No Runtime Typeof

Error

small-rules/no-runtime-typeof

Disallow runtime typeof checks; external values must be decoded into meaningful types at their I/O boundary.

Rationale

Ported from dmmulroy/anti-slop. A typeof check narrows a representation but doesn’t establish the value’s full contract. Parse at the I/O boundary instead. Set allowInTypeGuards only for schema-free codebases that use type-predicate helpers.

Diagnostic Messages

runtimeTypeof

A `typeof` check narrows a representation without establishing its contract. Parse input at its I/O boundary, then branch on the domain value.

Configuration

This rule accepts one options object after the severity.

allowInTypeGuardsOptional

Allow `typeof` inside functions whose return type is a type predicate (`value is T`) or an assertion predicate (`asserts value is T`).

Type

boolean

Default

false

.oxlintrc.json

{
  "jsPlugins": [
    "@pobammer-ts/small-rules"
  ],
  "rules": {
    "small-rules/no-runtime-typeof": [
      "error",
      {
        "allowInTypeGuards": false
      }
    ]
  }
}

oxlint.config.ts

import { defineConfig } from "oxlint";

export default defineConfig({
  jsPlugins: ["@pobammer-ts/small-rules"],
  rules: {
    "small-rules/no-runtime-typeof": [
      "error",
      {
        allowInTypeGuards: false,
      },
    ],
  },
});

Examples

ad hoc typeof narrowing

if (typeof input === 'string') use(input);

explicit type-guard exception

function isString(value: unknown): value is string { return typeof value === "string"; }