const test: string[] = [];
test[0].length
This code does not throw a TypeScript error. How can you let typescript warn against the fact that a string might actually not exist at a given index?
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I don't think that's typescript's responsibilitybaao– baao2019-10-04 08:25:14 +00:00Commented Oct 4, 2019 at 8:25
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Shouldn't it warn that an item at any index could be either a string or undefined? And since that's obviously not the default, what's the best way to manually configure that?Gersom– Gersom2019-10-04 08:26:35 +00:00Commented Oct 4, 2019 at 8:26
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2Very relevant issue in Github: micosoft/TypeScript#13778. It's still open and awaiting feedback, and some of the discussion in there involves what people have done to deal with the problem you're facing.jcalz– jcalz2019-10-04 18:50:16 +00:00Commented Oct 4, 2019 at 18:50
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2 Answers
You can use this workaround, setting array items as possible undefined:
const test: (string | undefined)[] = [];
test[0].length; // error
if(test[0]) {
test[0].length; // good
}
I didn't find any eslint rule which can answer to this need :(
4 Comments
Gersom
But in this case
.map(item => item.length) will also warn right? Having to assert there is not desirable.
Richard Haddad
Hmm yep you're right. I don't know good solution to this issue...
jcalz
@Gersom TypeScript doesn't do a good job distinguishing "missing" from "present-but-
undefined". So Array<string | undefined> adequately describes "numeric-keyed property might be missing" but also matches "numeric-keyed property might be undefined" as in ["hey", undefined, "you"]. So .map() needs to handle undefined also. If you want a different solution you'd basically have to redefine all the Array type definitions. I might be able to come up with something, but Array<string | undefined> is a pretty good answer.jcalz
Update: I can't come up with anything remotely reasonable for this other than
Array<string | undefined>.This is now possible using the following option in tsconfig.json:
{
"compilerOptions": {
"noUncheckedIndexedAccess": true
}
}