union with undefined not shown in type hint tooltip regression bug

[jessetabak]

🔎 Search Terms

"type hints union undefined"

🕗 Version & Regression Information

• This changed between versions 5.4.5 and 5.5.4

⏯ Playground Link

https://www.typescriptlang.org/play/?target=7&ts=5.5.4&ssl=10&ssc=1&pln=11&pc=1#code/C4TwDgpgBAShCOBXAlgJwgEwNIRAZwB4AVKCAD2AgDsM8oB7AIwCsIBjYAPigF4oBvAFBQRUANpYoyKlADWuegDNYCFOgzFOAXQBcUIhK0BuQQF8Tg0JCgAxevV4DBASACGAfj15gqaQHMTZ0ZPKCpEAFtGCFRAti8ff0CMPUZ7ABsIVxkAHyhEGghFaUwTU0FBAHoAKkEAUTJIDkwoKwg9S3BoACFXVEchUShXeN8qPyhc-IxC4owjKAICHj4pmapm5Dpwzbx-KRkAN2jd+hlOPgBWADprgBYoAApTtJB9loALaFaod+lgOmA6WAyDAAEphKJGHowpFohM8gUius5gslitEbMpFsdntpFAjqgTmdLjcrvcnlQXm9gJ8Wp0fn8AUCQeDBnEoN5RgEISJklBUvQMll4askSUzIIqhUOtYen0+HAkGhMDh8AQ7PROBYKhUoABhADyMBgtT1RGhDmiqHoqAANFAihA0hghkMaPyhugGGBgadXGlBGxTt4HfY9Br+jyoOyAOSMLIJmO2qN8nyICASyq6w3G03m0KW1DWu0O5BOl2uN0uxie6DoJXqfmIYAI6Zil2bIZpNL0ADumEDwZb8dQejlkcGw1ba0wycGUOn7bnolj8aoieXvL0aYzZSz+qNJrNeitNvtjudrqy1drUHragHQaoIZHACYx70Jyu9HGE+ukym26oOmEpAA

💻 Code

type RequiredKeys<T extends object> = {
    [K in keyof Required<T>]: T[K];
};

type Foo = {
	a?: string;
	b?: number;
	c: string;
	d: boolean | undefined;
}

/*
Expected type: 
type Bar = {
    a: string | undefined; <<== undefined is missing in version >= 5.5.4 (only in the type hints tooltip)
    b: number | undefined; <<== undefined is missing in version >= 5.5.4 (only in the type hints tooltip)
    c: string;
    d: boolean | undefined;
}
*/
type Bar = RequiredKeys<Foo>;

// CORRECT: no error, field a and b are optional
const foo: Foo = {
    c: 'banana',
    d: true
}

// CORRECT: no error, field a and b are required but undefined is allowed
const bar: Bar = {
    a: undefined,
    b: undefined,
    c: 'banana',
    d: true
}

// CORRECT: error, field a and b are required
const bar2: Bar = {
    c: 'banana',
    d: true
}

🙁 Actual behavior

The purpose of RequiredKeys is to make all optional fields required, but leave the union with undefined intact.
The actual behavior is that it also removes the union with undefined, but only in the type hint tooltip, the compiler and type instances do work.

This issue started from version 5.5.4 and up.

This is the incorrect type in the type hint tooltip, fields a and b should have a union with undefined:

type Bar = {
    a: string;
    b: number;
    c: string;
    d: boolean | undefined;
}

🙂 Expected behavior

It's expected that the type hint tooltip shows the union with undefined for fields a and b.

This is the correct type in the type hint tooltip, fields a and b have a union with undefined:

type Bar = {
    a: string | undefined;
    b: number | undefined;
    c: string;
    d: boolean | undefined;
}

Additional information about the issue

Toggle between versions 5.4.5 and 5.5.4 in the playground and hover over the Bar type to see the difference.

[Andarist]

duplicate of #59948

[typescript-bot]

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