I am trying to convince typescript that an item cannot be `null` / `undefined` when using a filter, but typescript does not trust me. Am I missing something? Is typescript stupid, or am I? 🤔 The only way I could get this working is by force-casting a type with `as`. Can you suggest a more elegant way?

test1-output.txt

src/test.ts:10:39 - error TS18049: 'item' is possibly 'null' or 'undefined'.

10 strings.forEach((item) => console.log(item.id, item.label))
                                         ~~~~

src/test.ts:10:48 - error TS18049: 'item' is possibly 'null' or 'undefined'.

10 strings.forEach((item) => console.log(item.id, item.label))
                                                  ~~~~


Found 2 errors in the same file, starting at: src/test.ts

test1.ts

const stringsAndNull = [
  { id: 1, label: 'foo' },
  { id: 2, label: 'bar' },
  null,
  { id: 3, label: 'test' },
  undefined,
  { id: 3, label: 'lol' },
]
const strings = stringsAndNull.filter((item) => item != null)
strings.forEach((item) => console.log(item.id, item.label))

test2-output.txt

src/test.ts:10:17 - error TS2345: Argument of type '(item: {    id: number;    label: string;}) => void' is not assignable to parameter of type '(value: { id: number; label: string; } | null | undefined, index: number, array: ({ id: number; label: string; } | null | undefined)[]) => void'.
  Types of parameters 'item' and 'value' are incompatible.
    Type '{ id: number; label: string; } | null | undefined' is not assignable to type '{ id: number; label: string; }'.
      Type 'undefined' is not assignable to type '{ id: number; label: string; }'.

10 strings.forEach((item: { id: number, label: string}) => console.log(item.id, item.label))
                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

test2.ts

const stringsAndNull = [
  { id: 1, label: 'foo' },
  { id: 2, label: 'bar' },
  null,
  { id: 3, label: 'test' },
  undefined,
  { id: 3, label: 'lol' },
]
const strings = stringsAndNull.filter((item) => item != null)
strings.forEach((item: { id: number, label: string }) => console.log(item.id, item.label))

How about this?

const isDefined = <T>(item: T | undefined | null): item is T => item != null;
const strings = stringsAndNull.filter(isDefined);
strings.forEach((item) => console.log(item.id, item.label));

This seems to work! 🤩

Thank you very much! 👍 👍 👍