Create a gradient line using an expression

Use the line-gradient paint property and an expression to visualize distance from the starting point of a line. See the example: https://docs.mapbox.com//mapbox-gl-js/example/line-gradient/

by cs09g

HTML

<script src="https://api.mapbox.com/mapbox-gl-js/v1.10.0/mapbox-gl.js"></script>
<link rel="stylesheet" href="https://api.mapbox.com/mapbox-gl-js/v1.10.0/mapbox-gl.css">

<div id="map"></div>

CSS

body { margin: 0; padding: 0; }
	#map { position: absolute; top: 0; bottom: 0; width: 100%; }

JavaScript

mapboxgl.accessToken = 'pk.eyJ1IjoiZmFyYWRheTIiLCJhIjoiTUVHbDl5OCJ9.buFaqIdaIM3iXr1BOYKpsQ';
    var map = (window.map = new mapboxgl.Map({
        container: 'map',
        style: 'mapbox://styles/mapbox/light-v10',
        center: [-77.035, 38.875],
        zoom: 12
    }));

    var geojson = {
        'type': 'FeatureCollection',
        'features': [
            {
                'type': 'Feature',
                'properties': {},
                'geometry': {
                    'coordinates': [
                        [-77.044211, 38.852924],
                        [-77.045659, 38.860158],
                        [-77.044232, 38.862326],
                        [-77.040879, 38.865454],
                        [-77.039936, 38.867698],
                        [-77.040338, 38.86943],
                        [-77.04264, 38.872528],
                        [-77.03696, 38.878424],
                        [-77.032309, 38.87937],
                        [-77.030056, 38.880945],
                        [-77.027645, 38.881779],
                        [-77.026946, 38.882645],
                        [-77.026942, 38.885502],
                        [-77.028054, 38.887449],
                        [-77.02806, 38.892088],
                        [-77.03364, 38.892108],
                        [-77.033643, 38.899926]
                    ],
                    'type': 'LineString'
                }
            }
        ]
    };

    map.on('load', function() {
        // 'line-gradient' can only be used with GeoJSON sources
        // and the source must have the 'lineMetrics' option set to true
        map.addSource('line', {
            type: 'geojson',
            lineMetrics: true,
            data: geojson
        });

        // the layer must be of type 'line'
        map.addLayer({
            type: 'line',
            source: 'line',
            id: 'line',
            paint: {
                'line-color': 'red',
                'line-width': 14,
           ...