Skip to main content

typescript/no-redundant-undefined-let

Disallow redundant undefined in let type unions when declarations are never reassigned and initialized with definitely-defined values.

Targeted pattern scope​

This rule targets let declarations that are:

  • initialized with a syntactically definitely-defined value (for example string, number, object, array, template literal, function, class, or new value),
  • explicitly typed as a union including undefined, and
  • never reassigned after declaration.

What this rule reports​

This rule reports declarations such as:

  • let value: string | undefined = "x";

when the variable is never reassigned and initializer is definitely defined. In those cases, | undefined is redundant.

Why this rule exists​

When let is used for values that are never reassigned and initialized with definite values, retaining | undefined adds type noise and weakens type precision.

❌ Incorrect​

let value: string | undefined = "x";
let count: number | undefined = 1 as const;

✅ Correct​

let value: string = "x";
let count: number = 1 as const;

Behavior and migration notes​

This rule is autofixable and also provides suggestions.

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

To avoid unsafe assumptions, this rule does not report cases where:

  • the initializer might evaluate to undefined, or
  • the variable is reassigned later.

Options​

This rule has no options.

Additional examples​

let value: string | undefined = "x";
value = "y";
// ✅ valid (reassigned)

const maybe = undefined as string | undefined;
let output: string | undefined = maybe;
// ✅ 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-let": "warn",
  },
 },
];

When not to use it​

Disable this rule if your team intentionally keeps explicit | undefined in never-reassigned let annotations for stylistic consistency.

Package documentation​

Rule catalog ID: R094

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.