Parameters<> utility type does not respect exactOptionalPropertyTypes

[nemosmithasf]

Bug Report

🔎 Search Terms

• exactOptionalPropertyTypes
• Parameters
• Utility Type

🕗 Version & Regression Information

4.7.4

⏯ Playground Link

Playground link with relevant code

💻 Code

const temp = function (param?: string) {}; // (param?: string) => void

type temp = Parameters<typeof temp>; // [param?: string | undefined]
                                                       // ^^^^^^^^^ Shouldn't be here

🙁 Actual behavior

undefined is included in the Parameters<> output even when exactOptionalPropertyTypes is on

🙂 Expected behavior

The output should match the function parameters exactly, so no undefined value

[MartinJohns]

That's a parameter, not a property. exactOptionalPropertyTypes does not matter here. Also, undefined is currently a valid value for an optional parameter, until #12400 is implemented.

This is perfectly valid:

const temp = function (param?: string) {}; // (param?: string) => void

temp(undefined);

When you hover over param you can also see that undefined is a valid type for it:

[nemosmithasf]

Hmm, strange. It seems 4.7.4 will not automatically append | undefined to optional params in the function type; but 4.6.4 does.

Also, I also see where you're coming from, but if you think about it intuitively: if you want a tuple representation of a set of parameters, then optional parameters should translate to optional properties... in which case exactOptionalPropertyTypes should apply to the optional properties of the tuple.

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.