I think we should avoid checks based on value of an entry unless we really mean the value is there and has a special boolean meaning. Better to use the "x in Y" style.  e.g. if(!(peerId in this.peers_)) { 
  // create peer
  ... 
  this.peers_[peerId] = ... 
}
return this.peers_[peerId];

This gives the code a tad compositionality in the form of copy/paste-ableness too :)