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View local GeoJSON (experimental)

View local GeoJSON with experimental File System Access API.

<!DOCTYPE html>
<html lang="en">
<head>
    <title>View local GeoJSON (experimental)</title>
    <meta property="og:description" content="View local GeoJSON with experimental File System Access API." />
    <meta charset='utf-8'>
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel='stylesheet' href='https://unpkg.com/maplibre-gl@5.2.0/dist/maplibre-gl.css' />
    <script src='https://unpkg.com/maplibre-gl@5.2.0/dist/maplibre-gl.js'></script>
    <style>
        body { margin: 0; padding: 0; }
        html, body, #map { height: 100%; }
    </style>
</head>
<body>
<style>
    #viewbutton {
        position: absolute;
        top: 0;
        left: 0;
    }
</style>
<div id="map"></div>
<button id="viewbutton">View local GeoJSON file</button>
<script>
    const map = new maplibregl.Map({
        container: 'map',
        style:
            'https://api.maptiler.com/maps/streets/style.json?key=get_your_own_OpIi9ZULNHzrESv6T2vL',
        center: [-8.3226655, 53.7654751],
        zoom: 8
    });

    const viewbutton = document.getElementById('viewbutton');

    async function buttonClickHandler() {
        const [fileHandle] = await window.showOpenFilePicker({
            // allow only single file
            multiple: false,

            // apply filter for GeoJSON files
            types: [
                {
                    description: 'GeoJSON',
                    accept: {'application/geo+json': ['.geojson']}
                }
            ],

            // start in download directory
            startIn: 'downloads'
        });

        // get file handle and read content
        const file = await fileHandle.getFile();
        const contents = await file.text();

        // parse file as json and add as source to the map
        map.addSource('uploaded-source', {
            'type': 'geojson',
            'data': JSON.parse(contents)
        });

        map.addLayer({
            'id': 'uploaded-polygons',
            'type': 'fill',
            'source': 'uploaded-source',
            'paint': {
                'fill-color': '#888888',
                'fill-outline-color': 'red',
                'fill-opacity': 0.4
            },
            // filter for (multi)polygons; for also displaying linestrings
            // or points add more layers with different filters
            'filter': ['==', '$type', 'Polygon']
        });
    }

    if ('showOpenFilePicker' in window) {
        viewbutton.addEventListener('click', buttonClickHandler);
    } else {
        viewbutton.innerText =
            'Your browser does not support File System Access API';
        // If you want a fallback, try <input type="file">; but this uses classical file upload
    }
</script>
</body>
</html>