Zod V4

Zod v4 Guidelines

Type Inference

Every schema MUST have inferred type above it:

export type User = z.infer<typeof User>
export const User = z.object({...})

Requirements:

• ALWAYS place type above schema
• ALWAYS use same name for type & schema
• NEVER use "Schema" suffix
• ALWAYS use JSDoc (/** */), never //
• NO EXCEPTIONS - even for internal/helper schemas

String Validation

String validations are standalone functions:

// WRONG: z.string().email()
// RIGHT: z.email(), z.url(), z.uuid(), z.ip()

Error Messages

Use error param sparingly - Zod's defaults are excellent:

// WRONG: z.email({error: "Invalid email"}) // Redundant!
// RIGHT: z.email() // Zod says "Invalid email"
// RIGHT: Only for business logic:
z.string().refine((val) => /[A-Z]/.test(val), {
  error: 'Must contain uppercase',
})

Number Types

• Use z.number() for general numbers
• z.int() for integers only (not z.number().int())
• z.int32(), z.float64() for specific types
• Numbers finite by default

Object Types

• z.object() - strips unknowns (default)
• z.strictObject() - rejects extras
• z.looseObject() - allows extras

Custom Validation

Use .check() for advanced validation, .refine() for simple validation:

Migration steps:

• .superRefine() → .check() for advanced validation with multiple issues
• In .check(): val → ctx.value, ctx.addIssue() → ctx.issues.push()
• z.ZodIssueCode.custom → 'custom' string
• Add input: ctx.value to issue object
• Use .refine() for simple boolean validation with single error

Error Formatting

• z.prettifyError() - Human-readable format
• z.treeifyError() - Tree structure format

Functions

Define function schemas with input/output types:

z.function({
  input: [z.string()],
  output: z.number(),
})

Records

z.record(keyType, valueType)

ISO Formats

• z.iso.datetime() - ISO 8601 datetime
• z.iso.date() - ISO 8601 date

Additional Features

• Default: .default() applies to output; use .prefault() for v3 behavior
• File validation: z.file().min(1024).max(5*1024*1024).mime(['image/jpeg'])
• Pipe: z.pipe(z.string(), z.number()) for transformations
• Async: Use .check(async (val) => {...}) for async validation
• Arrays: z.array(z.email()) or z.email().array()
• Optional: .optional(), .nullable(), .nullish()

Quick Reference: v3 → v4

Complete Example

import * as z from 'zod'

/** User registration */
export type UserReg = z.infer<typeof UserReg>
export const UserReg = z.object({
  email: z.email(),
  password: z
    .string()
    .min(8)
    .refine((pwd) => /[A-Z]/.test(pwd) && /\d/.test(pwd), { error: 'Need uppercase & number' }),
  age: z.number().min(18),
})

/** Function with input/output types */
export type ProcessUser = z.infer<typeof ProcessUser>
export const ProcessUser = z.function({
  input: [UserReg],
  output: z.object({
    id: z.string(),
    createdAt: z.iso.datetime(),
  }),
})

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