Type assertion / coercion with io-ts

use-decoder.ts

import * as D from "io-ts/Decoder";
import { extract } from "fp-ts/lib/Identity";
import { fold } from "fp-ts/lib/Either";
import { pipe } from "fp-ts/lib/function";

// Utility to assert/coerce type.
export const decodeType = <P>(type: D.Decoder<unknown, P>, value: unknown): P =>
  pipe(
    type.decode(value),
    fold(() => {
      throw new TypeError("Invalid input");
    }, extract)
  );

// Example interface
interface FooBar {
  id: number;
}

// Decoder for above interface
const TFooBar = D.type({
  id: D.number,
});

const getStuff = (input: { [key: string]: unknown }): FooBar => {
  // Do computation
  const result = { ...input };

  return decodeType<FooBar>(TFooBar, result);
};

// This will throw
getStuff({ id: "42" });

// This will return a FooBar
getStuff({ id: 42 });