Awesome

Type-safe JSON Decoder

A strongly typed JSON decoder and validator inspired by Elm, namely the Json.Decode package.

Installation

npm install --save type-safe-json-decoder

Introduction

Parsing JSON introduces an unfortunate any in to TypeScript programs. The objects returned from JSON.parse often become the data sources for entire applications, never once validated against the actual interfaces and classes which they go into. This module allows for the creation of decoders which perform runtime type checks on the input and return a fully typed result.

Given this JSON input:

const usersJSON = `{
  "users": [
    {"id": 1, "name": "Alice"},
    {"id": 2, "name": "Bob"}
  ]
}`

We can create a decoder that matches this expected structure:

import { Decoder, at, array, object, number, string } from 'type-safe-json-decoder'

interface User {
  id: number
  name: string
}

const usersDecoder: Decoder<User[]> = at(['users'], array(
  object(
    ['id', number()],
    ['name', string()],
    (id, name) => ({id, name})
  )
))

const users: User[] = usersDecoder.decodeJSON(usersJSON)

The important thing to note here is that decodeJSON does not return any. It returns a type assignable to User[].

A decoder will also a throw nice error message if it comes across an unexpected value at runtime:

const badJSON = `{
  "users": [{"id": "0", "name": "Mallory"}]
}`

usersDecoder.decodeJSON(badJSON)
// throws => error at .users[0].id: expected number, got string

Documentation

Detalied API documentation can be found here