Typescript: (0,eval) produces TS2695
TypeScript Version: 2.1.4
(0,eval)('var x = 10');
Expected behavior:
Compile fine.
Actual behavior:
error TS2695: Left side of comma operator is unused and has no side effects.
See also at TS Playground.
(0,eval) is a well-used convenience syntax for invoking eval with a global scope (as opposed to the scope of the caller).
One possible workaround is: (eval||null). But that's longer and most of all — hard to Google the reasoning behind. Whereas (0,eval) is easy to find a detailed explanation for.
Please make it not error.
mihailik
All 9 comments
(0,eval) is a hack, you can just use your own global eval like:
const myEval = eval;
myEval(`var x = 10`);
I'm personally fine with TS erroring out on this. It is a _really_ edgy case.
benjamingr
on 16 Dec 2016
Calling something hack or edge case isn't relevant. Infinity is real proper edge case, yet it produces no error: 1+Infinity. (function(){ })() is a definite absolute hack, yet no errors pop up.
What's relevant is the usage of a given pattern. (0,eval) is widely used, even in pretty much cutting edge libraries:
Widely-used well-known googlable "hacks" should not trigger errors. I think this check was added recently — I don't think it generated this noise in 2.0.6. It's a regression and may cause undue friction for migrating code from JS.
mihailik
on 16 Dec 2016
@mihailik SystemJS is a _module loader_, it's meant to evaluate code so it's the one place where you might find this hack outside of transpiled code.
I agree there is a bar here of what we want to honor or not. (function() {})() for scoping is hardly a hack in "idiomatic JS". Relying on whether or not eval is referenced directly or indirectly for defining global variables seems like a much much bigger stretch.
benjamingr
on 16 Dec 2016
Allowing strictly eval as a special case right operand here seems fine.
RyanCavanaugh
on 16 Dec 2016
Totally agree with eval being the only special exception here.
Certain type of code relies on eval: binding frameworks, dynamic code transforms, loaders and so on. Indirect (0,eval) is an important feature in such cases.
Couple more examples:
mihailik
on 16 Dec 2016
For reference, background to current behaviour can be found in #10814 which was prompted by the scenario in #10802.
yortus
on 17 Dec 2016
@yortus thanks for the providing the context. It seems a bit hand-holdy given that it's not a type level rule and that people can generally be expected to know the syntax for control flow constructs. The comma operator is definitely an easy thing to get wrong and probably something I would otherwise want to ban with a linter but not necessarily at the language level.
aluanhaddad
on 17 Dec 2016
Using void operator disables the check, so you can use (void 0,eval)('var x = 10');
mhegazy
on 24 Jan 2017
Accepting PRs to allow strictly the identifier eval as the right-side comma operand without error
RyanCavanaugh
on 24 Jan 2017
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Most helpful comment
(0,eval)is a hack, you can just use your own global eval like:I'm personally fine with TS erroring out on this. It is a _really_ edgy case.