Is the complexity actually quadratic, like amelius said?

The diffing operation is linear according to React documentation: https://facebook.github.io/react/docs/reconciliation.html

Is Array.prototype.concat linear, too? What about the ImmutableJS equivalent? I don't know a whole lot about algorithms, my intuition is that there would be a way to add an item to the end of the array without traversing the entire thing, but I guess it all depends on how the data structure is implemented.