Boolean Type Validation with JSON Schema

Validating Booleans in JSON Schema

The boolean type accepts exactly two values: true and false. Unlike many programming languages, JSON Schema does not treat 0, 1, "true", "false", or null as booleans — only the literal JSON tokens true and false pass validation.

Basic Boolean Schema

{ "type": "boolean" }

This rejects strings like "true", numbers like 1, and null. Only the bare keywords true and false are valid.

Enforcing a Specific Value with const

Sometimes you need a boolean that must always be true — for example, confirming that a user has accepted terms of service:

{
  "type": "boolean",
  "const": true
}

The const keyword restricts the value to a single allowed literal. If the user sends false, validation fails with a clear error message.

How the Example Schema Works

The schema defines a user preferences object with three boolean fields:

1. isActive — a required boolean indicating whether the account is active. Both true and false are valid.
2. emailNotifications — an optional boolean with default: true. If the client omits this field, the application should treat it as true. Note that JSON Schema validators do not insert defaults — the default keyword is metadata for documentation and code generators.
3. acceptedTerms — a required boolean constrained to true by the const keyword. This ensures the user explicitly accepts the terms before submission.

The test data passes because isActive is a boolean, emailNotifications is a boolean, and acceptedTerms is exactly true.

Common Mistakes

• Sending "true" as a string: Form data and query parameters often arrive as strings. Your API must parse them before validation, or the schema will reject them.
• Relying on default insertion: The default keyword does not modify data during validation. Your application code or a pre-processing step must apply defaults.
• Nullable booleans: If a boolean can also be null, use "type": ["boolean", "null"] instead of plain "type": "boolean".

When to Use const vs. enum

For booleans, const: true is equivalent to enum: [true]. Use const when there is exactly one allowed value — it produces clearer error messages and is easier to read.