Ah, yeah, perhaps I should have elaborated more.  It&#x27;s a subtle issue, but it&#x27;s one that comes up all the time when optimizing React by removing unnecessary renders.<p>With immutable data structures, you know that x == y implies, even if x was initialized a while ago, that the data is the same.  With mutable data, that&#x27;s not true -- perhaps the values referenced by x have changed value in the meantime, so x == y does not imply that the data is the same.