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typescript/no-redundant-undefined-return-type

Disallow redundant undefined in return type unions when a function deterministically returns a definitely-defined value.

Targeted pattern scope​

This rule targets functions with:

  • an explicit return type union including undefined, and
  • a function body that deterministically returns a syntactically definitely defined value.

The current deterministic patterns are intentionally conservative:

  • expression-bodied functions returning definitely defined expressions
  • block-bodied functions with exactly one return statement whose argument is definitely defined

What this rule reports​

This rule reports cases such as:

  • (): string | undefined => "x"
  • function f(): number | undefined { return 1; }

because the implementation deterministically returns a defined value, making | undefined redundant.

Why this rule exists​

When function implementations deterministically return defined values, keeping | undefined in the return type adds type noise and weakens API clarity.

❌ Incorrect​

const read = (): string | undefined => "x";

function count(): number | undefined {
 return 1;
}

class Box {
 get label(): string | undefined {
  return "box";
 }
}

✅ Correct​

const read = (): string => "x";

function count(): number {
 return 1;
}

class Box {
 get label(): string {
  return "box";
 }
}

Behavior and migration notes​

This rule is autofixable and also provides suggestions.

The fixer removes undefined from the top-level return type union while preserving remaining union members.

To avoid unsafe assumptions, this rule intentionally does not report complex control-flow cases where proving deterministic non-undefined return values is not trivial.

Options​

This rule has no options.

Additional examples​

const value: string | undefined = undefined;
const read = (): string | undefined => value;
// ✅ valid (returned identifier may be undefined)

function read(): string | undefined {
 const value = "x";
 return value;
}
// ✅ valid (multiple statements in block body are out of conservative scope)

ESLint flat config example​

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

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

When not to use it​

Disable this rule if your team intentionally keeps explicit | undefined in return types for stylistic consistency, even when current implementations are deterministically defined.

Package documentation​

Rule catalog ID: R098

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.