typescript don't warn about a potentially undefined array?

Question

I have a question about how could I leverage typescript to warn about an array that could cause a run time error ? I have the following code I usually use a question mark after the array .. but should typescript know that this might cause a runtime error ?

// cause run time erorr.. 
context.arr[props.id].includes(select)
// no run time erorr
context.arr[props.id]?.includes(select)

If it's the `context.arr[props.id]` that might be undefined, that's just fine — CertainPerformance, Nov 01 '21 at 02:04
@CertainPerformance it is created after an action... but the function that does the check runs as well , but shouldn't typescript be warning about this ? — Richardson, Nov 01 '21 at 02:07
@CertainPerformance it possible to warn about `context.arr[props.id]` potentially undefined by enabling `noUncheckedIndexedAccess`. It can introduce a lot of noise in existing code base, but still possible — Aleksey L., Nov 02 '21 at 06:18

score 2 · Answer 1 · answered Nov 02 '21 at 06:09

2

You can enable the desired behavior by turning on Checked Indexed Accesses (available since 4.1 version). Under this mode, indexed access is considered potentially undefined. To enable it - set noUncheckedIndexedAccess compiler option to true.

const arr = ['foo', 'bar']

// Expect error: Object is possibly 'undefined'.
arr[1].includes('test')

// OK
arr[1]?.includes('test')

Playground

answered Nov 02 '21 at 06:09

Aleksey L.

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--- Perfect --- – Magnus Jul 29 '22 at 08:53

score 1 · Accepted Answer · answered Nov 01 '21 at 02:10

?. is optional chaining, which will properly chain with what follows if the expression on the left-hand side is defined (not undefined/null), and will evaluate to undefined otherwise:

context.arr[props.id]?.includes(select)

// if context.arr[props.id] is undefined/null, equivalent to:
undefined
// (does not throw an error)


// if context.arr[props.id] is  notundefined/null, equivalent to:
context.arr[props.id].includes(select)

So, as long as TypeScript can see that, when context.arr[props.id] isn't undefined/null, it'll be an array, that statement is perfectly type-safe. (though, TypeScript has problems with .includes, but that's another issue)

typescript don't warn about a potentially undefined array?

2 Answers2