writing-json-schemas

name

writing-json-schemas

description

Authors JSON Schema definitions for use with z-schema validation. Use when the user needs to write a JSON Schema, define a schema for an API payload, create schemas for form validation, structure schemas with $ref and $defs, choose between oneOf/anyOf/if-then-else, design object schemas with required and additionalProperties, validate arrays with items or prefixItems, add format constraints, organize schemas for reuse, or write draft-2020-12 schemas.

Writing JSON Schemas for z-schema

Write correct, idiomatic JSON Schemas validated by z-schema. Default target: draft-2020-12.

Schema template

Start every schema with a $schema declaration and type:

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "properties": {},
  "required": [],
  "additionalProperties": false
}

Set additionalProperties: false explicitly when extra properties should be rejected — z-schema allows them by default.

Object schemas

Basic object with required fields

{
  "type": "object",
  "properties": {
    "name": { "type": "string", "minLength": 1 },
    "email": { "type": "string", "format": "email" },
    "age": { "type": "integer", "minimum": 0, "maximum": 150 }
  },
  "required": ["name", "email"],
  "additionalProperties": false
}

Nested objects

{
  "type": "object",
  "properties": {
    "address": {
      "type": "object",
      "properties": {
        "street": { "type": "string" },
        "city": { "type": "string" },
        "zip": { "type": "string", "pattern": "^\\d{5}(-\\d{4})?$" }
      },
      "required": ["street", "city"]
    }
  }
}

Dynamic property names

Use patternProperties to validate property keys by regex:

{
  "type": "object",
  "patternProperties": {
    "^x-": { "type": "string" }
  },
  "additionalProperties": false
}

Use propertyNames (draft-06+) to constrain all property key strings:

{
  "type": "object",
  "propertyNames": { "pattern": "^[a-z_]+$" }
}

Array schemas

Uniform array

{
  "type": "array",
  "items": { "type": "string" },
  "minItems": 1,
  "uniqueItems": true
}

Tuple validation (draft-2020-12)

Use prefixItems for positional types, items for remaining elements:

{
  "type": "array",
  "prefixItems": [{ "type": "string" }, { "type": "integer" }],
  "items": false
}

items: false rejects extra elements beyond the tuple positions.

Contains (draft-06+)

Require at least one matching item:

{
  "type": "array",
  "contains": { "type": "string", "const": "admin" }
}

With count constraints (draft-2019-09+):

{
  "type": "array",
  "contains": { "type": "integer", "minimum": 10 },
  "minContains": 2,
  "maxContains": 5
}

String constraints

{
  "type": "string",
  "minLength": 1,
  "maxLength": 255,
  "pattern": "^[A-Za-z0-9_]+$"
}

Format validation

z-schema has built-in format validators: date, date-time, time, email, idn-email, hostname, idn-hostname, ipv4, ipv6, uri, uri-reference, uri-template, iri, iri-reference, json-pointer, relative-json-pointer, regex, duration, uuid.

{ "type": "string", "format": "date-time" }

Format assertions are always enforced by default (formatAssertions: null). For vocabulary-aware behavior in draft-2020-12, set formatAssertions: true on the validator.

Numeric constraints

{
  "type": "number",
  "minimum": 0,
  "maximum": 100,
  "multipleOf": 0.01
}

Use exclusiveMinimum / exclusiveMaximum for strict bounds:

{ "type": "integer", "exclusiveMinimum": 0, "exclusiveMaximum": 100 }

Combinators

anyOf — match at least one

{
  "anyOf": [{ "type": "string" }, { "type": "number" }]
}

oneOf — match exactly one

{
  "oneOf": [
    { "type": "string", "maxLength": 5 },
    { "type": "string", "minLength": 10 }
  ]
}

allOf — match all

Use for schema composition. Combine base schemas with refinements:

{
  "allOf": [
    { "$ref": "#/$defs/base" },
    { "properties": { "extra": { "type": "string" } } }
  ]
}

not — must not match

{ "not": { "type": "null" } }

if / then / else (draft-07+)

Conditional validation — prefer over complex oneOf when the logic is "if X then require Y":

{
  "type": "object",
  "properties": {
    "type": { "type": "string", "enum": ["personal", "business"] },
    "company": { "type": "string" }
  },
  "if": { "properties": { "type": { "const": "business" } } },
  "then": { "required": ["company"] },
  "else": {}
}

When to use which combinator

Scenario	Use
Value can be multiple types	anyOf
Exactly one variant must match	oneOf
Compose inherited schemas	allOf
"if condition then require fields"	if/then/else
Exclude a specific shape	not

Prefer if/then/else over oneOf when the condition is a single discriminator field — it produces clearer error messages.

Schema reuse with $ref and $defs

Local definitions

{
  "$defs": {
    "address": {
      "type": "object",
      "properties": {
        "street": { "type": "string" },
        "city": { "type": "string" }
      },
      "required": ["street", "city"]
    }
  },
  "type": "object",
  "properties": {
    "home": { "$ref": "#/$defs/address" },
    "work": { "$ref": "#/$defs/address" }
  }
}

Cross-schema references

Compile an array of schemas and reference by ID:

import ZSchema from "z-schema";

const schemas = [
  {
    $id: "address",
    type: "object",
    properties: { city: { type: "string" } },
    required: ["city"]
  },
  {
    $id: "person",
    type: "object",
    properties: {
      name: { type: "string" },
      home: { $ref: "address" }
    },
    required: ["name"]
  }
];

const validator = ZSchema.create();
validator.validateSchema(schemas);
validator.validate({ name: "Alice", home: { city: "Paris" } }, "person");

Strict schemas with unevaluatedProperties (draft-2019-09+)

When combining schemas with allOf, additionalProperties: false in a sub-schema blocks properties defined in sibling schemas. Use unevaluatedProperties instead — it tracks all properties evaluated across applicators:

{
  "allOf": [
    {
      "type": "object",
      "properties": { "name": { "type": "string" } },
      "required": ["name"]
    },
    {
      "type": "object",
      "properties": { "age": { "type": "integer" } }
    }
  ],
  "unevaluatedProperties": false
}

This accepts { "name": "Alice", "age": 30 } but rejects { "name": "Alice", "age": 30, "extra": true }.

Validating the schema itself

Always validate schemas at startup:

const validator = ZSchema.create();
try {
  validator.validateSchema(schema);
} catch (err) {
  console.log("Schema errors:", err.details);
}

Common mistakes

• Forgetting additionalProperties: By default, extra properties are allowed. Set additionalProperties: false or use unevaluatedProperties: false to reject them.
• Using additionalProperties: false with allOf: This blocks properties from sibling schemas. Use unevaluatedProperties: false at the top level instead (draft-2019-09+).
• Array items in draft-2020-12: Use prefixItems for tuple validation. items now means "schema for remaining items".
• Missing $schema: Without it, z-schema uses its configured default draft. Include $schema for explicit draft targeting.
• definitions vs $defs: Both work, but $defs is the canonical form in draft-2019-09+. Use it consistently.