Required Properties Validation in JSON Schema

Q: What does this JSON Schema validate?

## Required Properties in JSON Schema The required keyword specifies which properties must be present in a JSON object. It takes an array of property names. If any listed property is missing from the object, validation fails. ### Basic Syntax json { "type": "object", "properties": { "name": { "type": "string" }, "age": { "type": "integer" } }, "required": ["name"] } Here name is required and age is optional. The object {"name": "Alice"} is valid, but {"age": 30} is not becau

Learn how to use the required keyword in JSON Schema to mandate specific properties. Understand the difference between required, optional, and nullable fields.

Object Constraints

JSON Schema

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "properties": {
    "id": {
      "type": "integer"
    },
    "name": {
      "type": "string",
      "minLength": 1
    },
    "email": {
      "type": "string",
      "format": "email"
    },
    "phone": {
      "type": "string"
    },
    "role": {
      "type": "string",
      "enum": ["admin", "editor", "viewer"],
      "default": "viewer"
    }
  },
  "required": ["id", "name", "email"]
}

Test Data

{
  "id": 1001,
  "name": "Alice Chen",
  "email": "alice@example.com",
  "role": "editor"
}

Detailed Explanation

Required Properties in JSON Schema

The required keyword specifies which properties must be present in a JSON object. It takes an array of property names. If any listed property is missing from the object, validation fails.

Basic Syntax

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

Here name is required and age is optional. The object {"name": "Alice"} is valid, but {"age": 30} is not because name is missing.

How the Example Schema Works

The schema defines a user record with five properties, three of which are required:

id (required) — an integer identifier. Must always be present.
name (required) — a non-empty string. The minLength: 1 prevents empty names.
email (required) — a valid email address.
phone (optional) — when provided, must be a string. When omitted, no error.
role (optional) — an enum with a default of "viewer". Clients may omit it.

The test data includes id, name, and email, satisfying all requirements. The phone property is absent, which is fine because it is not in the required array. The role is present and set to "editor", which is one of the allowed enum values.

Required vs. Present vs. Valid

A common misconception is that required validates the value. It does not — it only checks for the presence of the key:

Scenario	Required check	Type check
Key missing	Fails	Not evaluated
Key present, value `null`	Passes	Fails (unless nullable)
Key present, wrong type	Passes	Fails
Key present, correct type	Passes	Passes

Dynamic Required Properties

In advanced schemas, you can use if/then to make requirements conditional:

{
  "if": { "properties": { "role": { "const": "admin" } } },
  "then": { "required": ["phone"] }
}

This makes phone required only when role is "admin". See the conditional-if-then-else example for more on this pattern.

Best Practices

List all mandatory fields in required. Do not rely on consumers reading documentation.
Combine with minLength: 1 for required strings to reject empty values.
Keep required at the object level, not inside individual property definitions — this is a common syntax mistake.

Use Case

Use required properties in every API request/response schema where certain fields are mandatory. This is the foundation of input validation for user registration, order placement, configuration APIs, and any endpoint where missing data would cause downstream failures.

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