Tools/JSON Typing

JSON Typing Practice

JSON syntax rules, all six data types, real-world patterns, and the most common mistakes developers make — with typing practice links.

JSON Data Types

JSON supports exactly six value types — nothing more, nothing less.

Type	Example	Description
`string`	`"Hello, World!"`	Sequence of Unicode characters in double quotes
`number`	`42 \| 3.14 \| -7 \| 1e10`	Integer or floating-point — no distinction
`boolean`	`true \| false`	Lowercase literals only (not True/False)
`null`	`null`	Explicit absence of a value
`array`	`[1, "two", true, null]`	Ordered list of values of any type
`object`	`{"key": "value"}`	Unordered collection of key-value pairs

JSON Syntax Rules

✓ Keys must be strings enclosed in double quotes
✓ Key-value pairs are separated by a colon :
✓ Pairs in an object are separated by commas
✓ Arrays use square brackets []; objects use curly braces {}
✓ Whitespace (spaces, tabs, newlines) outside strings is ignored
✗ No trailing commas — ever
✗ No comments — JSON is data, not code
✗ No single quotes — always double quotes

Common JSON Mistakes

Trailing comma— JSON is strict — no trailing commas allowed

Wrong

{"a": 1, "b": 2,}

Correct

{"a": 1, "b": 2}

Single quotes— Both keys and strings must use double quotes

Wrong

{'key': 'value'}

Correct

{"key": "value"}

Unquoted keys— Keys must always be quoted strings

Wrong

{name: 'Alice'}

Correct

{"name": "Alice"}

Comments— JSON does not support comments of any kind

Wrong

// This is JSON
{}

Correct

{}

Undefined value— undefined is a JavaScript concept, not valid JSON

Wrong

{"x": undefined}

Correct

{"x": null}

Functions / regex— JSON is data-only — no functions or regex literals

Wrong

{"fn": function(){}}

Correct

{"fn": null}

NaN / Infinity— NaN and Infinity are not valid JSON numbers

Wrong

{"n": NaN}

Correct

{"n": null}

Unescaped newline— Newlines inside strings must be escaped as \n

Wrong

{"text": "line1
line2"}

Correct

{"text": "line1\nline2"}

Real-World JSON Patterns

These are the JSON structures you encounter every day as a developer. Type them until the structure feels natural.

REST API Response

{
  "status": "success",
  "data": {
    "id": 12345,
    "username": "saurav_dev",
    "email": "dev@example.com",
    "created_at": "2024-01-15T09:30:00Z",
    "is_pro": true,
    "stats": {
      "wpm": 87,
      "accuracy": 97.3,
      "tests_completed": 142
    }
  },
  "meta": {
    "request_id": "req_8f2a4d",
    "duration_ms": 48
  }
}

package.json (Node.js)

{
  "name": "my-app",
  "version": "1.0.0",
  "description": "A Next.js application",
  "main": "index.js",
  "scripts": {
    "dev": "next dev",
    "build": "next build",
    "start": "next start",
    "lint": "next lint"
  },
  "dependencies": {
    "next": "^14.2.0",
    "react": "^18.3.0",
    "react-dom": "^18.3.0"
  },
  "devDependencies": {
    "typescript": "^5.4.0",
    "@types/react": "^18.3.0",
    "tailwindcss": "^3.4.0"
  }
}

App Config File

{
  "app": {
    "name": "CodeSpeedTest",
    "version": "2.1.0",
    "environment": "production",
    "debug": false
  },
  "database": {
    "host": "db.example.com",
    "port": 5432,
    "name": "codespeedtest_prod",
    "pool_size": 10,
    "ssl": true
  },
  "features": {
    "ai_suggestions": true,
    "leaderboard": true,
    "certificates": true,
    "race_mode": false
  },
  "limits": {
    "free_tests_per_day": 10,
    "max_wpm_record": 300,
    "snippet_max_length": 2000
  }
}

Array of Objects

{
  "languages": [
    {
      "id": 1,
      "name": "Python",
      "slug": "python",
      "paradigm": ["object-oriented", "functional", "procedural"],
      "typing": "dynamic",
      "popular": true
    },
    {
      "id": 2,
      "name": "TypeScript",
      "slug": "typescript",
      "paradigm": ["object-oriented", "functional"],
      "typing": "static",
      "popular": true
    },
    {
      "id": 3,
      "name": "Rust",
      "slug": "rust",
      "paradigm": ["systems", "functional"],
      "typing": "static",
      "popular": false
    }
  ],
  "total": 3,
  "page": 1
}

Deeply Nested Object

{
  "user": {
    "profile": {
      "personal": {
        "name": "Alice",
        "age": 28
      },
      "professional": {
        "role": "Senior Engineer",
        "company": {
          "name": "Acme Corp",
          "size": "enterprise",
          "location": {
            "city": "San Francisco",
            "country": "US"
          }
        }
      }
    },
    "preferences": {
      "theme": "dark",
      "notifications": {
        "email": true,
        "push": false,
        "sms": false
      }
    }
  }
}

Practice typing these in CodeSpeedTest →

JSON is the lingua franca of the web. API responses, config files, package manifests — typing JSON quickly and accurately is a core developer skill. Start typing real code now.

Take the coding speed test →Browse all languages

Tools/JSON Typing

JSON Typing Practice

JSON syntax rules, all six data types, real-world patterns, and the most common mistakes developers make — with typing practice links.

JSON Data Types

JSON supports exactly six value types — nothing more, nothing less.

Type	Example	Description
`string`	`"Hello, World!"`	Sequence of Unicode characters in double quotes
`number`	`42 \| 3.14 \| -7 \| 1e10`	Integer or floating-point — no distinction
`boolean`	`true \| false`	Lowercase literals only (not True/False)
`null`	`null`	Explicit absence of a value
`array`	`[1, "two", true, null]`	Ordered list of values of any type
`object`	`{"key": "value"}`	Unordered collection of key-value pairs

JSON Syntax Rules

✓ Keys must be strings enclosed in double quotes
✓ Key-value pairs are separated by a colon :
✓ Pairs in an object are separated by commas
✓ Arrays use square brackets []; objects use curly braces {}
✓ Whitespace (spaces, tabs, newlines) outside strings is ignored
✗ No trailing commas — ever
✗ No comments — JSON is data, not code
✗ No single quotes — always double quotes

Common JSON Mistakes

Trailing comma— JSON is strict — no trailing commas allowed

Wrong

{"a": 1, "b": 2,}

Correct

{"a": 1, "b": 2}

Single quotes— Both keys and strings must use double quotes

Wrong

{'key': 'value'}

Correct

{"key": "value"}

Unquoted keys— Keys must always be quoted strings

Wrong

{name: 'Alice'}

Correct

{"name": "Alice"}

Comments— JSON does not support comments of any kind

Wrong

// This is JSON
{}

Correct

{}

Undefined value— undefined is a JavaScript concept, not valid JSON

Wrong

{"x": undefined}

Correct

{"x": null}

Functions / regex— JSON is data-only — no functions or regex literals

Wrong

{"fn": function(){}}

Correct

{"fn": null}

NaN / Infinity— NaN and Infinity are not valid JSON numbers

Wrong

{"n": NaN}

Correct

{"n": null}

Unescaped newline— Newlines inside strings must be escaped as \n

Wrong

{"text": "line1
line2"}

Correct

{"text": "line1\nline2"}

Real-World JSON Patterns

These are the JSON structures you encounter every day as a developer. Type them until the structure feels natural.

REST API Response

{
  "status": "success",
  "data": {
    "id": 12345,
    "username": "saurav_dev",
    "email": "dev@example.com",
    "created_at": "2024-01-15T09:30:00Z",
    "is_pro": true,
    "stats": {
      "wpm": 87,
      "accuracy": 97.3,
      "tests_completed": 142
    }
  },
  "meta": {
    "request_id": "req_8f2a4d",
    "duration_ms": 48
  }
}

package.json (Node.js)

{
  "name": "my-app",
  "version": "1.0.0",
  "description": "A Next.js application",
  "main": "index.js",
  "scripts": {
    "dev": "next dev",
    "build": "next build",
    "start": "next start",
    "lint": "next lint"
  },
  "dependencies": {
    "next": "^14.2.0",
    "react": "^18.3.0",
    "react-dom": "^18.3.0"
  },
  "devDependencies": {
    "typescript": "^5.4.0",
    "@types/react": "^18.3.0",
    "tailwindcss": "^3.4.0"
  }
}

App Config File

{
  "app": {
    "name": "CodeSpeedTest",
    "version": "2.1.0",
    "environment": "production",
    "debug": false
  },
  "database": {
    "host": "db.example.com",
    "port": 5432,
    "name": "codespeedtest_prod",
    "pool_size": 10,
    "ssl": true
  },
  "features": {
    "ai_suggestions": true,
    "leaderboard": true,
    "certificates": true,
    "race_mode": false
  },
  "limits": {
    "free_tests_per_day": 10,
    "max_wpm_record": 300,
    "snippet_max_length": 2000
  }
}

Array of Objects

{
  "languages": [
    {
      "id": 1,
      "name": "Python",
      "slug": "python",
      "paradigm": ["object-oriented", "functional", "procedural"],
      "typing": "dynamic",
      "popular": true
    },
    {
      "id": 2,
      "name": "TypeScript",
      "slug": "typescript",
      "paradigm": ["object-oriented", "functional"],
      "typing": "static",
      "popular": true
    },
    {
      "id": 3,
      "name": "Rust",
      "slug": "rust",
      "paradigm": ["systems", "functional"],
      "typing": "static",
      "popular": false
    }
  ],
  "total": 3,
  "page": 1
}

Deeply Nested Object

{
  "user": {
    "profile": {
      "personal": {
        "name": "Alice",
        "age": 28
      },
      "professional": {
        "role": "Senior Engineer",
        "company": {
          "name": "Acme Corp",
          "size": "enterprise",
          "location": {
            "city": "San Francisco",
            "country": "US"
          }
        }
      }
    },
    "preferences": {
      "theme": "dark",
      "notifications": {
        "email": true,
        "push": false,
        "sms": false
      }
    }
  }
}

Practice typing these in CodeSpeedTest →

JSON is the lingua franca of the web. API responses, config files, package manifests — typing JSON quickly and accurately is a core developer skill. Start typing real code now.

Take the coding speed test →Browse all languages