JSON Null & Boolean Values: When and How to Use Them

JSON Boolean Values

JSON has exactly two boolean values: true and false. They must be lowercase — no quotes, no capitalization. Booleans represent binary states: yes/no, on/off, enabled/disabled.

{
  "isActive": true,
  "isVerified": false,
  "hasSubscription": true,
  "emailOptIn": false,
  "darkMode": true
}

Boolean Naming Conventions

Good boolean field names read naturally as questions. Prefix them with is, has, can, or should to make their meaning clear:

{
  "isEnabled": true,
  "hasPermission": false,
  "canEdit": true,
  "shouldNotify": false,
  "isPublished": true
}

Avoid ambiguous names like "status": true— it's unclear what "true status" means. "isActive": true is immediately understandable.

Invalid Boolean Formats

These are all invalid in JSON and will cause parse errors:

// ❌ All of these are INVALID JSON
{
  "a": True,       // Capital T — must be lowercase
  "b": FALSE,      // All caps — must be lowercase
  "c": "true",     // Quoted — this is a string, not a boolean
  "d": 1,          // Number — JSON has no truthy/falsy concept
  "e": yes         // Not a JSON type at all
}

In JSON, "true" (with quotes) is a string containing four characters, not a boolean. Many bugs come from accidentally quoting boolean values. A consumer checking if (data.isActive === true) will get false when the value is the string "true".

JSON Null Values

The null value represents an intentional absence of data. It is always lowercase and unquoted:

{
  "user": {
    "id": 42,
    "name": "Jane Doe",
    "middleName": null,
    "phone": null,
    "deletedAt": null,
    "referredBy": null
  }
}

In this example, middleName is nullbecause the user doesn't have one, phone hasn't been provided yet, and deletedAt is nullbecause the account hasn't been deleted. Each null conveys "this field exists but currently has no value."

Null vs Undefined vs Missing Keys

This distinction trips up many developers. JSON only has null — it does not have undefined. But there are three different ways a value can be "absent":

// 1. Key present with null value — "exists but empty"
{
  "phone": null
}

// 2. Key missing entirely — "not provided"
{
}

// 3. Key present with empty string — "provided but blank"
{
  "phone": ""
}

These three states have different semantic meanings:

• null — the field is known but has no value. The user explicitly left it empty or it was cleared.
• Missing key— the field was not included in the response. This might mean the consumer doesn't have permission to see it, or the API version doesn't include it.
• Empty string "" — the field has a value, and that value is an empty string. This is a valid string, not absence.

The String "null" Trap

Watch out for the difference between null and "null":

{
  "correctNull": null,
  "stringNull": "null",
  "stringUndefined": "undefined"
}

The value "null" is a string containing four characters. It is not the same as null. This commonly happens when code serializes a null value incorrectly, converting it to the string "null" before inserting it into JSON.

Using Null and Booleans Together

A practical example showing both types working together in a feature flag system:

{
  "features": {
    "darkMode": {
      "enabled": true,
      "rolloutPercentage": 100,
      "overrideUserId": null
    },
    "betaSearch": {
      "enabled": false,
      "rolloutPercentage": 0,
      "overrideUserId": null
    },
    "newCheckout": {
      "enabled": true,
      "rolloutPercentage": 25,
      "overrideUserId": 1042
    }
  }
}

Here, enabled is a boolean controlling whether the feature is on, rolloutPercentage is a number for gradual rollout, and overrideUserId is either a specific user ID or nullwhen there's no override.

Common Mistakes to Avoid

• Capitalizing booleans: True, False, TRUE, FALSE are all invalid. Only lowercase true and false work.
• Quoting null or booleans: "null" and "true" are strings, not the actual null and boolean types.
• Using 0/1 for booleans: JSON supports real booleans. Don't use 0 and 1as substitutes — they're numbers.
• Confusing null with undefined: undefined is not a valid JSON value. If you serialize a JavaScript object with undefined properties, they are silently dropped.
• Inconsistent null handling: Decide whether missing fields in your API responses should return null or be omitted entirely, then follow that convention everywhere.