Order of z.object props with preprocess gives wrong parse results

[lucasbozzo]

Version: 3.22.4

It seems that Zod is unable of properly validating props with preprocess if they are not placed at the beginning of the Zod Object.

The following schema should always return an error for second.

const schemaWrong = z.object({
  first: z.string(),
  second: z.preprocess(() => undefined, z.string()),
});

schemaWrong.safeParse({}) // Shows only 1 error in `first`

If we switch the order of the props, then it works:

const schemaFixed = z.object({
  second: z.preprocess(() => undefined, z.string()),
  first: z.string(),
});

schemaWrong.safeParse({}) // Correctly shows 2 errors.

Playground: https://stackblitz.com/edit/typescript-wqj5ep?file=schemas%2FschemaWithDefaultType.ts

Note: Version 3.21 works correctly

[selimb]

Was just investigating this as well. I believe the behavior changed with #2426, specifically this bit (all comments are mine):

      // This runs preprocess
      const processed = effect.transform(ctx.data, checkCtx);
      // Early return if there are any issues on `ctx`
      if (ctx.common.issues.length) {
        return {
          status: "dirty",
          value: ctx.data,
        };
      }

      if (ctx.common.async) {
        return Promise.resolve(processed).then((processed) => {
          return this._def.schema._parseAsync({
            data: processed,
            path: ctx.path,
            parent: ctx,
          });
        });
      } else {
        // This runs the "inner" schema
        return this._def.schema._parseSync({
          data: processed,
          path: ctx.path,
          parent: ctx,
        });
      }
    }

[JacobWeisenburger]

Perhaps this will help:

const schemaFixed = z.object( {
    first: z.string(),
    second: z.any().transform( () => undefined ).pipe( z.string() ),
} )

// works as expected
const result = schemaFixed.safeParse( {} )
!result.success && console.log( result.error.issues )
// [
//     {
//         code: "invalid_type",
//         expected: "string",
//         received: "undefined",
//         path: [ "first" ],
//         message: "Required",
//     }, {
//         code: "invalid_type",
//         expected: "string",
//         received: "undefined",
//         path: [ "second" ],
//         message: "Required",
//     }
// ]

const schemaFixed = z.object( {
    second: z.any().transform( () => undefined ).pipe( z.string() ),
    first: z.string(),
} )

// works as expected
const result = schemaFixed.safeParse( {} )
!result.success && console.log( result.error.issues )
// [
//     {
//         code: "invalid_type",
//         expected: "string",
//         received: "undefined",
//         path: [ "second" ],
//         message: "Required",
//     }, {
//         code: "invalid_type",
//         expected: "string",
//         received: "undefined",
//         path: [ "first" ],
//         message: "Required",
//     }
// ]

If you found my answer satisfactory, please consider supporting me. Even a small amount is greatly appreciated. Thanks friend! 🙏
https://github.com/sponsors/JacobWeisenburger

[selimb]

z.any().transform().pipe() can indeed be used to replace z.preprocess! But isn't this still a regression (which has an easy fix, see #3124).

[aaronadamsCA]

This issue appears to be a duplicate of #2687.

Here is a one-line fix you can apply as a local patch to the Zod package:

• For Deno users
• For everyone else

I would suggest lending your 👍 to #2912 to encourage a fix to be merged.