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typescript/no-redundant-undefined-const

Disallow redundant undefined in const declaration union types when the initializer is definitely defined.

Targeted pattern scope​

This rule targets const declarations that combine:

  • a union type including undefined, and
  • a syntactically definitely defined initializer (for example string, number, object, array, template literal, function, class, or new value).

What this rule reports​

This rule reports declarations such as:

  • const value: string | undefined = "x";
  • const count: number | undefined = 0;

because the initializer is definitely defined, making | undefined redundant.

Why this rule exists​

For definitely defined const initializers, keeping | undefined in the annotation adds type noise and obscures intent without improving safety.

❌ Incorrect​

const value: string | undefined = "x";
const count: number | undefined = 0;
const config: { enabled: boolean } | undefined = { enabled: true };

✅ Correct​

const value: string = "x";
const count: number = 0;
const config: { enabled: boolean } = { enabled: true };

Behavior and migration notes​

This rule is autofixable and also provides suggestions.

The fixer removes undefined from the union annotation and preserves the other constituents.

To avoid unsafe assumptions, this rule intentionally does not report initializers that may evaluate to undefined (for example identifiers, calls, or other non-guaranteed-defined expressions).

This rule has no options.

Additional examples​

const maybe = Math.random() > 0.5 ? "x" : undefined;
const value: string | undefined = maybe;
// ✅ valid (initializer can be undefined)

let mutableValue: string | undefined = "x";
// ✅ valid (rule only targets const declarations)

ESLint flat config example​

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

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

When not to use it​

Disable this rule if your team intentionally keeps explicit | undefined on const declarations as a style convention.

Package documentation​

Rule catalog ID: R092

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.