Ensuring TypeScript's strict null check on a field within an object that is part of an

When using TypeScript and checking for null on a nullable field inside an object array (where strictNullCheck is set to true), the compiler may still raise an error saying that 'Object is possibly undefined'. Here's an example:

interface IA {
    d?: Date
}
const arr : IA[] = [{d: new Date()}];

console.error(arr[0].d == null ? "" : arr[0].d.toString());  
                                           // ^ complains that "Object is possibly 'undefined'

Try it on TS PlayGround (with strictNullCheck enabled)

Strangely enough, if we write:

const a : IA = {d: new Date()};
console.error(a.d == null ? "" : a.d.toString());

then the compiler does not show any errors.

Why does this happen? And what would be the best approach in this scenario if we want to keep strictNullCheck activated?

typescript

Answer №1

TypeScript does not keep track of array elements based on their index, meaning the type system does not recognize that two expressions like arr[0] are necessarily referencing the same object.

This lack of tracking is actually a positive feature because the order of array elements is not guaranteed to remain consistent between different observations. Even if it appears that you did not use the sort function between accessing two elements, in other cases where expressions are not evaluated consecutively, there is no certainty that arr[0] is pointing to the same object both times.

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