static getNumber(domElementOrSelector, callback = null) {
        return this._get(domElementOrSelector, 'getNumber', callback);
    }
Return the element unformatted value as a real Javascript number, from the given DOM element or query selector.
Warning: This can lead to precision problems with big numbers that should be stored as strings.

@param {HTMLElement|string} domElementOrSelector
@param {function|null} callback
@returns {number|null}