The RTCDataChannel.onmessage property stores an event handler which specifies a function to be called when the message event is fired on the channel. This event is represented by the MessageEvent interface. This event is sent to the channel when a message is received from the other peer. 
RTCDataChannel.onmessage = function;
 A function which the browser will call to handle the message event. The function receives as its sole input parameter a MessageEvent object describing the event. 
 This code snippet creates a peer connection, adds a data channel to it, and starts creating new <p> (paragraph) elements each time a message arrives, with the message's contents displayed inside it. The new elements are then attached to the end of the document. 
let pc = new RTCPeerConnection(); let dc = pc.createDataChannel(); dc.onmessage = function(event) { var el = document.createElement("p"); var txtNode = document.createTextNode(event.data); el.appendChild(txtNode); receiveBox.appendChild(el); }
| Specification | 
|---|
| WebRTC 1.0: Real-Time Communication Between Browsers (WebRTC 1.0) # dom-rtcdatachannel-onmessage  | 
| Desktop | Mobile | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | WebView Android | Chrome Android | Firefox for Android | Opera Android | Safari on IOS | Samsung Internet | |
onmessage | 
24  | 
79  | 
22  | 
No  | 
15  | 
11  | 
≤37  | 
25  | 
22  | 
14  | 
11  | 
1.5  | 
RTCPeerConnectionRTCDataChannelmessage event and its type, MessageEvent.
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    https://developer.mozilla.org/en-US/docs/Web/API/RTCDataChannel/onmessage