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typescript/no-redundant-undefined-default-parameter

Disallow redundant undefined in default parameter union types when the default initializer is definitely defined.

Targeted pattern scope​

This rule targets parameters that have:

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

What this rule reports​

This rule reports defaulted parameters such as:

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

because a definitely defined initializer means runtime undefined input is already handled by the default assignment.

Why this rule exists​

Keeping | undefined on parameters with definitely defined default values adds type noise and can obscure intent. In these cases, the annotation is usually clearer as the non-undefined type.

❌ Incorrect​

function read(value: string | undefined = "x") {
 return value;
}

const pick = (count: number | undefined = 0) => count;

class Box {
 constructor(private label: string | undefined = "box") {}
}

✅ Correct​

function read(value: string = "x") {
 return value;
}

const pick = (count: number = 0) => count;

class Box {
 constructor(private label: string = "box") {}
}

Behavior and migration notes​

This rule is autofixable and also provides suggestions.

The fixer removes undefined from the parameter union type and preserves all remaining union constituents.

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

Options​

This rule has no options.

Additional examples​

const maybe = undefined as string | undefined;
function read(value: string | undefined = maybe) {
 return value;
}
// ✅ valid (initializer may be undefined)

ESLint flat config example​

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

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

When not to use it​

Disable this rule if your team intentionally keeps explicit | undefined for defaulted parameters for stylistic consistency.

Package documentation​

Rule catalog ID: R120

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.