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filter(Boolean) removes more than null and undefined: it also drops 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 check when you mean “not null or undefined”
For an array where zero, false, or an empty string must be preserved, filter with an explicit nullish condition:
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const present = values.filter(value => value != null);In JavaScript and TypeScript, value != null matches both null and undefined. If your codebase avoids loose equality, the equivalent explicit form is:
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);For reuse, define a type predicate whose condition exactly matches its type claim:
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function isNonNullish<T>(value: T): value is NonNullable<T> {
return value !== null && value !== undefined;
}
const present = values.filter(isNonNullish);What filter(Boolean) actually removes
Array.prototype.filter calls its callback for each element and keeps the element when the callback result is truthy. Passing Boolean directly converts each element to a boolean; it is a truthiness test, not a nullish test. JavaScript falsy values include:
false0and-00nNaN""(the empty string)nullundefined
Thus, a truthiness filter is appropriate only when every falsy value should be discarded. The MDN Boolean reference documents the conversion behavior and falsy values.
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Why the distinction matters for TypeScript arrays
Consider counts where zero is a meaningful value:
const counts: Array<number | null | undefined> = [0, 1, null, undefined];
const truthy = counts.filter(Boolean); // runtime result: [1]
const nonNullish = counts.filter(value => value != null); // [0, 1]The first filter silently loses the valid count 0. The same problem arises if an array’s domain treats false or "" as meaningful. Choose the predicate according to what the data means, rather than using truthiness as shorthand for “present.”
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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsFor object-only arrays whose optional members are null or undefined, truthiness can be an effective runtime filter: ordinary objects are truthy. But an explicit nullish check states the intended exclusion more precisely and remains correct if the element type later includes meaningful primitive values.
What TypeScript infers depends on the check and version
Runtime filtering and static type narrowing are related but distinct. JavaScript decides which elements survive based on the callback’s result. TypeScript analyzes callback forms under its type-predicate inference rules; a boolean-returning callback is not automatically a type guard in every case.
TypeScript 5.5 introduced inferred type predicates for suitable functions, and its release notes demonstrate filtering an optional value with .filter(score => score !== undefined). They also explain the scope of truthiness inference: “A truthiness check will infer a type predicate for object types, where there’s no ambiguity.” That object-specific qualification does not make a truthiness check a general-purpose nullish filter for primitive unions. See the TypeScript 5.5 release notes for the version-specific behavior.
For broader guidance on truthiness narrowing and explicit null checks, consult the official TypeScript Handbook section on narrowing.
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