prefer-type-fest-set-optional
Require TypeFest SetOptional<T, Keys> over imported aliases like PartialBy.
Targeted pattern scopeโ
This rule scopes matching to imported legacy aliases that model the same semantics as SetOptional<T, Keys>.
- Type references that resolve to imported
PartialByaliases.
Locally defined lookalikes or unrelated type references are excluded unless they resolve to the targeted imported alias.
What this rule reportsโ
This rule reports imported alias usages that should migrate to SetOptional<T, Keys>.
- Type references that resolve to imported
PartialByaliases.
Why this rule existsโ
SetOptional is the canonical TypeFest utility for making selected keys optional. Standardizing on it improves discoverability and keeps utility naming consistent across projects.
โ Incorrectโ
import type { PartialBy } from "type-aliases";
type PartialUser = PartialBy<User, "email">;
โ Correctโ
import type { SetOptional } from "type-fest";
type PartialUser = SetOptional<User, "email">;
Behavior and migration notesโ
SetOptional<T, Keys>marks a selected subset of keys optional while preserving all other key modifiers.- This rule targets imported alias names that represent the same semantics (
PartialBy). - Use this utility for draft/update shapes where only specific fields become optional.
Additional examplesโ
โ Incorrect โ Additional exampleโ
import type { PartialBy } from "type-aliases";
type Draft = PartialBy<User, "email">;
โ Correct โ Additional exampleโ
import type { SetOptional } from "type-fest";
type Draft = SetOptional<User, "email">;
โ Correct โ Repository-wide usageโ
type PartialAddress = SetOptional<Address, "line2">;
ESLint flat config exampleโ
import typefest from "eslint-plugin-typefest";
export default [
{
plugins: { typefest },
rules: {
"typefest/prefer-type-fest-set-optional": "error",
},
},
];
When not to use itโ
Disable this rule if external contracts require preserving existing aliases.
Package documentationโ
TypeFest package documentation:
Source file: source/set-optional.d.ts
/**
* Create a type that makes the given keys optional. The remaining keys are kept
* as is. The sister of the `SetRequired` type.
*
* Use-case: You want to define a single model where the only thing that changes
* is whether or not some of the keys are optional.
*
* @category Object
*
* @example
* ```
* import type {SetOptional} from 'type-fest';
*
* type Foo = {
* a: number;
* b?: string;
* c: boolean;
* };
*
* type SomeOptional = SetOptional<Foo, 'b' | 'c'>;
* // type SomeOptional = {
* // a: number;
* // b?: string; // Was already optional and still is.
* // c?: boolean; // Is now optional.
* // }
* ```;
*/
Rule catalog ID: R063