill", () => {
  map.getCanvas().style.cursor = "pointer";
});
map.on("mouseleave", "parks-fill", () => {
  map.getCanvas().style.cursor = "";
});

Why good: Events scoped to a specific layer (not the whole map), cursor change signals interactivity

See examples/core.md Pattern 5 for feature-state hover highlighting.

Pattern 8: 3D Terrain and Fog

Add elevation with a raster-dem source and atmospheric effects with fog.

const TERRAIN_EXAGGERATION = 1.5;
const TERRAIN_MAX_ZOOM = 14;
const TERRAIN_TILE_SIZE = 512;

map.on("style.load", () => {
  map.addSource("mapbox-dem", {
    type: "raster-dem",
    url: "mapbox://mapbox.mapbox-terrain-dem-v1",
    tileSize: TERRAIN_TILE_SIZE,
    maxzoom: TERRAIN_MAX_ZOOM,
  });

  map.setTerrain({ source: "mapbox-dem", exaggeration: TERRAIN_EXAGGERATION });

  map.setFog({
    range: [-1, 2],
    "horizon-blend": 0.3,
    color: "white",
    "high-color": "#add8e6",
    "space-color": "#d8f2ff",
    "star-intensity": 0.0,
  });
});

Why good: Named exaggeration constant, terrain source separate from visual layers, fog adds atmospheric depth

See examples/interaction.md Pattern 1-2 for fog presets and fill-extrusion 3D buildings.

Pattern 9: v3 Standard Style Configuration

Customize the Standard style's built-in appearance without replacing it.

// At initialization
const map = new mapboxgl.Map({
  container: "map",
  style: "mapbox://styles/mapbox/standard",
  config: {
    basemap: {
      lightPreset: "dusk",
      showPointOfInterestLabels: false,
    },
  },
});

// At runtime
map.setConfigProperty("basemap", "lightPreset", "night");
map.setConfigProperty("basemap", "showPlaceLabels", true);

Why good: Configuration API modifies the Standard style's built-in features without needing to understand its internal layer structure

<decision_framework>

Decision Framework

Choosing a Layer Type

What geometry are you displaying?
|
+-> Points?
|   +-> Few (<100) with custom HTML? -> Markers (DOM-based)
|   +-> Many or data-driven styling? -> circle layer or symbol layer
|   +-> Heatmap visualization? -> heatmap layer
|
+-> Lines/routes?
|   +-> line layer (width, color, dash patterns)
|
+-> Polygons?
|   +-> Flat colored areas? -> fill layer
|   +-> 3D extruded shapes? -> fill-extrusion layer
|
+-> Raster imagery?
    +-> raster layer (satellite, custom tiles)

Choosing a Source Type

Where is your data?
|
+-> Local/API GeoJSON? -> type: "geojson"
|   +-> Dynamic updates? -> Use map.getSource(id).setData(newData)
|   +-> Large point dataset? -> Enable cluster: true
|
+-> Mapbox tileset or third-party vector tiles? -> type: "vector"
|
+-> Elevation data? -> type: "raster-dem"
|
+-> Image overlay? -> type: "image" (with coordinates bounds)

Markers vs Circle Layers

How many points?
|
+-> < 100 with custom HTML/interaction? -> Markers (DOM elements)
+-> 100-10,000? -> circle layer (GPU-rendered)
+-> 10,000+? -> circle layer with clustering enabled on source

Styling Approach

Is the style static (same for all features)?
|
+-> YES -> Use literal paint values: "circle-color": "#e74c3c"
+-> NO -> Does it depend on a data property?
    +-> Discrete categories? -> "match" expression
    +-> Continuous range? -> "interpolate" expression
    +-> Conditional logic? -> "case" expression
    +-> Zoom-dependent? -> "interpolate" with ["zoom"]

</decision_framework>

<red_flags>

RED FLAGS

High Priority Issues:

• Calling addSource/addLayer before the load event -- style is not ready, throws error
• Adding a layer that references a source that doesn't exist -- must add source first
• Not calling map.remove() on component unmount -- leaks GPU memory, WebGL contexts, and event listeners
• Using Popup.setHTML() with unsanitized user input -- XSS vulnerability. Use setText() or setDOMContent() for user data
• Iterating features to set individual styles instead of using expressions -- defeats GPU rendering, O(n) JavaScript vs O(1) GPU expressions

Medium Priority Issues:

• Using Markers for large datasets (100+ points) -- DOM elements are expensive, use circle/symbol layers instead
• Not scoping layer events to a specific layer -- map.on("click", handler) fires for any click, map.on("click", "layer-id", handler) targets one layer
• Missing cursor feedback on interactive layers -- users don't know features are clickable without mouseenter/mouseleave cursor changes
• Hardcoding coordinates, zoom levels, or style values -- use named constants
• Using @types/mapbox-gl package -- types are included in mapbox-gl since v3

Gotchas & Edge Cases:

• Coordinates are [longitude, latitude] -- reversed from the common [lat, lng] order used by some libraries
• queryRenderedFeatures only returns features currently visible in the viewport -- for all features use querySourceFeatures
• GeoJSON source setData() replaces the entire dataset -- for partial updates use featureState via map.setFeatureState()
• style.load fires every time the style changes (including setStyle), load fires only once -- use style.load for operations that must survive style switches
• Expression property access returns null for missing properties -- always provide fallback values in match/case/coalesce
• Popup setHTML does not sanitize HTML -- any user-provided content must be sanitized before passing
• flyTo with essential: true overrides prefers-reduced-motion -- use only for critical navigation, not decorative animations
• Clustered sources automatically add point_count and cluster_id properties -- do not create these manually
• getClusterExpansionZoom is async (callback-based) -- handle errors and check if map still exists before calling easeTo
• map.getSource() returns undefined if the source doesn't exist -- always guard the return value
• Layer slot property only works with the Standard style -- classic styles use beforeId parameter in addLayer
• fill-extrusion layers require fill-extrusion-height property -- without it extrusions are flat (0 height)

</red_flags>

<critical_reminders>

CRITICAL REMINDERS

All code must follow project conventions in CLAUDE.md

(You MUST add sources before layers that reference them -- adding a layer without its source throws a runtime error)

(You MUST listen for load or style.load before calling addSource/addLayer -- the style is not ready on construction)

(You MUST clean up map instances with map.remove() on unmount -- leaks GPU memory and event listeners)

(You MUST use named constants for coordinates, zoom levels, and style values -- NO magic numbers)

(You MUST use expressions for data-driven styling instead of iterating features and setting styles individually)

Failure to follow these rules will cause runtime errors, memory leaks, and XSS vulnerabilities.

</critical_reminders>