prefer-type-fest-less-than-or-equal

Require TypeFest LessThanOrEqual<A, B> over boolean wrappers built from GreaterThan<A, B>.

Targeted pattern scope

This rule targets equivalent conditional wrappers such as GreaterThan<A, B> extends true ? false : true and infer-wrapped variants.

What this rule reports

Conditional types equivalent to LessThanOrEqual<A, B> that are composed from GreaterThan<A, B>.

Why this rule exists

LessThanOrEqual is the canonical TypeFest numeric comparison helper for <= checks. Using it directly improves readability and avoids repeated wrapper patterns.

❌ Incorrect

import type { GreaterThan } from "type-fest";

type IsLessOrEqual = GreaterThan<1, 2> extends true ? false : true;

✅ Correct

import type { LessThanOrEqual } from "type-fest";

type IsLessOrEqual = LessThanOrEqual<1, 2>;

Behavior and migration notes

This rule targets wrappers equivalent to LessThanOrEqual<A, B>.
It handles direct and infer-wrapped conditional forms.
It does not report wrappers whose boolean branches do not match <= semantics.

Additional examples

❌ Incorrect — Additional example

import type { GreaterThan } from "type-fest";

type IsLessOrEqual =
    GreaterThan<4, 9> extends infer Result
        ? Result extends true
            ? false
            : true
        : never;

✅ Correct — Additional example

import type { LessThanOrEqual } from "type-fest";

type IsLessOrEqual = LessThanOrEqual<4, 9>;

✅ Correct — Repository-wide usage

type IsNonPositive<N extends number> = LessThanOrEqual<N, 0>;

ESLint flat config example

import typefest from "eslint-plugin-typefest";

export default [
    {
        plugins: { typefest },
        rules: {
            "typefest/prefer-type-fest-less-than-or-equal": "error",
        },
    },
];

When not to use it

Disable this rule if your codebase intentionally standardizes on custom numeric wrapper types.

Package documentation

TypeFest package documentation:

Source file: source/less-than-or-equal.d.ts

/**
Returns a boolean for whether a given number is less than or equal to another number.
*/
export type LessThanOrEqual<A extends number, B extends number> = ...

Rule catalog ID: R085

prefer-type-fest-less-than-or-equal

Targeted pattern scope

What this rule reports

Why this rule exists

❌ Incorrect

✅ Correct

Behavior and migration notes

Additional examples

❌ Incorrect — Additional example

✅ Correct — Additional example

✅ Correct — Repository-wide usage

ESLint flat config example

When not to use it

Package documentation

Further reading

Adoption resources

Targeted pattern scope​

What this rule reports​

Why this rule exists​

❌ Incorrect​

✅ Correct​

Behavior and migration notes​

Additional examples​

❌ Incorrect — Additional example​

✅ Correct — Additional example​

✅ Correct — Repository-wide usage​

ESLint flat config example​

When not to use it​

Package documentation​

Further reading​

Adoption resources​

Targeted pattern scope

What this rule reports

Why this rule exists

❌ Incorrect

✅ Correct

Behavior and migration notes

Additional examples

❌ Incorrect — Additional example

✅ Correct — Additional example

✅ Correct — Repository-wide usage

ESLint flat config example

When not to use it

Package documentation

Further reading

Adoption resources