JSON Serialization and Deserialization Explained

What Are Serialization and Deserialization?

Serialization converts an in-memory object into a format that can be stored or transmitted — in this case, a JSON string. Deserialization is the reverse: parsing a JSON string back into an in-memory object. These are the most fundamental JSON operations in any programming language.

What Gets Lost in Serialization

JSON supports only 6 types: string, number, boolean, null, array, and object. These types are not preserved during round-trip serialization:

Custom Serialization in JavaScript

class User {
  constructor(public name: string, public createdAt: Date) {}

  toJSON() {
    return {
      name: this.name,
      createdAt: this.createdAt.toISOString(),
    };
  }
}

const user = new User('Alice', new Date());
console.log(JSON.stringify(user));
// {"name":"Alice","createdAt":"2026-06-14T12:00:00.000Z"}

// Reviver for deserialization
const parsed = JSON.parse(jsonStr, (key, value) => {
  if (key === 'createdAt') return new Date(value);
  return value;
});

Custom Serialization in Python

import json
from datetime import datetime
from dataclasses import dataclass, asdict

@dataclass
class User:
    name: str
    created_at: datetime

class DateTimeEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, datetime):
            return obj.isoformat()
        return super().default(obj)

user = User('Alice', datetime.now())
print(json.dumps(asdict(user), cls=DateTimeEncoder))

Serialization Performance Tips

• Avoid serializing large objects unnecessarily — be selective about what goes in the payload
• Use streaming serializers for large datasets (instead of building the full string in memory)
• Cache serialized strings when the object does not change frequently
• In Python, orjson is 5-10x faster than the standard json module
• In JavaScript, JSON.stringify is already highly optimized — prefer it over third-party alternatives

FAQ