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filter(Boolean) removes more than null and undefined: it also drops values such as 0, false, NaN, 0n, and the empty string. If those values are valid data, use values.filter(value => value != null) to remove only nullish entries.
Use a nullish predicate when only null and undefined should be removed
For example, this keeps zero while excluding both nullish values:
const counts: Array<number | null | undefined> = [0, 1, null, undefined];
const present = counts.filter(value => value != null);
// [0, 1]In JavaScript and TypeScript, the loose comparison value != null intentionally matches both null and undefined. If you prefer to avoid loose equality, the equivalent explicit check is:
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);Choose the predicate according to what the data means. If an empty string, zero, or false is a valid value, do not use a truthiness filter to clean the array.
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What filter(Boolean) removes at runtime
Array.prototype.filter calls its callback for each array element and retains the element when the callback result is truthy. Passing Boolean directly converts each value to a boolean, so the filter discards every value that converts to false—not just nullish ones. See the JavaScript Boolean reference for the conversion rules.
Falsy values include:
false0and-00nNaN"", the empty stringnullundefined
For instance, [0, 1, null].filter(Boolean) produces [1]. If zero represents a real count, that is data loss rather than null cleanup.
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How TypeScript’s type inference affects the result
Runtime filtering and static typing are related but separate. JavaScript determines which elements remain using truthiness. TypeScript analyzes the callback to decide whether it can narrow the resulting array’s element type.
TypeScript 5.5 introduced inferred type predicates for qualifying functions, including suitable callbacks passed to filter. Its 5.5 release notes demonstrate filtering out undefined and explain the limits of truthiness checks: “A truthiness check will infer a type predicate for object types, where there’s no ambiguity.” That qualification matters. With primitive unions, a truthiness check also excludes valid falsy values, so it does not express a general “not nullish” test.
Do not assume every boolean-returning callback is a type guard, or that type inference makes filter(Boolean) behave like a nullish check. The explicit comparison states the intended runtime condition and gives TypeScript a check it can analyze under its inference rules.
When truthiness filtering is appropriate
Truthiness filtering can be appropriate when every falsy value is meant to be discarded. For example, a collection of objects with optional nullish entries may be a suitable case because ordinary objects are truthy. But for a collection that can contain strings, numbers, or booleans, decide whether empty strings, zero, or false are meaningful before filtering by truthiness.
If you want a reusable nullish check, make its type predicate match its implementation:
Do these 3 things before closing this tab:
1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minutefunction isNonNullish<T>(value: T): value is NonNullable<T> {
return value !== null && value !== undefined;
}
const present = values.filter(isNonNullish);The TypeScript Handbook’s narrowing guide covers truthiness narrowing and explicit null checks. Its introduction describes the Handbook as a comprehensive guide to TypeScript features and behavior, rather than a complete language specification.
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