JSON Cheat Sheet — Syntax, Parsing & Common Mistakes

{
  "name": "Alice",
  "age": 30,
  "active": true,
  "email": null
}

["apple", "banana", "cherry"]

[1, "mixed", true, null, {"nested": "object"}]

[]

{
  "user": {
    "name": "Alice",
    "address": {
      "street": "123 Main St",
      "city": "Springfield",
      "coordinates": [39.7817, -89.6501]
    },
    "tags": ["admin", "verified"],
    "scores": [
      {"subject": "math", "grade": 95},
      {"subject": "science", "grade": 88}
    ]
  }
}

const data = JSON.parse('{"name": "Alice", "age": 30}');
console.log(data.name); // "Alice"

// With a reviver function to transform values during parsing
const parsed = JSON.parse('{"date": "2026-04-12"}', (key, value) => {
  if (key === 'date') return new Date(value);
  return value;
});

const obj = { name: 'Alice', age: 30, active: true };

JSON.stringify(obj);
// '{"name":"Alice","age":30,"active":true}'

// Pretty-print with 2-space indentation
JSON.stringify(obj, null, 2);

// Filter keys with a replacer array
JSON.stringify(obj, ['name', 'age']);
// '{"name":"Alice","age":30}'

// Transform values with a replacer function
JSON.stringify(obj, (key, value) => {
  if (typeof value === 'string') return value.toUpperCase();
  return value;
});
// '{"name":"ALICE","age":30,"active":true}'

function safeJsonParse(text) {
  try {
    return { data: JSON.parse(text), err: null };
  } catch (err) {
    return { data: null, err };
  }
}

const { data, err } = safeJsonParse(userInput);
if (err) console.error('Invalid JSON:', err.message);

# Pretty-print a file
jq . data.json

# Extract nested fields from an API response
curl -s https://api.example.com/users | jq '.data[] | {name, email}'

# Filter and count
jq '[.[] | select(.status == "active")] | length' users.json

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "required": ["name", "age"],
  "properties": {
    "name": {
      "type": "string",
      "minLength": 1
    },
    "age": {
      "type": "integer",
      "minimum": 0
    },
    "email": {
      "type": "string",
      "format": "email"
    },
    "tags": {
      "type": "array",
      "items": { "type": "string" },
      "uniqueItems": true
    }
  },
  "additionalProperties": false
}

// INVALID - trailing comma after last element
{
  "name": "Alice",
  "age": 30,
}

// VALID
{
  "name": "Alice",
  "age": 30
}

// INVALID
{'name': 'Alice'}

// VALID
{"name": "Alice"}

// INVALID - no comments allowed in JSON
{
  // this is a comment
  "name": "Alice"  /* inline comment */
}

// VALID - no comments
{
  "name": "Alice"
}

// INVALID
{name: "Alice", age: 30}

// VALID
{"name": "Alice", "age": 30}

// JavaScript loses precision with large integers
JSON.parse('{"id": 9007199254740993}');
// { id: 9007199254740992 } -- precision lost!

// Solution: use strings for large IDs
// {"id": "9007199254740993"}

// Or use BigInt-aware parsing (Node.js example)
// JSON.parse(text, (key, value, { source }) => {
//   if (typeof value === 'number' && source.length > 15) return BigInt(source);
//   return value;
// });