Omitting a property on a discriminated union type

[Trekiros]

Bug Report

When using Omit on a discriminated union type, the properties unique to each "branch" of the union type are lost.

🔎 Search Terms

Omit, union, discriminated union

🕗 Version & Regression Information

Tested on TypeScript 4.8.4 and on the nightly version (June 5th, 2023)

⏯ Playground Link

demo

Alternative demo (a different way to do discriminated unions, which is also affected)

💻 Code

type T = {
    name: string,
    type: 1,
    prop1: string,
} | {
    name: string,
    type: 2,
    prop2: string,
}

type T2 = Omit<T, 'name'>

🙁 Actual behavior

T2 is interpreted as such by TypeScript:

type T2 = {
    type: 1 | 2;
}

And the following object is seen as invalid:

const obj: T2 = {
    type: 2,
    prop2: 'abcd', // "prop2 does not exist on type T2"
}

🙂 Expected behavior

T2 should be interpreted like this instead:

type T2 = {
    type: 1;
    prop1: string;
} | {
    type: 2;
    prop2: string;
}

And the object described above should be seen as valid.

[MartinJohns]

This is unfortunately working as intended. Omit does not distribute over the union. There are many implementations of a distributive omit out there, just search for it using "distributive omit". See also #54451.

[nurulakbaral]

You can create your own Omit with Distributive Conditional Types:

type TheOmit<T, K extends string | number | symbol> = T extends any ? Omit<T, K> : never

[MartinJohns]

@nurulakbaral Be aware that this version will not warn you if you mistype the property name, while the version suggested in #54451 will error, but only allows to omit properties common to all types of the union.

[nurulakbaral]

@nurulakbaral Be aware that this version will not warn you if you mistype the property name, while the version suggested in #54451 will error, but only allows to omit properties common to all types of the union.

type TheOmit<T, K extends keyof T> = T extends any ? Omit<T, K> : never

How about this?

[jcalz]

Duplicate of #31501

[microsoft-github-policy-service]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow or the TypeScript Discord community.

[typescript-bot]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow or the TypeScript Discord community.

[microsoft-github-policy-service]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow or the TypeScript Discord community.