No Known Value Widening

Error

small-rules/no-known-value-widening

Disallow syntactically established values from flowing into explicitly broad or anonymous target types that discard useful evidence.

Rationale

Ported from dmmulroy/anti-slop. A written unknown, object, open map, or unnamed object type can wipe what an object you wrote still shows. Let the type be what TS infers. Or use satisfies to check a wider contract and keep the known keys.

Diagnostic Messages

widening

The explicit {{target}} type on {{subject}} discards known type evidence. Keep inference, validate with `satisfies`, or use a named owner contract.

Configuration

This rule does not accept options.

.oxlintrc.json

{
  "jsPlugins": [
    "@pobammer-ts/small-rules"
  ],
  "rules": {
    "small-rules/no-known-value-widening": "error"
  }
}

oxlint.config.ts

import { defineConfig } from "oxlint";

export default defineConfig({
  jsPlugins: ["@pobammer-ts/small-rules"],
  rules: {
    "small-rules/no-known-value-widening": "error",
  },
});

Examples

known value annotated unknown

const value: unknown = {};

satisfies keeps the known keys

type Command = () => void;
const startCommand = () => {};
const commands = { start: startCommand } satisfies Record<string, Command>;