Pre-projection for Leaflet

project.js

// Based on Leaflet's projection/transformation functions
var project = (function () {
    var R = 6378137, d = Math.PI / 180, max = 1 - 1E-15;
    var a = 0.5 / (Math.PI * R);

    function _project(lat, lng) {
        var sin = Math.max(Math.min(Math.sin(lat * d), max), -max);
        return [R * lng * d, R * Math.log((1 + sin) / (1 - sin)) / 2];
    }

    function _transform(x, y) {
        return [a * x + 0.5, -a * y + 0.5];
    }

    function _scale(zoom) {
        return 256 * Math.pow(2, zoom);
    }

    return function(lat, lng) {
        var p = _project(lat, lng);
        return _transform(p[0], p[1]).map(function (n) { return n * _scale(0); });
    };
})();

function projectPolygon(rings) {
    rings.forEach(function (ring) {
        ring.forEach(function (point, i) {
            ring[i] = project(point[1], point[0]);
        });
    });
}

geojson.features.forEach(function (feature) {
    var coordinates = feature.geometry.coordinates;
    
    switch (feature.geometry.type) {
      case 'Polygon':
        coordinates = [coordinates];
      case 'MultiPolygon':
        coordinates.forEach(projectPolygon);
        break;

      // TODO: other features
    }
});