Chrome JS Console only pre-evaluates (displays a gray result before you hit enter on a line of JavaScript code) expressions that don't have side effects. How does Chrome tell? Side effects tested: changing closured(non-local) variables, altering objects pointed to by non-local variables (including accessing properties with get() attributes that have side effects), async code (setTimeout), console.log, etc.
V8 has massive lists denoting which internal operations ("bytecode handlers", "intrinsics", and "builtins") are considered side effect free for this feature's purposes. See src/debug/debug-evaluate.cc for all the gory details.
(This distinction is in no way exposed to, or relevant for, regular JavaScript operation; it's purely for debugger features.)