mutation_observer.js

/*
Mutation Observer object

Used to detect and inspect changes to the DOM

See:
    https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver
*/

mutationObserver = {
    observer: null,
    mutationObserverInit: {
        childList: true,
        attributes: true,
        characterData: true,
        subtree: true,
        attributeOldValue: true,
        characterDataOldValue: true,
        attributeFilter: []
    },
    mutationRecords: [],
    init: function () {
        if (!this.observer) {
            this.observer = new MutationObserver(this.mutationRecordCallback.bind(this));
            this.isRunning = false;
        }
    },
    hasMutationRecord: function () {
        return this.mutationRecords.length > 0;
    },
    mutationRecordCallback: function (records, theObserver) {
        this.mutationRecords = records;
    },
    deleteMutationRecord: function () {
        this.mutationRecords = [];
    },
    observe: function (node) {
        if (!node) {
            node = document;
        }
        this.observer.observe(node, this.mutationObserverInit);
        this.isRunning = true;
    },
    isObserverRunning: function () {
        return this.isRunning;
    },
    disconnect: function (kill) {
        this.deleteMutationRecord();
        this.observer.disconnect();
        this.isRunning = false;
        if (kill) {
            this.observer = null;
        }
    }
};

mutation_observer.py

class MutationObserver(object):
    """
    Class for observing changes in the DOM.
    """

    def __init__(self, driver):
        self._driver = driver

    def init_observer(self):
        """
        Initialize the Mutation Observer class.

        :return: None
        """
        script = "mutationObserver.init();"
        # Inject the Mutation Observer object
        self._driver.execute_sync_script(get_mutation_observer())  # get_mutation_observer() reads the mutation_observer.js for the wrapper
        # Initialize it
        self._driver.execute_sync_script(script)  # this errors out with: WebDriverException: Message: ReferenceError: mutationObserver is not defined
        

@BeyondEvil Try to change mutationObserver = {...} to window.mutationObserver = {...} (in the JS file) and initialize it like this window.mutationObserver.init();.