JSON Validator and Linter Guide

Validating JSON ensures your data is correctly formatted before processing.

Basic Validation

function isValidJSON(str) {
  try {
    JSON.parse(str);
    return true;
  } catch (e) {
    return false;
  }
}

// Usage
console.log(isValidJSON('{"name":"John"}')); // true
console.log(isValidJSON('{name:John}'));     // false

Detailed Error Messages

function validateJSON(str) {
  try {
    const parsed = JSON.parse(str);
    return { valid: true, data: parsed };
  } catch (e) {
    const match = e.message.match(/position (\d+)/);
    const position = match ? parseInt(match[1]) : null;
    
    return {
      valid: false,
      error: e.message,
      position
    };
  }
}

Using ajv for Schema Validation

import Ajv from 'ajv';

const ajv = new Ajv();

const schema = {
  type: 'object',
  properties: {
    name: { type: 'string' },
    age: { type: 'integer' },
    email: { type: 'string', format: 'email' }
  },
  required: ['name', 'email']
};

const validate = ajv.compile(schema);

const data = { name: 'John', age: 30, email: 'john@example.com' };
const valid = validate(data);

if (!valid) {
  console.log(validate.errors);
}

Common JSON Errors

| Error | Cause | Fix | |-------|-------|-----| | Unexpected token | Invalid character | Check for typos | | Unexpected end | Unclosed bracket | Count your [] | | Missing comma | Between elements | Add commas | | Trailing comma | After last item | Remove it | | Single quotes | Only double allowed | Use " not ' |

Formatter + Validator

function formatAndValidate(str) {
  try {
    const parsed = JSON.parse(str);
    return {
      valid: true,
      formatted: JSON.stringify(parsed, null, 2)
    };
  } catch (e) {
    return { valid: false, error: e.message };
  }
}

CLI Tools

# Validate JSON
cat file.json | python -m json.tool

# Pretty print
node -e "console.log(JSON.stringify(JSON.parse(require('fs').readFileSync(0)), null, 2))"

# jq
cat file.json | jq .

Online Validators

• jsonlint.com
• jsonformatter.curiousconcept.com
• jsonvalidator.org

Best Practices

1. Validate at input - Check JSON before processing
2. Validate at output - Ensure correct format
3. Use schemas - For complex data structures
4. Add validation in CI - Catch errors early

Lint Rules

// Example: no-trailing-commas
// Invalid
{
  "name": "John",
}

// Valid
{
  "name": "John"
}

Schema Example

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "type": "object",
  "properties": {
    "username": {
      "type": "string",
      "minLength": 3,
      "maxLength": 30
    },
    "email": {
      "type": "string",
      "format": "email"
    },
    "age": {
      "type": "integer",
      "minimum": 0,
      "maximum": 150
    }
  },
  "required": ["username", "email"]
}

Conclusion

Always validate JSON before processing. Use schema validation for complex data. Add validation in your build process and CI pipeline.