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Building a javascript engine is super simple. But to build one that has performance on par with the current crop will take you - as a single person - a lifetime and by then the state of the art will have moved on.

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The performance is hard, but the bug-for-bug compatibility is what will kill you.

You could just leave them out ;)

Actually, for JS engines this is a much smaller problem than for other parts of the web platform. There are two nasty warts I'm aware of: the "function inside if" mess, and the fact that the spec says enumeration order is undefined but actual web pages depend on some things about enumeration order that all browsers implement; chances are this will make it into the spec at some point. But by and large JS engines agree with each other and with the spec. Much more so than, say, DOM or CSS implementations.

Enumeration order was indeed (finally) added to the ES2015 spec. Here's a link to the release candidate spec for the [[OwnPropertyKeys]] algorithm:

https://people.mozilla.org/~jorendorff/es6-draft.html#sec-or...

[[OwnPropertyKeys]] is not invoked by a for-in loop. [[Enumerate]] is instead, and while it defines that the set of iterated names is the same as that returned by [[OwnPropertyKeys]] it explicitly says that order is not defined.

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