TypeScript strict null check on field of object in array

Question

In TypeScript, despite checking for null on a nullable field in an object array (with strictNullCheck set to true), the compiler still complains that the 'Object is possibly undefined'. Consider the following:

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'

Link to TS PlayGround (set strictNullCheck on)

However, if we have:

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

then the compiler is happy.

Why is this the desired behavior? And what would be the correct practice here if I don't want to turn strictNullCheck off?

Ryan Cavanaugh · Accepted Answer · 2018-04-21 22:31:30Z

5

TypeScript doesn't track array elements by their index; in other words there's no mechanism in the type system that identifies that two expressions arr[0] are necessarily pointing to the same object.

In general this is a good assumption because the ordering of array elements is not guaranteed to remain the same between any two observations of the same element. We can tell from inspection that e.g. you didn't call sort between those two accesses, but in any other case where two expressions aren't evaluated right after the other, it's not known for sure that arr[0] points to the same object both times.

answered Apr 21, 2018 at 22:31

Ryan Cavanaugh

223k59 gold badges280 silver badges235 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

stav Over a year ago

Thanks, but here the expressions are evaluated one after the other, so I'm just missing the point on why this is different from a var (which can also be mutated by a nested function or such)

Ryan Cavanaugh Over a year ago

You know that, but TypeScript has not implemented a special case to identify that situation

stav Over a year ago

OK, don't complete understand still, but maybe that's not the objective.. what is the recommended practice here then (if I don't want to suppress strictNullCheck)?

Ryan Cavanaugh Over a year ago

Extract it to a const or use the postfix ! operator (arr[0].d!.toString())

Ryan Cavanaugh Over a year ago

It's easier to understand if your mental model of how TS work is "A set of heuristics and tracking rules identify when two expressions resolve to the same object" instead of "TS starts with a perfect model of the universe and discards data on purpose"

Collectives™ on Stack Overflow

TypeScript strict null check on field of object in array

1 Answer 1

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related