UnwrapDateTimeFormat = function (dtf) {
            let hiddenObject = platform.getHiddenObject(dtf);
            if ((!hiddenObject || !hiddenObject.initializedDateTimeFormat) && dtf instanceof DateTimeFormat) {
                dtf = dtf[platform.FallbackSymbol];
            }

            if (typeof dtf !== "object") {
                platform.raiseNeedObjectOfType("DateTimeFormat.prototype.format", "DateTimeFormat");
            }

            hiddenObject = platform.getHiddenObject(dtf);
            if (!hiddenObject.initializedDateTimeFormat) {
                platform.raiseNeedObjectOfType("DateTimeFormat.prototype.format", "DateTimeFormat");
            }

            return hiddenObject;
        }
The Intl.DateTimeFormat constructor

ECMA-402: #sec-intl.datetimeformat

@param {String|String[]} locales
@param {Object} options