.omit() inoperative with .passthrough()

[adrien-febvay]

.omit() doesn't do anything when using .passthrough(): the omitted keys are still there.

import { z } from 'zod'; // zod@3.20.6

// Our schema, just two known keys `a` and `b`
const schema = z.object({ a: z.number(), b: z.number() });

// We want to omit `b`, but preserve unknown keys
// No luck here: we get { a: 1, b: 2, x: 654 }
console.log(schema.omit({ b: true }).passthrough().parse({ a: 1, b: 2, x: 654 }));

// Still not what we want: { a: 1, b: 2, y: 286 }
console.log(schema.passthrough().omit({ b: true }).parse({ a: 1, b: 2, y: 286 }));

// Workaround for expected result: { a: 1, z: 123 }
console.log(schema.passthrough().transform(({ b, ...res }) => res).parse({ a: 1, b: 2, z: 123 }));

Am I doing something wrong? Am I not supposed to chain those two methods?

I understood that .passthrough() would just pass unknown keys, not omitted ones:

https://www.npmjs.com/package/zod#passthrough
By default Zod object schemas strip out unrecognized keys during parsing.
Instead, if you want to pass through unknown keys, use .passthrough().

[JacobWeisenburger]

https://github.com/colinhacks/zod#pickomit
.omit omits keys from the schema, not from the parsed/validated data. Therefore it makes b an unknown key, thus causing it to be passed through.

This is the way that I would do what you are trying to do:

const schema = z.object( { a: z.number(), b: z.number() } )
    .passthrough()
    .transform( ( { b, ...rest } ) => rest )

console.log( schema.parse( { a: 1, b: 2, z: 123 } ) )
// { a: 1, z: 123 }

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