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typescript/no-redundant-undefined-readonly-property

Disallow redundant undefined in readonly property union types when the initializer is definitely defined.

Targeted pattern scope​

This rule targets class fields that are:

  • declared readonly,
  • not optional (? is absent),
  • annotated as a union including undefined, and
  • initialized with a syntactically definitely defined value (for example string, number, object, array, template literal, function, class, or new value).

What this rule reports​

This rule reports declarations such as:

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

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

Why this rule exists​

Readonly fields with definitely defined initializers already have a concrete value at declaration time. Keeping | undefined in those annotations adds type noise and weakens type precision.

❌ Incorrect​

class Box {
 readonly value: string | undefined = "x";
 static readonly count: number | undefined = 0;
}

✅ Correct​

class Box {
 readonly value: string = "x";
 static readonly count: number = 0;
}

Behavior and migration notes​

This rule is autofixable and also provides suggestions.

The fixer removes undefined from the readonly property union type while preserving the remaining union constituents.

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

This rule has no options.

Additional examples​

const maybe = undefined as string | undefined;

class Box {
 readonly value: string | undefined = maybe;
 // ✅ valid (initializer may be undefined)

 value2: string | undefined = "x";
 // ✅ valid (not readonly)
}

ESLint flat config example​

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

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

When not to use it​

Disable this rule if your team intentionally preserves explicit | undefined on readonly property annotations for stylistic consistency.

Package documentation​

Rule catalog ID: R097

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.