how do you exactly know how javascript works inside out?
Javascript has pretty detailed specifications. They're not exhaustive, but the article here does mention that in some of the dark corner cases they compared behavior against other existing Javascript engines.
There's also a very comprehensive test suite (Test262) that comprises almost 40,000 unit tests that exercise various aspects of the language that a new implementation can use to get a pretty good view of how much their new engine conforms to the specification.
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Not the author, but:
Javascript has pretty detailed specifications. They're not exhaustive, but the article here does mention that in some of the dark corner cases they compared behavior against other existing Javascript engines.
There's also a very comprehensive test suite (Test262) that comprises almost 40,000 unit tests that exercise various aspects of the language that a new implementation can use to get a pretty good view of how much their new engine conforms to the specification.