Using JavaScript to handle "Uncaught ReferenceError: map is not defined" in a PyQt5 Interactive Map

Addressing Map Initialization Issues in PyQt5 Web Applications

When developing applications with PyQt5, integrating dynamic content such as interactive maps can enhance the user experience. However, it is not uncommon to encounter errors when combining different technologies like Python and JavaScript. One such error is the "Uncaught ReferenceError: map is not defined," which occurs when trying to manipulate a map using JavaScript within PyQt5.

In this particular scenario, the issue arises from initializing a Leaflet map through Folium in Python, and embedding it in a PyQt5 application using QtWebEngineWidgets. As the application loads, JavaScript attempts to reference a map object that hasn't been properly initialized, leading to errors in both rendering and functionality.

Another common issue, "Map instance not initialized," happens when trying to interact with the map before the DOM has fully loaded. Ensuring that the map instance is available for JavaScript to control is crucial for adding features like location changes or interactive buttons.

This article aims to dissect these issues, explore the root causes, and provide solutions for properly initializing and controlling the map in PyQt5. We’ll also demonstrate how to link JavaScript functionality with Python, ensuring smooth interaction between the two languages.

Overcoming Map Initialization Issues in PyQt5 Applications

The Python script integrated with JavaScript serves to create an interactive map using PyQt5 and Folium. The key functionality here is the ability to change map locations based on user input via radio buttons. In the load_map function, Folium is used to create the map object, which is then embedded into the PyQt5 interface. This map is interactive and allows adding custom buttons through HTML, which is later linked to JavaScript functions. The Folium library makes it easier to create maps and integrate HTML-based elements like buttons, which trigger actions when clicked.

The second major part of the script is the JavaScript code embedded in the HTML of the map. The initializeMap function ensures that a map instance is properly initialized and available globally. This addresses the issue of the "map is not defined" error by ensuring the JavaScript variable map_instance is assigned the Leaflet map object created by Folium. By using the DOMContentLoaded event listener, the map instance is initialized only when the page has fully loaded, which prevents any errors related to undefined variables during page rendering.

The next significant portion of the script is the moveToLocation JavaScript function. This function is responsible for smoothly panning and zooming the map to specific coordinates when called. By utilizing the flyTo method from Leaflet.js, the map smoothly transitions to a new location when the user selects a different radio button. This interaction between Python and JavaScript is achieved by calling the runJavaScript method from PyQt5, which allows Python to execute JavaScript functions within the WebView component.

The last part of the code handles user input through the radio buttons. When a user selects a radio button, the update_label function is called to check which button is selected and trigger the corresponding map movement. For each location, the script sends a JavaScript command through runJavaScript to change the map's view. This structure allows seamless interaction between the Python backend and the JavaScript front-end, making the interface responsive and interactive for users.

from PyQt5 import QtCore, QtWebEngineWidgets
import folium, os
class UI_MainWindow:
    def load_map(self):
        center_lat, center_lng = 18.45, -66.08
        self.map_obj = folium.Map(location=[center_lat, center_lng], zoom_start=15, min_zoom=14, max_zoom=17, control_scale=True)
        # JavaScript to move the map
        move_js = """
        <script>
        var map_instance;
        function initializeMap() { map_instance = map; }
        function moveToLocation(lat, lng) { if (map_instance) { map_instance.flyTo([lat, lng], 16); } }
        </script>
        """
        self.map_obj.get_root().html.add_child(folium.Element(move_js))
        # Assign map path
        map_path = os.path.join(os.getcwd(), "map_buttons.html")
        self.map_obj.save(map_path)
        self.webView.setUrl(QtCore.QUrl.fromLocalFile(map_path))
    def update_label(self, radio_button):
        if radio_button.isChecked():
            if radio_button == self.radio:  # PO1
                self.webView.page().runJavaScript("moveToLocation(18.45, -66.08);")
            elif radio_button == self.radio2:  # PO2
                self.webView.page().runJavaScript("moveToLocation(18.46, -66.07);")

from PyQt5 import QtCore, QtWebEngineWidgets
import folium, os
class UI_MainWindow:
    def load_map(self):
        center_lat, center_lng = 18.45, -66.08
        self.map_obj = folium.Map(location=[center_lat, center_lng], zoom_start=15, min_zoom=14, max_zoom=17)
        # Initialize map instance in JavaScript
        init_map_js = """
        <script>
        document.addEventListener("DOMContentLoaded", function() { initializeMap(); });
        </script>
        """
        self.map_obj.get_root().html.add_child(folium.Element(init_map_js))
        map_path = os.path.join(os.getcwd(), "map_buttons.html")
        self.map_obj.save(map_path)
        self.webView.setUrl(QtCore.QUrl.fromLocalFile(map_path))
    def update_label(self, radio_button):
        if radio_button.isChecked():
            if radio_button == self.radio:
                self.webView.page().runJavaScript("moveToLocation(18.45, -66.08);")
            elif radio_button == self.radio2:
                self.webView.page().runJavaScript("moveToLocation(18.46, -66.07);")

Understanding JavaScript Integration with Folium in PyQt5

One critical aspect when working with PyQt5 and Folium is the seamless integration of Python and JavaScript. Folium, a Python library, simplifies the creation of Leaflet maps, which are rendered as HTML. This makes it easy to display interactive maps within PyQt5 applications, which use QtWebEngineWidgets to display web content. However, a common challenge arises when trying to control these maps with JavaScript. The error “Uncaught ReferenceError: map is not defined” is caused by improper initialization of the map instance within the JavaScript code.

The best way to resolve this issue is by ensuring the map object is properly initialized in the JavaScript section. This is achieved by creating an initializeMap function, which assigns the Leaflet map object to a global JavaScript variable once the page’s DOM is fully loaded. Using event listeners like document.addEventListener, we can ensure the map is ready before any attempts to interact with it, eliminating the “map instance not initialized” error. This approach ensures the map can be smoothly panned or zoomed as required.

Additionally, ensuring smooth communication between Python and JavaScript is vital. The PyQt5 function runJavaScript allows executing JavaScript functions directly from Python, making it possible to control the map through PyQt5 widgets like radio buttons. This level of integration not only resolves the map initialization issue but also provides a powerful way to build interactive applications where Python handles the backend logic and JavaScript manages the front-end functionality.