zerojs_language tests

Language and host-facing behaviour scripts rely on: optional chaining (?.), nullish coalescing (??) and the logical assignments (??=, ||=, &&=), what an error prints as, what console writes for an object, an array or an error, and what Date.now() and performance.now() read.

Every language case runs on both tiers a VM can execute with, the compiled tier and the interpreter, and expects the same answer from each.

Test	What it holds
optional_chaining_reads_through_nil	a?.b, a?.[k], f?.(x) give undefined for a null or undefined base, skip the rest of the chain and the arguments, keep this, and parse a?.5:1 as a conditional.
optional_chaining_is_refused_as_a_target	A chain is a SyntaxError as an assignment, update or new target and before a tagged template.
nullish_coalescing_keeps_false_and_zero	?? replaces only null and undefined, never evaluates its right side otherwise, and refuses mixing with || and && unparenthesised.
logical_assignment_assigns_only_when_it_applies	??=, ||= and &&= evaluate the right side and write only when the current value calls for it.
an_error_prints_its_message	String(e), `${e}`, "" + e and e.toString() read <name>: <message> for built-in errors and errors the runtime raises.
console_log_prints_objects_and_arrays_readably	objects, arrays, nested values, errors, functions, Map, Set and cycles print the way a JavaScript console prints them, bounded in depth and length.
date_now_moves_within_one_frame	Date.now() follows the wall clock between two reads with no engine frame between them.
date_now_tracks_the_wall_clock	Date.now() stays inside the wall second os.time() reports.
date_now_moves_across_an_await	Date.now() read before and after an awaited timer differs by about the timer.
performance_now_is_monotonic	performance.now() never goes backwards and counts milliseconds.