Bugs in negated `instanceof` expressions

[gorosgobe]

in and instanceof expressions in JS

a in obj;
a instanceof C;

can be negated by grouping them and applying the ! operator, i.e.

!(a in obj);
!(a instanceof C);

Applying the ! operator incorrectly (on the LHS operand) leads to bugs:

!a in obj; // will evaluate to false, unless obj has a "true" or "false" key
!a instanceof C; // will evaluate to false, unless C overrides instanceof with a @@hasInstance method

For more information, please see these MDN docs and the no-unsafe-negation recommended Eslint rule.

I have found several potentially problematic instances of the above bugs in your codebase: https://sourcegraph.com/search?q=context:global+repo:%5Egithub%5C.com/gkz/LiveScript%24+lang:javascript+/%5C%21%5B%5B:alnum:%5D%5D%2B+instanceof+%5B%5B:alnum:%5D%5D%2B/+-file:%5C.min%5C.js%24+count:all&patternType=standard&sm=1&groupBy=repo

[vendethiel]

LS has not instance/not in composite keywords.

[gorosgobe]

This has nothing to do with LS as the language, but rather with its implementation - see sourcegraph link above.

[vendethiel]

I thought this was a compilation error (wrong level), but the source has it as-is: https://github.com/gkz/LiveScript/blob/master/src/ast.ls#L2515