Continually polls for a change in a simple value (eg. primitive) returned by a function. Good for setting up a fake listener on something doesn't have another way to add a real event listener.

pollChange.js

/**
 * Continually calls `getValue()` with a state object that can be modified and
 * the returned value will be compared to what was previously returned.  If the
 * return value is different than the previous, the `callback()` function will
 * be called.
 * @param {(currValue: any, state: State) => Value} getValue
 *   A function that is called after every interval (as specified by
 *   `msInterval`).  The arguments passed will be (1) the previous value
 *   returned by this function and (2) the state object (which on the initial
 *   call starts off as an empty object but can be augmented).  The return value
 *   is then compared to the previous return value to determine if `callback()`
 *   should be called.
 * @param {(newValue: Value, oldValue: Value, state: State) => void} callback
 *   A function that is called only if the last return value of `getValue()` is
 *   different from the one before that.
 * @param {?number=} msInterval
 *   Optional, defaults to 100.  The amount of milliseconds to wait between
 *   polls.
 * @returns {number}
 *   An ID that identifies the created timer.  This value can be passed to
 *   `clearInterval()` to cancel the interval.
 * @template State
 * @template Value
 */
function pollChange(getValue, callback, msInterval) {
    const state = {};
    let lastValue = getValue(undefined, state);
    return setInterval(() => {
        const newValue = getValue(lastValue, state);
        if (lastValue !== lastValue ? newValue === newValue : lastValue !== newValue) {
            callback(newValue, lastValue, state);
        }
        lastValue = newValue;
    }, msInterval ?? 100);
}

~example.js

// Any time the hidden input field's value changes call triggerHiddenInputEvent().
pollChange(
  () => document.getElementById('hiddenInput').value,
  () => triggerHiddenInputEvent(document.getElementById('hiddenInput'))
);