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prefer-ts-extras-is-defined-filter

Require isDefined from ts-extras in Array.prototype.filter callbacks instead of inline undefined checks.

Targeted pattern scopeโ€‹

This rule only inspects inline .filter(...) predicates that perform explicit undefined checks.

  • Inline undefined predicates inside .filter(...), including:
  • filter((value) => value !== undefined)
  • filter((value) => typeof value !== "undefined")
  • filter((value): value is T => value !== undefined)

Named predicate references and broader callback logic are not matched unless they keep this exact inline-check structure.

What this rule reportsโ€‹

This rule reports inline filter predicates that encode undefined checks and can be normalized with isDefined.

  • Inline undefined predicates inside .filter(...), including:
  • filter((value) => value !== undefined)
  • filter((value) => typeof value !== "undefined")
  • filter((value): value is T => value !== undefined)

Why this rule existsโ€‹

filter(isDefined) is a canonical narrowing form that removes repeated callback boilerplate.

  • Filter chains are shorter and easier to scan.
  • Narrowing to defined values is consistent.
  • Inline predicate duplication is eliminated.

โŒ Incorrectโ€‹

const values = inputs.filter((value) => value !== undefined);

โœ… Correctโ€‹

const values = inputs.filter(isDefined);

Behavior and migration notesโ€‹

  • array.filter(isDefined) keeps elements where value is not undefined.
  • Manual predicate bodies with additional side effects should not be auto-converted.
  • Non-filter undefined checks belong to prefer-ts-extras-is-defined.

Additional examplesโ€‹

โŒ Incorrect โ€” Additional exampleโ€‹

const ids = maybeIds.filter((id) => id !== undefined);

โœ… Correct โ€” Additional exampleโ€‹

const ids = maybeIds.filter(isDefined);

โœ… Correct โ€” Repository-wide usageโ€‹

const values = readings.filter(isDefined);

ESLint flat config exampleโ€‹

import typefest from "eslint-plugin-typefest";

export default [
    {
        plugins: { typefest },
        rules: {
            "typefest/prefer-ts-extras-is-defined-filter": "error",
        },
    },
];

When not to use itโ€‹

Disable this rule if your filters intentionally use named local predicates for domain-specific semantics.

Package documentationโ€‹

ts-extras package documentation:

Source file: source/is-defined.ts

/**
Check whether a value is defined, meaning it is not `undefined`.

This can be useful as a type guard, as for example, `[1, undefined].filter(Boolean)` does not always type-guard correctly.

@example
```
import {isDefined} from 'ts-extras';

[1, undefined, 2].filter(isDefined);
//=> [1, 2]
```

@category Type guard
*/

Rule catalog ID: R015

Further readingโ€‹

Adoption resourcesโ€‹