🔎 Search Terms
"exactOptionalPropertyTypes", "strict type equality"
🕗 Version & Regression Information
- Tested in TypeScript 5.8.2
⏯ Playground Link
https://www.typescriptlang.org/play/?exactOptionalPropertyTypes=true#code/PTAEFMA8EMGMBcDyAHeBLA9gO2gGwAoBOGy4h8AngCoWkDOAXAFBOWmhUCMoAvKAN5NQoZMWQB+BqDrxCaLAHMANEwC+rWuA4AmXgKEixk0AFcsAE3AAzeeHOgAPtNnzlajewD6egNoGQoIAy5Bzc0BY6oNCEWrDYdGiW0fYycgi4FBAAjiZ4BgCCdHRk8AA8AKLZeCUGwlxKOgYAfI0qwgFckeFUulFaWBjwoLFY8Yl2oLgYGEXpWTm4+YXFJQByAyUAMlNFFfPVwrWc9d1Nza2g7bpodKD9g9CFaAo4AEa4WvAYIYtF5KvrBXiz2gb3A+wOdQawmaLX8YA610ij2BoNAnyhoEBy0BT1e73BtW0x04pxUAF0ANwsNhaLF-TxUCCQeDgCw3WQmcCNPQANwwCSpAUAvBuAI-2OKkANZor4uBQKMjOVKDGlzPBoSgAOlAAGUMCZCLBwFIABbweDIRggBTq40mF4a2IAW2AAFk0LBiHQMFZ4MAaKRtR60KhgNc6Jy6MBtAB2AAM2gALABiMOcp2O1nwAC0Ce0nAArNoAJwANnzsY8Wl2VQAGvUAJrcngGAAUJSojRbAEpeNzGVAWWzQDXQOJQNwpNoewPWeY6K3252ezw+0zB3PQPXR+PQJOe2OOVopFY8EUDFSVVtpuBq7gSnXN02DD4a2S17Obj562SDGOW1+3xnIcXzfA9CE5XdQBPXAii7Axj1PcBz0rUA1lKAAhd8hxeKZ3jCJ82jAAADW8iNAF5wFgaATCKUAAHcwmVL5oD5BJUPWMIKG5CjXFAIjoKKMiMEIPjDyIgxbxKdD6kPbkgI3Q8am3ASkIOSCW0k6SoMQuTmQ-NFwK0MCIKkLBwB5Mg4OEC9NExZE8TBKgonleB6l1fVDSbUAfHcg1wEAvTgKcwgXNAgyTO0mDwCpIA
💻 Code
// exactOptionalPropertyTypes:
type T1 = {
prop?: string,
}
type T2 = {
prop?: undefined | string,
}
type _ = [
// ❌ T1 and T2 are considered strictly equal
Assert<Equal<
T1, T2
>>,
// T2 is not assignable to T1
Assert<Not<Assignable<
T1, T2
>>>,
// T1 is assignable to T2
Assert<Assignable<
T2, T1
>>,
// T1 and T2 are not considered loosely equal
Assert<Not<LooseEqual<
T1, T2
>>>,
];
type Assert<_T extends true> = void;
// 🟢 Trick to trigger strict type equality. Source: https://github.com/Microsoft/TypeScript/issues/27024#issuecomment-421529650
type Equal<X, Y> =
(<T>() => T extends X ? 1 : 2) extends
(<T>() => T extends Y ? 1 : 2) ? true : false
;
type LooseEqual<X, Y> =
[X] extends [Y]
? ([Y] extends [X] ? true : false)
: false
;
type Not<B extends boolean> =
// `Equal` because want to avoid Not<any> being `false` or `true`
Equal<B, true> extends true
? false
: (Equal<B, false> extends true ? true : never)
;
type Assignable<Target, Source> = [Source] extends [Target] ? true : false;🙁 Actual behavior
T1 and T2 are considered to be strictly equal – even though T2 is not assignable to T1.
🙂 Expected behavior
T1 and T2 should not be considered strictly equal.
Additional information about the issue
🔎 Search Terms
"exactOptionalPropertyTypes", "strict type equality"
🕗 Version & Regression Information
⏯ Playground Link
https://www.typescriptlang.org/play/?exactOptionalPropertyTypes=true#code/PTAEFMA8EMGMBcDyAHeBLA9gO2gGwAoBOGy4h8AngCoWkDOAXAFBOWmhUCMoAvKAN5NQoZMWQB+BqDrxCaLAHMANEwC+rWuA4AmXgKEixk0AFcsAE3AAzeeHOgAPtNnzlajewD6egNoGQoIAy5Bzc0BY6oNCEWrDYdGiW0fYycgi4FBAAjiZ4BgCCdHRk8AA8AKLZeCUGwlxKOgYAfI0qwgFckeFUulFaWBjwoLFY8Yl2oLgYGEXpWTm4+YXFJQByAyUAMlNFFfPVwrWc9d1Nza2g7bpodKD9g9CFaAo4AEa4WvAYIYtF5KvrBXiz2gb3A+wOdQawmaLX8YA610ij2BoNAnyhoEBy0BT1e73BtW0x04pxUAF0ANwsNhaLF-TxUCCQeDgCw3WQmcCNPQANwwCSpAUAvBuAI-2OKkANZor4uBQKMjOVKDGlzPBoSgAOlAAGUMCZCLBwFIABbweDIRggBTq40mF4a2IAW2AAFk0LBiHQMFZ4MAaKRtR60KhgNc6Jy6MBtAB2AAM2gALABiMOcp2O1nwAC0Ce0nAArNoAJwANnzsY8Wl2VQAGvUAJrcngGAAUJSojRbAEpeNzGVAWWzQDXQOJQNwpNoewPWeY6K3252ezw+0zB3PQPXR+PQJOe2OOVopFY8EUDFSVVtpuBq7gSnXN02DD4a2S17Obj562SDGOW1+3xnIcXzfA9CE5XdQBPXAii7Axj1PcBz0rUA1lKAAhd8hxeKZ3jCJ82jAAADW8iNAF5wFgaATCKUAAHcwmVL5oD5BJUPWMIKG5CjXFAIjoKKMiMEIPjDyIgxbxKdD6kPbkgI3Q8am3ASkIOSCW0k6SoMQuTmQ-NFwK0MCIKkLBwB5Mg4OEC9NExZE8TBKgonleB6l1fVDSbUAfHcg1wEAvTgKcwgXNAgyTO0mDwCpIA
💻 Code
🙁 Actual behavior
T1andT2are considered to be strictly equal – even thoughT2is not assignable toT1.🙂 Expected behavior
T1andT2should not be considered strictly equal.Additional information about the issue