Create a gradient line using an expression | Mapbox GL JS

Use the line-gradient paint property and an expression to visualize distance from the starting point of a line.
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Create a gradient line using an expression</title>
<meta name="viewport" content="initial-scale=1,maximum-scale=1,user-scalable=no">
<link href="https://api.mapbox.com/mapbox-gl-js/v2.1.1/mapbox-gl.css" rel="stylesheet">
<script src="https://api.mapbox.com/mapbox-gl-js/v2.1.1/mapbox-gl.js"></script>
<style>
body { margin: 0; padding: 0; }
#map { position: absolute; top: 0; bottom: 0; width: 100%; }
</style>
</head>
<body>
<div id="map"></div>
 
<script>
	// TO MAKE THE MAP APPEAR YOU MUST
	// ADD YOUR ACCESS TOKEN FROM
	// https://account.mapbox.com
	mapboxgl.accessToken = '<your access token here>';
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,
// 'line-gradient' must be specified using an expression
// with the special 'line-progress' property
'line-gradient': [
'interpolate',
['linear'],
['line-progress'],
0,
'blue',
0.1,
'royalblue',
0.3,
'cyan',
0.5,
'lime',
0.7,
'yellow',
1,
'red'
]
},
layout: {
'line-cap': 'round',
'line-join': 'round'
}
});
});
</script>
 
</body>
</html>