Update a choropleth layer by zoom level
Using 2014 census data, display state or county population depending on zoom level.
<!DOCTYPE html>
<html>
<head>
    <meta charset='utf-8' />
    <title></title>
    <meta name='viewport' content='initial-scale=1,maximum-scale=1,user-scalable=no' />
    <script src='https://api.tiles.mapbox.com/mapbox-gl-js/v0.34.0/mapbox-gl.js'></script>
    <link href='https://api.tiles.mapbox.com/mapbox-gl-js/v0.34.0/mapbox-gl.css' rel='stylesheet' />
    <style>
        body { margin:0; padding:0; }
        #map { position:absolute; top:0; bottom:0; width:100%; }
    </style>
</head>
<body>

<style>

.legend {
    background-color: #fff;
    border-radius: 3px;
    bottom: 30px;
    box-shadow: 0 1px 2px rgba(0,0,0,0.10);
    font: 12px/20px 'Helvetica Neue', Arial, Helvetica, sans-serif;
    padding: 10px;
    position: absolute;
    right: 10px;
    z-index: 1;
}

.legend h4 {
    margin: 0 0 10px;
}

.legend div span {
    border-radius: 50%;
    display: inline-block;
    height: 10px;
    margin-right: 5px;
    width: 10px;
}

</style>

<div id='map'></div>

<div id='state-legend' class='legend'>
    <h4>Population</h4>
    <div><span style='background-color: #723122'></span>25,000,000</div>
    <div><span style='background-color: #8B4225'></span>10,000,000</div>
    <div><span style='background-color: #A25626'></span>7,500,000</div>
    <div><span style='background-color: #B86B25'></span>5,000,000</div>
    <div><span style='background-color: #CA8323'></span>2,500,000</div>
    <div><span style='background-color: #DA9C20'></span>1,000,000</div>
    <div><span style='background-color: #E6B71E'></span>750,000</div>
    <div><span style='background-color: #EED322'></span>500,000</div>
    <div><span style='background-color: #F2F12D'></span>0</div>
</div>

<div id='county-legend' class='legend' style='display: none;'>
    <h4>Population</h4>
    <div><span style='background-color: #723122'></span>1,000,000</div>
    <div><span style='background-color: #8B4225'></span>500,000</div>
    <div><span style='background-color: #A25626'></span>100,000</div>
    <div><span style='background-color: #B86B25'></span>50,000</div>
    <div><span style='background-color: #CA8323'></span>10,000</div>
    <div><span style='background-color: #DA9C20'></span>5,000</div>
    <div><span style='background-color: #E6B71E'></span>1,000</div>
    <div><span style='background-color: #EED322'></span>100</div>
    <div><span style='background-color: #F2F12D'></span>0</div>
</div>

<script>
mapboxgl.accessToken = '<your access token here>';
var map = new mapboxgl.Map({
    container: 'map',
    style: 'mapbox://styles/mapbox/light-v9',
    center: [-98, 38.88],
    minZoom: 3,
    zoom: 3
});

var zoomThreshold = 4;

map.on('load', function() {

    map.addSource('population', {
        'type': 'vector',
        'url': 'mapbox://mapbox.660ui7x6'
    });

    map.addLayer({
        'id': 'state-population',
        'source': 'population',
        'source-layer': 'state_county_population_2014_cen',
        'maxzoom': zoomThreshold,
        'type': 'fill',
        'filter': ['==', 'isState', true],
        'paint': {
            'fill-color': {
                property: 'population',
                stops: [
                    [0, '#F2F12D'],
                    [500000, '#EED322'],
                    [750000, '#E6B71E'],
                    [1000000, '#DA9C20'],
                    [2500000, '#CA8323'],
                    [5000000, '#B86B25'],
                    [7500000, '#A25626'],
                    [10000000, '#8B4225'],
                    [25000000, '#723122']
                ]
            },
            'fill-opacity': 0.75
        }
    }, 'waterway-label');

    map.addLayer({
        'id': 'county-population',
        'source': 'population',
        'source-layer': 'state_county_population_2014_cen',
        'minzoom': zoomThreshold,
        'type': 'fill',
        'filter': ['==', 'isCounty', true],
        'paint': {
            'fill-color': {
                property: 'population',
                stops: [
                    [0, '#F2F12D'],
                    [100, '#EED322'],
                    [1000, '#E6B71E'],
                    [5000, '#DA9C20'],
                    [10000, '#CA8323'],
                    [50000, '#B86B25'],
                    [100000, '#A25626'],
                    [500000, '#8B4225'],
                    [1000000, '#723122']
                ]
            },
            'fill-opacity': 0.75
        }
    }, 'waterway-label');

});

var stateLegendEl = document.getElementById('state-legend');
var countyLegendEl = document.getElementById('county-legend');
map.on('zoom', function() {
    if (map.getZoom() > zoomThreshold) {
        stateLegendEl.style.display = 'none';
        countyLegendEl.style.display = 'block';
    } else {
        stateLegendEl.style.display = 'block';
        countyLegendEl.style.display = 'none';
    }
});

</script>

</body>
</html>
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