The read-only RTCDataChannel property ordered indicates whether or not the data channel guarantees in-order delivery of messages; the default is true, which indicates that the data channel is indeed ordered. This is set when the RTCDataChannel is created, by setting the ordered property on the object passed as RTCPeerConnection.createDataChannel()'s options parameter.
A boolean value which is true if in-order delivery is guaranteed and is otherwise false.
const pc = new RTCPeerConnection();
const dc = pc.createDataChannel("my channel");
if (!dc.ordered) {
// Handle unordered messaging
}