JavaScript objects look like hash maps, but engines don't treat them that way. How hidden shapes and inline caches turn property access into a single memory load — and the coding habits that quietly switch that optimization off.

Local variables vanish when a function returns; objects outlive it. The two regions of memory behind that difference, why assigning an object never copies it, and what it means for closures, performance, and leaks.

Machine clocks drift, so timestamps can't tell you which of two events in a distributed system came first. Logical clocks can — Lamport clocks give a consistent order, vector clocks tell you whether an order exists at all.

The key prop isn't there to silence a console warning. It's how React decides whether an element is the same thing it rendered last time — and getting it wrong moves state to the wrong row. How the diff works, and how to pick a key.