Booleans and Null

Learning Focus

true, false, and null look trivial but cause frequent bugs when languages apply different coercion semantics.

Boolean

Written in lowercase only — capitals cause a parse error.

{
  "isActive":  true,
  "isDeleted": false,
  "hasAccess": true
}

Language Mapping

JSON	Python	JavaScript	Go	PHP
true	True	true	true	true
false	False	false	false	false

Null

Represents the intentional absence of a value. Also lowercase only.

{ "middleName": null, "deletedAt": null }

null vs Missing Key

{ "a": null }   // key exists, value is null
{ }             // key does not exist at all

These are semantically different. Handle both cases explicitly:

data = {"a": None}
print("a" in data)   # True  — key present, value null
print("b" in data)   # False — key truly missing

Type Coercion Traps (JavaScript)

// ⚠️ Dangerous: loose equality
if (!apiResponse.value) {
  // Triggers for null, false, 0, "" — not just null!
}

// ✅ Safe: explicit check
if (apiResponse.value === null)  { /* ... */ }
if (apiResponse.value === false) { /* ... */ }

Concept Map

Concept Flow

JSON Literals
  ├── true / false  — lowercase only (parse error otherwise)
  └── null          — lowercase only
        └── null ≠ missing key ≠ empty string ≠ 0

Common Pitfalls

Pitfall	Consequence	Prevention
True / False in raw JSON	Parse error	Use lowercase in JSON strings
Treating null as missing key	Logic errors	Check key existence separately
Coercing 0 / "" to boolean	Silent logic bugs	Use strict equality
Python None via f-string	Produces "None" string, not null	Always use json.dumps()

What's Next

• Previous: Strings and Numbers — Review primitive types.
• Next: JSON Objects — The key-value container type.
• Section Overview — Return to the Data Types module index.