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Filter objects by a property
filter() calls its callback for each array element and returns a new array containing the elements for which the callback returns a truthy value. The callback receives an object here, so access the property you want to test:
type User = { id: number; name: string; active: boolean };
const users: User[] = [
{ id: 1, name: "Ada", active: true },
{ id: 2, name: "Lin", active: false },
];
const activeUsers = users.filter(user => user.active);
// User[]Write an explicit comparison when it makes the rule clearer: user.role === "admin", user.age >= 18, or user.name.startsWith("A"). Because this selects some User values without changing their shape, the result is still User[]; a custom type guard is unnecessary. TypeScript supports both User[] and the equivalent generic form Array<User> in its object types documentation.
Filter a union and narrow its result type
If an array can contain different object shapes and you want only one of them, a boolean callback may select the right values at runtime without giving TypeScript enough information to narrow the result. Give the callback a type predicate, which has the form parameterName is Type:
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type Fish = { swim: () => void; name: string };
type Bird = { fly: () => void; name: string };
type Pet = Fish | Bird;
function isFish(pet: Pet): pet is Fish {
return "swim" in pet;
}
const pets: Pet[] = [];
const fish = pets.filter(isFish); // Fish[]The runtime check must justify the type you claim. TypeScript trusts an explicit predicate; it does not prove that the implementation correctly identifies every value of that type. The narrowing handbook covers type predicates and narrowing with in.
Use a literal tag for a discriminated union
When union members have a shared tag property with distinct literal values, compare that property directly. For example, to keep errors from a result array:
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type Result =
| { kind: "success"; value: string }
| { kind: "error"; message: string };
const results: Result[] = [];
const errors = results.filter(
(result): result is Extract<Result, { kind: "error" }> =>
result.kind === "error"
);The equality check tests the tag’s value, which makes the intended branch explicit. An in check instead tests whether an object has a property; if that property is optional in a union, its presence may not separate the members completely.
Remove null or undefined without losing valid values
For TypeScript 5.5 and later, qualifying callbacks can have their type predicates inferred from the check. For example, this comparison removes only undefined and narrows the result to number[]:
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const values: Array<number | undefined> = [1, undefined, 3];
const definedValues = values.filter(value => value !== undefined);
// number[] with TypeScript 5.5 predicate inferenceThe same approach works for nullable objects:
const maybeUsers: Array<User | null> = [];
const presentUsers = maybeUsers.filter(user => user !== null);
// User[] when the predicate is inferredTypeScript 5.5’s release notes describe inferred predicates for checks such as comparisons with null or undefined. With an older compiler, or when you want a reusable guard, declare the predicate explicitly:
function isDefined<T>(value: T | undefined): value is T {
return value !== undefined;
}
const defined = values.filter(isDefined);Why filter(Boolean) can be wrong
A truthiness check discards every falsy value, not only missing values. For numbers, filter(Boolean) also removes 0; for other data it can remove false or the empty string. If those are valid values, compare against the exact value you intend to exclude, such as value !== undefined, value !== null, or item.enabled === true. The handbook’s narrowing guidance explains why truthiness checks can be ambiguous for primitive values.
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Account for TypeScript version when filtering unions of arrays
TypeScript 5.2 improved method calls on unions of arrays. For a value typed as string[] | number[], calling methods such as filter() can be handled by treating it as (string | number)[]. Consequently, the method result can be an array of the union element type rather than a union of the original array types. See the TypeScript 5.2 release notes for this version-specific behavior.
When a reusable filtering helper is useful
If you repeatedly apply the same abstraction, a small generic helper can accept any array and a boolean predicate:
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function filterItems<T>(
items: T[],
predicate: (item: T) => boolean
): T[] {
return items.filter(predicate);
}This helper preserves the input element type but does not narrow it. If the helper needs to return a narrower type, its callback type must preserve a type predicate rather than only returning boolean. For a one-off filter, calling array.filter() directly is simpler. The TypeScript functions handbook recommends avoiding unnecessary generic type parameters.
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