JSON objects — syntax, nesting, and common mistakes

Syntax rules

{ "key": value, "key2": value2 }

• Curly braces wrap the object: { and }
• Keys must be strings in double quotes — not single quotes, not bare words
• A colon separates each key from its value
• Commas separate pairs; no trailing comma after the last one
• Values may be any JSON type: string, number, boolean, null, array, or object
• An empty object {} is valid

Examples

// Simple
{ "name": "Ada", "age": 36, "active": true }

// Nested objects
{
  "user": {
    "name": "Ada",
    "address": { "city": "London", "zip": "SW1A" }
  }
}

// Object containing an array of objects
{
  "team": "platform",
  "members": [
    { "id": 1, "name": "Ada" },
    { "id": 2, "name": "Grace" }
  ]
}

// Empty object
{}

Accessing values

// JavaScript
const data = JSON.parse('{"user":{"name":"Ada","tags":["a","b"]}}');
data.user.name;            // "Ada"
data.user.tags[1];         // "b"
data.user?.address?.city;  // undefined — safe optional chaining
Object.keys(data.user);    // ["name", "tags"]

# Python
import json
data = json.loads('{"user":{"name":"Ada","tags":["a","b"]}}')
data["user"]["name"]           # "Ada"
data["user"].get("address")    # None — no KeyError
list(data["user"].keys())      # ['name', 'tags']

Common mistakes

Unquoted or single-quoted keys

// ✗ Invalid — bare key (this is a JS object literal, not JSON)
{ name: "Ada" }

// ✗ Invalid — single quotes
{ 'name': 'Ada' }

// ✓ Valid
{ "name": "Ada" }

Trailing comma

// ✗ Invalid
{ "a": 1, "b": 2, }

// ✓ Valid
{ "a": 1, "b": 2 }

Duplicate keys

Legal but dangerous — most parsers silently keep the last value. See duplicate keys in JSON for what each parser does.

{ "id": 1, "id": 2 }   // parses to { "id": 2 } in most languages

Python/JS literals that aren't JSON

// ✗ None / True / False are Python, not JSON
{ "value": None, "ok": True }

// ✓ JSON uses lowercase and null
{ "value": null, "ok": true }

Objects vs arrays — which to use

Use an object when each value has a meaningful name. Use an array when you have a list of like items and order matters. A very common API shape combines both: an object of metadata wrapping an array of records.

{
  "total": 2,
  "page": 1,
  "results": [
    { "id": 1, "name": "Ada" },
    { "id": 2, "name": "Grace" }
  ]
}

See the companion guide on JSON arrays for the list side of this.