The RTCIceCandidate interface's read-only foundation property is a string which uniquely identifies the candidate across multiple transports.

The foundation can therefore be used to correlate candidates that are present on multiple RTCIceTransport objects

A string which uniquely identifies the candidate across all RTCIceTransports on which it is available.

Consider this SDP attribute line (a-line) which describes an ICE candidate:

a=candidate:4234997325 1 udp 2043278322 192.0.2.172 44323 typ host

The field "4234997325" is the foundation.

This code snippet uses the foundation of two candidates to determine if they're actually the same candidate.

if (candidate1.foundation === candidate2.foundation) {
  /* the two candidates are the same, even if they're on
     different transports */
}