In TypeScript there are several ways to declare a property with ``++undefined++`` value: adding ``++| undefined++`` in the property type or using optional property syntax (``++?++`` after its name). Use ``++| undefined++`` syntax when you want to be explicit that an object has that property, in that case TypeScript compiler will not allow omitting it:

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

let John = { name: "John" }; // will not compile
let John = { name: "John", address: undefined }; // will compile, we want to be explicit when person does not have home
----

Use optional property syntax for properties holding some additional information.

----
interface Person {
  name: string;
  pet?: string;
}

let John = { name: "John" }; // will compile
let John = { name: "John", pet: undefined }; // will compile, there is no pet like for the object on previous line
let John = { name: "John", pet: "Benji" }; // will compile
----

Using ``++| undefined++`` for optional property is redundant, it can be omitted without change to the actual type. Still if you want to force the property in the object consider using only ``++| undefined++`` without ``++?++``.


== Noncompliant Code Example

----
interface Person {
  name: string;
  address? : string | undefined;   // Noncompliant, "?" should be removed
  pet?: Animal | undefined; // Noncompliant, "undefined" should be removed
}
----


== Compliant Solution

----
interface Person {
  name: string;
  address: string | undefined;
  pet?: Animal;
}
----

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== Implementation Specification
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