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id schema-decode-encode
title Decode and Encode Data
category getting-started
skillLevel beginner
tags
schema
decode
encode
parsing
serialization
lessonOrder 16
rule
description
Decode and Encode Data using Schema.
summary You receive data as JSON and need to convert it to typed objects. Later, you need to serialize those objects back to JSON for storage or API responses. Without a consistent approach, you write...

Problem

You receive data as JSON and need to convert it to typed objects. Later, you need to serialize those objects back to JSON for storage or API responses. Without a consistent approach, you write parsing code in one place and serialization code elsewhere—they drift apart and bugs creep in.

Solution

import { Schema } from "effect"

// Define schema once—use for both decode and encode
const Product = Schema.Struct({
  id: Schema.String,
  name: Schema.String,
  price: Schema.Number,
  inStock: Schema.Boolean,
})

type Product = typeof Product.Type

// Create decoder and encoder from same schema
const decode = Schema.decodeUnknownSync(Product)
const encode = Schema.encodeSync(Product)

// DECODE: unknown → typed Product
const rawJson = {
  id: "prod_123",
  name: "Widget",
  price: 29.99,
  inStock: true
}

const product: Product = decode(rawJson)
console.log(`Decoded: ${product.name} - $${product.price}`)
// Output: Decoded: Widget - $29.99

// Use the typed product in your code
if (product.inStock) {
  console.log("✅ Available for purchase")
}

// ENCODE: typed Product → unknown (for JSON serialization)
const jsonOutput = encode(product)
console.log(`Encoded: ${JSON.stringify(jsonOutput)}`)
// Output: Encoded: {"id":"prod_123","name":"Widget","price":29.99,"inStock":true}

Why This Works

Concept Explanation
Single source of truth One schema defines both parsing and serialization
decodeUnknownSync Parses and validates unknown data to typed value
encodeSync Converts typed value back to serializable format
Type safety TypeScript knows the exact type after decode
Bidirectional Same schema handles input and output

When to Use

  • Receiving JSON from APIs and returning JSON responses
  • Reading config files and writing them back
  • Caching objects (serialize) and restoring them (deserialize)
  • Any round-trip data transformation

Related Patterns