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Right. Technically Mug will conform to the ECMAScript 3 Compact Profile: http://www.ecma-international.org/publications/files/ECMA-ST... Which doesn't include eval() or the with(){} statement (both used infrequently enough that it's not worth the overhead).

A REPL would still be possible, however, it would require some work to maintain the current scope between input.

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Is there a reason you are not targeting ECMA5 (with the exclusions you mentioned)?

Much of the runtime has been developing by trying out existing code, then adding support gradually. Of course most of the code I tried out was ECMA3 compliant.

I expect to add in support for ECMAScript 5's Object extensions API (defineProperty, getters/setters) as Mug supports them internally, just as Mug already has a global JSON object. The features which could cause incompatibility with existing code is something I'd have to consider separately, so it depends when a majority of authors start writing ECMA5-compliant code (no function.callee, implicit this object bound to global, etc.)

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