The functionality of verifying the type of an item in a list using Typescript is not functioning

In my TypeScript code, I am working with a type called NameValue and another one called MixedStuff.

type NameValue = { name: string; value: string };
type MixedStuff = NameValue | string;

function stripTwoChars(stuffs: MixedStuff[]): string {

    let stuffZero = stuffs[0];
    return (typeof stuffZero === "string" ?
        stuffZero.slice(0, 2) :
        stuffZero.name.slice(0, 2)); // This works fine

    return (typeof stuffs[0] === "string" ?
        stuffs[0].slice(0, 2) : // Here error: no slice on NameValue
        stuffs[0].name.slice(0, 2)); // Here error: no name on string
}

In the function, I first assign the first item in the list to a variable and then perform a typeof check on it, which functions as expected.

However, when I directly perform the typeof check on the first element of the list, it does not work correctly.

I am curious to know if this behavior is intentional or if there could be a bug causing this inconsistency.

Answer №1

Indeed, this behavior is intentional. While the idea of type-guards for array access expressions was considered, it was ultimately decided against. More details can be found in the discussion here

The decision was based on performance considerations. It is generally recommended to use const j = list[i] as an alternative approach, which should not pose significant challenges.

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