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typescript/no-redundant-undefined-optional

Disallow redundant undefined in optional parameter and tuple member union types.

Targeted pattern scope​

This rule targets optional signatures where undefined is already implied:

  • optional function/method parameters, for example value?: string | undefined
  • optional tuple members, for example [value?: string | undefined]
  • optional tuple element syntax, for example [(string | undefined)?]

It intentionally does not report optional object/class properties, because that scope is handled by consistent-optional-props.

What this rule reports​

This rule reports optional parameters and tuple members whose type unions include undefined explicitly.

Why this rule exists​

Optional parameters and optional tuple members already include undefined semantics. Keeping | undefined in those unions is usually redundant noise that hurts readability.

❌ Incorrect​

function parse(value?: string | undefined) {}

type Pair = [value?: string | undefined];

type MaybeName = [(string | undefined)?];

✅ Correct​

function parse(value?: string) {}

type Pair = [value?: string];

type MaybeName = [string?];

Behavior and migration notes​

This rule is autofixable and also provides suggestions.

The fixer removes undefined from the matching union and preserves remaining constituents in place.

Options​

This rule has no options.

Additional examples​

function load(id?: string | number | undefined) {
 return id;
}
// ❌ reported

function load(id?: string | number) {
 return id;
}
// ✅ valid

ESLint flat config example​

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

export default [
 {
  plugins: { "etc-misc": etcMisc },
  rules: {
   "etc-misc/typescript/no-redundant-undefined-optional": "warn",
  },
 },
];

When not to use it​

Disable this rule if your team intentionally keeps explicit | undefined in optional parameters or tuple members for stylistic consistency.

Package documentation​

Rule catalog ID: R095

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.