TypeScript type narrowing does not work with Array.filter for null/undefined

Problem

After filtering out null/undefined values from an array, TypeScript still considers the result type as (T | null)[] instead of T[].

Symptoms

• Type still includes null after filter
• Object is possibly null after filtering
• Boolean filter does not narrow type

Stack

Solution

Use a type predicate in the filter callback. TypeScript cannot infer type narrowing from Boolean or truthy checks in filter().

Code

const items: (string | null)[] = ["a", null, "b", null, "c"];

// BAD: result is still (string | null)[]
const bad = items.filter(Boolean);
const bad2 = items.filter(x => x !== null);

// GOOD: use type predicate
const good = items.filter((x): x is string => x !== null);
// good is string[]

// Reusable helper:
function nonNullable<T>(value: T): value is NonNullable<T> {
  return value !== null && value !== undefined;
}
const result = items.filter(nonNullable); // string[]

Caveats

TypeScript 5.5+ may improve type narrowing in filter() callbacks, but type predicates remain the most reliable approach.