Why is this an issue?

In TypeScript, there are two ways to define properties or parameters that are potentially undefined:

• Union with undefined: Adding | undefined in the property type makes the property required, but can be undefined. Use this syntax when you want to be explicit that an object should provide that property, in which case the TypeScript compiler will not allow omitting it.

interface Person {
  name: string;
  address: string | undefined;
}

let John = { name: "John", address: undefined };

• Optional property syntax (? after its name): The property is optional, which means that an object can omit it and let the TypeScript compiler provide it as being undefined.

interface Person {
  name: string;
  address?: string;
}

let John = { name: "John" };

This rule checks for optional property declarations that use both the ? syntax and unions with undefined.

interface Person {
  name: string;
  address?: string | undefined;   // Noncompliant: using both syntaxes is redundant
}

Choose one of the syntaxes to declare optional properties and remove the other one. Consider using only | undefined if you want to make the property explicit in the object.

interface Person {
  name: string;
  address?: string;
}

The rule does not raise any issues when the TypeScript compiler option exactOptionalPropertyTypes is enabled because this option ensures that undefined does not become redundant in this context.

Resources

Documentation

• TypeScript Documentation - Optional Properties
• TypeScript Documentation - Union Types
• TypeScript Documentation - exactOptionalPropertyTypes