Simple JSON validation example using RamdaJS and fp-ts with Typescript

index.ts

import { both, converge, equals as eq, ifElse, not, pipe, toLower, type, keys, curry } from "ramda"
import { Either, left, right, isLeft, isRight, getOrElse } from "fp-ts/lib/Either"

export type Json = string|number|boolean|Date|JsonObject 

interface JsonObject {
    [x: string]: Json;
}

interface JsonArray extends Array<Json> { }

export type Schema = { [id: string] : string }

const nameAndAgeSchema: Schema = {
    name: "string",
    age: "number"
}

const validRequest: Json = {
    name: "Michael",
    age: 48
}

const invalidRequest: Json = {
    name: 123,
    age: "Hello"
}


const validType: (t: string) => (value: Json) => Either<string[], Json> = t =>
ifElse(
    both(pipe(eq(undefined), not), pipe(type, toLower, eq(t))),
    right,
    a => left([`${a} is not a valid ${t}`]))


function validInput(schema: Schema, input: Json): Either<string[], Json> {
    return keys(input)
        .map(k => validType(schema[k])(input[k]))
        .reduce((acc, element): Either<string[], Json> => 
            element.fold(
                e => isLeft(acc) ? acc.mapLeft(l => l.concat(e)) : left(e),
                j => acc
            )
        , right(input))
}

const validNameAndAge = curry(validInput)(nameAndAgeSchema)

console.log(validNameAndAge(validRequest).value) // => { name: 'Michael', age: 48 }
console.log(validNameAndAge(invalidRequest).value) // => [ '123 is not a valid string', 'Hello is not a valid number' ]