FYI: In V8, Error objects keep closures alive until the err.stack property is accessed, which prevents collection of the closure (and associated objects) until the Error objects die.

Mocha creates a long-living Error for each Runnable:

Would it make sense to either 1. store a string message instead of an Error, or 2. clear Error.stack, or 3. format Error.stack eagerly instead?

See also: https://crbug.com/v8/7142 and https://twitter.com/wSokra/status/935790750188625920.