JSON Schema Validation — Cheat Sheet

**By Dev Tools Weekly** | jsonformat.co
Common patterns, error types, and debugging tips.

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JSON Syntax Rules (The Basics)

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JSON Data Types

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Common Parse Errors & Fixes

1. Unexpected Token

SyntaxError: Unexpected token ' in JSON at position 5

Cause: Single quotes instead of double quotes.

Fix: Replace all ' with " for strings.

2. Trailing Comma

SyntaxError: Unexpected token } in JSON at position 28

Cause: {"a": 1, "b": 2,} — comma before closing brace/bracket.

Fix: Remove the last comma.

3. Unescaped Special Characters

SyntaxError: Unexpected token (newline) in JSON

Cause: String contains literal newline, tab, or backslash.

Fix: Escape them: \n, \t, \\.

4. Unquoted Keys

SyntaxError: Unexpected token k in JSON

Cause: {key: "value"} — keys must be quoted.

Fix: {"key": "value"}.

5. Missing Comma

SyntaxError: Expected ',' or '}' after property value

Cause: Missing comma between properties.

Fix: Add commas between all key-value pairs.

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JSON Schema — Quick Reference

Basic Structure

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "properties": {
    "name": { "type": "string" },
    "age": { "type": "integer", "minimum": 0 },
    "email": { "type": "string", "format": "email" }
  },
  "required": ["name", "email"]
}

Type Keywords

String Formats (Built-in)

Composition Keywords

// Must match ALL schemas
{ "allOf": [{ "type": "string" }, { "minLength": 1 }] }
// Must match at least ONE
{ "anyOf": [{ "type": "string" }, { "type": "number" }] }
// Must match EXACTLY one
{ "oneOf": [{ "type": "string" }, { "type": "integer" }] }
// Must NOT match
{ "not": { "type": "null" } }
// Conditional
{ "if": { "properties": { "type": { "const": "business" }}},
  "then": { "required": ["company"] },
  "else": { "required": ["firstName"] }
}

Common Patterns

Enum (fixed values):

{ "type": "string", "enum": ["active", "inactive", "pending"] }

Nested object:

{
  "type": "object",
  "properties": {
    "address": {
      "type": "object",
      "properties": {
        "street": { "type": "string" },
        "city": { "type": "string" },
        "zip": { "type": "string", "pattern": "^[0-9]{5}$" }
      },
      "required": ["street", "city"]
    }
  }
}

Array of objects:

{
  "type": "array",
  "items": {
    "type": "object",
    "properties": {
      "id": { "type": "integer" },
      "name": { "type": "string" }
    },
    "required": ["id", "name"]
  },
  "minItems": 1,
  "uniqueItems": true
}

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Debugging JSON — Pro Tips

1. **Validate first, debug second.** Paste into [jsonformat.co](https://jsonformat.co) — it shows the exact line and column of errors.
2. **Minified JSON unreadable?** Pretty-print it. Most errors become obvious with indentation.
3. **Copy from API response?** Watch for trailing content — some APIs return multiple JSON objects (NDJSON).
4. **Unicode issues?** JSON requires UTF-8. BOM (\uFEFF) at the start will break parsers.
5. **Large numbers?** JavaScript loses precision above 2^53 - 1 (9007199254740991). Use strings for big IDs.

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jq Cheat Sheet (CLI JSON Processing)

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