Nothing interesting, per current spec. a and b both get 1 assigned to them, in V8, JSC, and current Spidermonkey, and the value of undefined doesn't change. C

Nothing interesting, per current spec. a and b both get 1 assigned to them, in V8, JSC, and current Spidermonkey, and the value of undefined doesn't change. Carakan seems to still implement the old spec version, so it assigns 1 to all three. The presence or absence of var is not relevant. The only reason a and b get 1 assigned to them is that the value of an assignment expression is always the RHS, no matter what actually happens to the assignment. Since "undefined" on the global is readonly, the assignment itself is just silently ignored. All this without strict mode, of course; in strict mode the assignment to undefined throws.