== 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.

[source,javascript]
----
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`.

[source,javascript]
----
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`.

[source,javascript,diff-id=1,diff-type=noncompliant]
----
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.

[source,javascript,diff-id=1,diff-type=compliant]
----
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 - https://www.typescriptlang.org/docs/handbook/2/objects.html#optional-properties[Optional Properties]
* TypeScript Documentation - https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#union-types[Union Types]
* TypeScript Documentation - https://www.typescriptlang.org/tsconfig/#exactOptionalPropertyTypes[exactOptionalPropertyTypes]

ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

Consider removing 'undefined' type or '?' specifier, one of them is redundant.


=== Highlighting

Primary: "?"

Secondary: "undefined"


endif::env-github,rspecator-view[]
