What is the best way to apply color to individual polygons based on a specific parameter using javascript?

I attempted to customize the colors of polygons loaded from a geojson file on Google Maps based on the "population density" data. However, my script failed to execute as intended.

I tried to extract the "population density" information within a loop function and used conditionals to assign colors to each polygon, but encountered an error stating "Cannot read property 'feature' of undefined."

function loadBoundariesFromGeoJson(geo_json_url) {
        initializeDataLayer();
        $.getJSON(geo_json_url, function (data) {
            if (data.type == "FeatureCollection") {
                if (data.features) {
                    for (var i = 0; i < data.features.length; i++) {
                        var boundary_id = i + 1;
                        var new_boundary = {};
                        if (!data.features[i].properties) data.features[i].properties = {};
                        data.features[i].properties.boundary_id = boundary_id;
                        data_layer.addGeoJson(data.features[i], { idPropertyName: 'boundary_id' });
                        new_boundary.feature = data_layer.getFeatureById(boundary_id);
                        if (data.features[i].properties.name) new_boundary.name = data.features[i].properties.name;
                        if (data.features[i].properties.popDensity) new_boundary.popDensity = data.features[i].properties.popDensity;
                        my_boundaries[boundary_id] = new_boundary;
                        if (data.features[i].properties.popDensity < 500) {
                            data_layer.overrideStyle(my_boundaries[i].feature, {
                                fillColor: '#00ff00',
                                fillOpacity: 0.9
                            });
                        } else {
                            data_layer.overrideStyle(my_boundaries[i].feature, {
                                fillColor: '#ff0000',
                                fillOpacity: 0.9
                            });
                        }
                    }
                }
            }
        });
    }

I need assistance in resolving this issue. Can anyone provide guidance on how to fix this error?

Answer №1

An exception may occur if my_boundaries[i] is empty in the following code snippet.

data_layer.overrideStyle(my_boundaries[i].feature, {
    fillColor: '#00ff00',
    fillOpacity: 0.9
});

Upon inspecting the array my_boundaries.

You add new_boundary to the array using my_boundaries[boundary_id] = new_boundary; where boundary_id is i+1;

However, based on the code above, you are attempting to access the feature property of my_boundaries[i], which would initially be pointing to position 0 where there is no data.

The recommended solution is to check if my_boundaries[i] is empty or null before assigning a property to it, or utilize my_boundaries[boundary_id].

if (data.features[i].properties.popDensity < 500) {
    if (typeof my_boundaries[i] !== "undefined"){
        data_layer.overrideStyle(my_boundaries[i].feature, {
            fillColor: '#00ff00',
            fillOpacity: 0.9
        });
    }
}

OR

if (data.features[i].properties.popDensity < 500) {
    data_layer.overrideStyle(my_boundaries[boundary_id].feature, {
        fillColor: '#00ff00',
        fillOpacity: 0.9
    });
}

I hope this guidance assists you.

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