When looking at the following code snippet
type stringUndefined = "string" | undefined;
type What<T> = T extends undefined ? "true" : "false";
const no : What<stringUndefined> = "";
The value of 'no' ends up being
"true" | "false""true" as anticipated.
You can try running this code in TS-Playground
https://i.sstatic.net/kWgQS.png
Edit:
It is important to note that strict null checks are enabled
"string" | undefined does not have the same extension as undefined, as it includes the possibility of being "string".
On the other hand, undefined does extend "string" | undefined because the components of a union type are considered extensions or refinements of the union itself. Therefore:
type StringLiteralOrUndefined = "string" | undefined;
type What<T> = undefined extends T ? true : false;
type X = What<StringLiteralOrUndefined>;
// ^? type X = true
type UnrelatedType = string | number;
type Y = What<UnrelatedType>;
// ^? type Y = false
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