Runtime typing

index.ts

import Ajv from 'ajv'
import { FromSchema } from 'json-schema-to-ts'

const productSchema = {
  type: 'object',
  properties: {
    id: { type: 'number' },
    name: { type: 'string' },
    quantity: { type: 'number' },
    type: {
      type: 'string',
      enum: [ 'FURNITURE', 'BOOK' ]
    }
  },
  additionalProperties: false,
  required: [ 'id', 'name', 'quantity', 'type' ]
} as const


type Product = FromSchema<typeof productSchema>
/*
type Product = {
    id: number;
    name: string;
    quantity: number;
    type: "FURNITURE" | "BOOK";
}
*/

const product: Product = {
  id: 0,
  name: '',
  quantity: 0,
  type: 'BOOK'
}

const bad: any = {
  id: '',
  name: 0,
  type: 'FISH'
}

const ajv = new Ajv()

const isProduct = ( value: any ): value is Product =>
  ajv.validate( productSchema, value )

const assertProduct = ( value: unknown ) => {
  if( !isProduct( value ) ){
    throw Error( ajv.errorsText( ajv.errors ) )
  }

  return value
}

console.log( isProduct( product ) )
console.log( isProduct( bad ) )

try {
  const p = assertProduct( bad )

  // if p passed assertion, it's a Product
  const { id, name, type, quantity } = p
  console.log( 'p is a Product', { id, name, type, quantity } )
} catch( err ){
  console.error( err )
}