Question 287898 in Javascript Question posted in
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Javascript – Why is TypeScript not detecting implicit use of any in nested object/array properties?

Why is Typescript not detecting that I’ve checked only part of unknown type, and not it’s nested properties thus resulting in runtime error?

interface User {
    name: string,
    age: number
}

const externalData: unknown = [1, 2, 3];

(Array.isArray(externalData) ? externalData : [])
    .reduce<User[]>((acc, curr) => {
        return [
            ...acc,
            {
                name: curr.some.nested.property, // not detecting that these properties may not exist on type 'any'
                age: curr.some.nested.another.propety // not detecting that these properties may not exist on type 'any'
            }
        ]
    }, []);
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2

Answers


  1. 0

    noImplicit will yell at you when there are no typings.
    For example function (foo) { ... }, foo has no type so it’s implicitly any.

    In your case (Array.isArray(externalData) ? externalData : []) is inferred to any.

    because the empty array is any[].

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  2. 0

    It looks like TypeScript is narrowing your unknown externalData to any[] when Array.isArray(externalData) is true. (I would have expected unknown[].) Here are a couple of simpler tests:

    const d: unknown = ["something"];
const a = Array.isArray(d) ? d : null;
//    ^? const a: any[] | null

    Notice how it’s clearly the true part of the conditional that’s contributing the any[] type to the union.

    Just going by the documentation (I don’t have a deep history in this part of TypeScript, or really any part), that would seem to be a case that should be caught:

    In some cases where no type annotations are present, TypeScript will fall back to a type of any for a variable when it cannot infer the type…Turning on noImplicitAny however TypeScript will issue an error whenever it would have inferred any…

    I don’t see any issue for this in the issue list, although there are several outstanding issues with the result of narrowing array types (such as this one where ReadonlyArray<T> is narrowed to any[]).

    Above my pay grade whether it’s a bug or limitation, but it’s that narrowing that appears to be what isn’t getting caught.

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