SP.ClientContext.traceCorrelationId property (sp.js)
Gets or sets the trace correlation ID used by the server.
Applies to: apps for SharePoint | SharePoint Foundation 2013 | SharePoint Server 2013
var value = SP.ClientContext.get_traceCorrelationId()
The trace correlation ID used by the server. It may be null when the request has not been executed or if the server does not return a correlation ID.