Formatting errors

Zod emphasizes completeness and correctness in its error reporting. In many cases, it's helpful to convert the $ZodError to a more useful format. Zod provides some utilities for this.

Consider this simple object schema.

import { z } from "zod/v4";
 
const schema = z.strictObject({
  username: z.string(),
  favoriteNumbers: z.array(z.number()),
});

Attempting to parse this invalid data results in an error containing two issues.

const result = schema.safeParse({
  username: 1234,
  favoriteNumbers: [1234, "4567"],
  extraKey: 1234,
});
 
result.error!.issues;
[
  {
    expected: 'string',
    code: 'invalid_type',
    path: [ 'username' ],
    message: 'Invalid input: expected string, received number'
  },
  {
    expected: 'number',
    code: 'invalid_type',
    path: [ 'favoriteNumbers', 1 ],
    message: 'Invalid input: expected number, received string'
  },
  {
    code: 'unrecognized_keys',
    keys: [ 'extraKey' ],
    path: [],
    message: 'Unrecognized key: "extraKey"'
  }
];

`z.treeifyError()`

To convert ("treeify") this error into a nested object, use z.treeifyError().

const tree = z.treeifyError(result.error);
 
// =>
{
  errors: [ 'Unrecognized key: "extraKey"' ],
  properties: {
    username: { errors: [ 'Invalid input: expected string, received number' ] },
    favoriteNumbers: {
      errors: [],
      items: [
        undefined,
        {
          errors: [ 'Invalid input: expected number, received string' ]
        }
      ]
    }
  }
}

The result is a nested structure that mirrors the schema itself. You can easily access the errors that occurred at a particular path. The errors field contains the error messages at a given path, and the special properties properties and items let you traverse deeper into the tree.

tree.properties?.username?.errors;
// => ["Invalid input: expected string, received number"]
 
tree.properties?.favoriteNumbers?.items?.[1]?.errors;
// => ["Invalid input: expected number, received string"];

Be sure to use optional chaining (?.) to avoid errors when accessing nested properties.

`z.prettifyError()`

The z.prettifyError() provides a human-readable string representation of the error.

const pretty = z.prettifyError(result.error);

This returns the following string:

✖ Unrecognized key: "extraKey"
✖ Invalid input: expected string, received number
  → at username
✖ Invalid input: expected number, received string
  → at favoriteNumbers[1]

`z.formatError()`

This has been deprecated in favor of z.treeifyError().

`z.flattenError()`

While z.treeifyError() is useful for traversing a potentially complex nested structure, the majority of schemas are flat—just one level deep. In this case, use z.flattenError() to retrieve a clean, shallow error object.

const flattened = z.flattenError(result.error);
// { errors: string[], properties: { [key: string]: string[] } }
 
{
  formErrors: [ 'Unrecognized key: "extraKey"' ],
  fieldErrors: {
    username: [ 'Invalid input: expected string, received number' ],
    favoriteNumbers: [ 'Invalid input: expected number, received string' ]
  }
}

The formErrors array contains any top-level errors (where path is []). The fieldErrors object provides an array of errors for each field in the schema.

flattened.fieldErrors.string; // => [ 'Invalid input: expected string, received number' ]
flattened.fieldErrors.numbers; // => [ 'Invalid input: expected number, received string' ]

Note — If you have issues with a path longer than one level, this function throws away some error information.

Formatting errors

z.treeifyError()

z.prettifyError()

z.formatError()

Show docs

z.flattenError()

On this page

`z.treeifyError()`

`z.prettifyError()`

`z.formatError()`

`z.flattenError()`