Sindre Sorhus/type-fest

IsEqual identity issues

2 years ago

👍

There are two self-identity bugs that can pretty simply be fixed: TS Playground

Both of these evaluate to false:

IsEqual<{a: 1} & {a: 1}, {a: 1}>
IsEqual<{a: 1} | {a: 1}, {a: 1}>

The fix is to expand the check a touch:

type IsEqual<A, B> =
    (<G>() => G extends A & G | G ? 1 : 2) extends
    (<G>() => G extends B & G | G ? 1 : 2)
    ? true
    : false;

There's also this intersection bug but I'm unaware of a fix: TS Playground

This evaluates to false:

IsEqual<{a: 1} & {b: 2}, {a: 1, b: 2}>

sindresorhus / type-fest

A collection of essential TypeScript types

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