I am currently working with Typescript version 3.9.x
Let's say I have the following interface:
interface mytype {
foo: Foo
bar: Bar
baz: Baz
}
I aim to create a OnlyOneOfType<T> type that allows only one member within the interface.
For example:
const test1: OnlyOneOfType<mytype> = {foo: 'FOO'}; // Should be valid
const test2: OnlyOneOfType<mytype> = {bar: 'BAR'}; // Should be valid
const test3: OnlyOneOfType<mytype> = {foo: 'FOO', bar: 'BAR'}; // Should fail
Is this the specific type you're referencing?
type EnsureOne<T, Keys extends keyof T = keyof T> = Pick<T, Exclude<keyof T, Keys>> &
{
[K in Keys]-?: Required<Pick<T, K>> & Partial<Record<Exclude<Keys, K>, undefined>>;
}[Keys];
type OneOfTypeOnly<T> = EnsureOne<T, keyof T>
interface myInterface {
apple: 'APPLE'
orange: 'ORANGE'
banana: 'BANANA'
}
const check1: OneOfTypeOnly<myInterface> = { apple: 'APPLE' }; // PASS
const check2: OneOfTypeOnly<myInterface> = { orange: 'ORANGE' }; // PASS
const check3: OneOfTypeOnly<myInterface> = { apple: 'APPLE', orange: 'ORANGE' }; // FAIL
This mirrors KPD's solution that can be found in the following question...url
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