Class Diagnostics

Use assert to ensure an assumption at runtime. When running with assertions enabled, the check expression is evaluated immediately. A check expression evaluating to false signals a violation of invariant that should never happen. If it does, a hard error is output unconditionally to the console and an AssertionError is thrown.

Do not use assertions as a means of ordinary error checking. Here are some valid examples of assertions:

    assert(() => Date.now() > 0, 'current time millis can not be before 1970 start of time!');
    assert(() => total_price >= item_price,
                 'total is sum of individual prices and thus can not be less than each one!');
    assert(() => num*num >= 0, 'squared num is less than zero!');