Transforming and Preprocessing Data with Zod

Why Transform or Preprocess?

Sometimes raw input isn’t in the shape you want:

• A date string that needs to become a Date object
• A number stored as a string
• A string that needs trimming or formatting

Zod offers .transform() and .preprocess() to fix, convert, or sanitize input during validation.

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Using transform()

Use .transform() after validation to shape or derive new values.

const schema = z.string().transform((val) => val.trim().toUpperCase());

const result = schema.parse("  hello  ");
// ➜ "HELLO"

You can also change the output type:

const NumSchema = z.string().transform((val) => parseInt(val));

type Result = z.infer<typeof numschema>;
// Result is now `number`, not `string`

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Using preprocess()

Use .preprocess() to manipulate values before validation happens.

Example: Accept a number or a string and turn it into a number before validating:

const schema = z.preprocess((val) => Number(val), z.number().min(0));

schema.parse("42"); // ✅ 42
schema.parse(42); // ✅ 42
schema.parse("abc"); // ❌ Not a number

This is great for working with:

• Query strings
• Form data
• Environment variables

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Differences Between transform and preprocess

Feature	.transform()	.preprocess()
Runs after validation	✅	❌
Runs before validation	❌	✅
Can change output type	✅	✅
Use case	Reformat/derive	Sanitize/parse raw input

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Practical Examples

1. Convert date string to Date object

const DateSchema = z
  .string()
  .refine((val) => !isNaN(Date.parse(val)), {
    message: "Invalid date format",
  })
  .transform((val) => new Date(val));

2. Trim and lowercase email

const EmailSchema = z
  .string()
  .email()
  .transform((val) => val.trim().toLowerCase());

3. Parse string to number and validate

const AgeSchema = z.preprocess(
  (val) => Number(val),
  z.number().int().positive()
);

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Summary

• Use .transform() to clean or reformat values after validation
• Use .preprocess() to sanitize input before validation
• Both are powerful tools to adapt real-world input to strict, type-safe schemas
• Great for working with forms, APIs, and config data

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Next: Lesson 10 – Working with Records, Maps, Sets, and Enums in Zod

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