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consistent-optional-props

Disallow redundant undefined unions on optional properties.

Targeted pattern scope​

This rule targets optional properties that redundantly include undefined in their union type.

Covered nodes include:

  • interface/type property signatures (TSPropertySignature), and
  • class property definitions (PropertyDefinition).

What this rule reports​

This rule reports ? properties whose type union explicitly includes undefined.

Example pattern:

name?: string | undefined;

Because ? already implies undefined, the explicit union member is redundant.

Why this rule exists​

Removing redundant undefined unions keeps type declarations smaller and easier to read.

❌ Incorrect​

interface User {
 name?: string | undefined;
}

class Config {
 timeoutMs?: number | undefined;
}

✅ Correct​

interface User {
 name?: string;
}

class Config {
 timeoutMs?: number;
}

Behavior and migration notes​

This rule reports only and does not provide an autofix.

Migration is straightforward: remove | undefined from optional properties.

Options​

This rule has no options.

Additional examples​

type Options = {
 verbose?: boolean | undefined;
};
// ❌ reported

type OptionsFixed = {
 verbose?: boolean;
};
// ✅ valid

ESLint flat config example​

import etcMisc from "eslint-plugin-etc-misc";

export default [
 {
  plugins: { "etc-misc": etcMisc },
  rules: {
   "etc-misc/consistent-optional-props": "error",
  },
 },
];

When not to use it​

Disable this rule if your codebase intentionally keeps explicit | undefined unions for documentation style.

Package documentation​

Rule catalog ID: R008

Further reading​

Adoption resources​

  • Start at warning level in CI, then move to error after cleanup.
  • Use focused codemods/autofix batches per package or directory.