Io-ts: How to describe enums and still get typescript types?

[JayAndCatchFire]

I've studied the various approaches in #67 and I've done a bit of my own code to try to crack this nut, but I can't seem to figure out how to represent enums with io-ts and also get all the necessary typescript hints. The various approaches suggested in that ticket and they work at runtime, but I lose all type hints at compile time. The result from a t.TypeOf or a decode on my enum types only show up to typescript as any. Furthermore, the values that are accepted by a function, for example, at compile time that expects the type of the t.TypeOf my io-ts enum accept _all_ numbers or _all_ strings instead of only the valid values associated with the given enum.

Is this a lost cause or is there some way to accurately get the full benefits of io-ts with enums?

[jrunning]

Literally the first thing I tried to do after installing this module today was to try to define an enum. I'm flummoxed, and wondering if it's worth trying to move forward with io-ts at all.

[smpalileo]

They need to be literals of your valid values.

For example:
enum = ['a', 'b', 'c']

would have to be implemented like this:

a = t.literal('a')
b = t.literal('b')
c = t.literal('c')

enumVar = t.union([a,b,c])

[sgronblo]

I just stumbled upon this issue when looking for searching for enums. But isn't the example proposed by @smpalileo the one that is shown as a bad practice here: https://github.com/gcanti/io-ts#union-of-string-literals ?

[omerkhawaja]

Firstly, io-ts library is ground-breaking.

It's my first week using TypeScript, but how about this implementation?

Using this, I'm able to get all benefits of type inference for autocomplete.

// Define a regular enum
enum TestEnum {
  Foo = 'Foo',
  Bar = 'Bar',
}

// Create an Enum-like validator
const TestEnumV = t.keyof({  // using "keyof" for better performance instead of "union"
  [TestEnum.Foo]: null,
  [TestEnum.Bar]: null,
});

// Create a Product type validator
const ProductV = t.type({
  id: t.number,
  name: t.string,
  quantity: t.number,
  type: TestEnumV,
});

type Product = t.TypeOf<typeof ProductV>;
const item: Product = {
  id: 3,
  name: 'yo',
  quantity: 123,
  type: TestEnum.Foo,  // Use TypeScript's "enum" here instead of "TestEnumV"
};

const result = ProductV.is(item);
console.log('result', result); // => true

[noe132]

I wrote this to check enum types, works for both string or number enums, since enums are just simple objects

update 2020-4-19: add enum reverse mapping check for number enums

/* EnumType.ts */
import * as t from 'io-ts'

// EnumType Class
export class EnumType<A> extends t.Type<A> {
  public readonly _tag: 'EnumType' = 'EnumType'
  public enumObject!: object
  public constructor(e: object, name?: string) {
    super(
      name || 'enum',
      (u): u is A => {
        if (!Object.values(this.enumObject).find((v) => v === u)) {
          return false
        }
        // enum reverse mapping check
        if (typeof (this.enumObject as any)[u as string] === 'number') {
          return false
        }

        return true
      },
      (u, c) => (this.is(u) ? t.success(u) : t.failure(u, c)),
      t.identity,
    )
    this.enumObject = e
  }
}

// simple helper function
export const createEnumType = <T>(e: object, name?: string) => new EnumType<T>(e, name)

/* fruit.ts */
// fruit enum
enum FRUIT {
  APPLE = 'APPLE',
  BANANA = 'BANANA',
}

// fruit codec
const FruitItem = t.type({
  fruit: createEnumType<FRUIT>(FRUIT, 'FRUIT'),
  quatity: t.number,
})

const result1 = FruitItem.is({
  fruit: 'BANANA',
  quatity: 10,
})
expect(result1).toBe(true)

const result2 = FruitItem.is({
  fruit: 'apple',
  quatity: 8,
})
expect(result2).toBe(false)


type FruitStaticType = t.TypeOf<typeof FruitItem>
// FruitStaticType is expected to be same as below
type FruitStaticType = {
  fruit: FRUIT
  quatity: number
}

[samhh]

In the meantime, here's a relatively clean alternative using fromRefinement in io-ts-types:

enum Thing {
    A = 'a',
    B = 'b',
}

const isThing: Refinement<unknown, Thing> = (arg): arg is Thing =>
    Object.values<unknown>(Thing).includes(arg);

const thingCodec = fromRefinement<Thing>('thing', isThing);

[geekflyer]

I took @SamHH's idea, removed the dependency on io-ts-types and turned this into a fromEnum utility function. Works great so far.

// this utility function can be used to turn a TypeScript enum into a io-ts codec.
export function fromEnum<EnumType>(enumName: string, theEnum: Record<string, string | number>) {
  const isEnumValue = (input: unknown): input is EnumType => Object.values<unknown>(theEnum).includes(input);

  return new t.Type<EnumType>(
    enumName,
    isEnumValue,
    (input, context) => (isEnumValue(input) ? t.success(input) : t.failure(input, context)),
    t.identity
  );
}

here's a usage example:

import { PathReporter } from "io-ts/lib/PathReporter";

enum Thing {
  FOO = "foo",
  BAR = "bar"
}

const ThingCodec = fromEnum<Thing>("Thing", Thing);

console.log(PathReporter.report(ThingCodec.decode('invalidvalue')));
// prints [ 'Invalid value "invalidvalue" supplied to : Thing' ]
console.log(PathReporter.report(ThingCodec.decode('foo')));
// prints [ 'No errors!' ]

[haysmike]

If you're only using string enums, I found a very slight improvement that allows omission of the type argument without it creating any types:

export function fromEnum<EnumType extends string>(
  enumName: string,
  theEnum: Record<string, EnumType>,
): t.Type<EnumType, EnumType, unknown> {
  const isEnumValue = (input: unknown): input is EnumType =>
    Object.values<unknown>(theEnum).includes(input);

  return new t.Type<EnumType>(
    enumName,
    isEnumValue,
    (input, context) =>
      isEnumValue(input) ? t.success(input) : t.failure(input, context),
    t.identity,
  );
}

Usage:

enum Thing {
  FOO = 'foo',
  BAR = 'bar',
}

const ThingCodec = fromEnum('Thing', Thing); // Can omit the type argument
type ThingType = t.TypeOf<typeof ThingCodec>;

const thing1: ThingType = Thing.FOO;

const thing2: ThingType = 'foo';
// `tsc` error TS2322: Type '"foo"' is not assignable to type 'Thing'.

console.log(PathReporter.report(ThingCodec.decode('invalidvalue')));
// prints [ 'Invalid value "invalidvalue" supplied to : Thing' ]
console.log(PathReporter.report(ThingCodec.decode('foo')));
// prints [ 'No errors!' ]

Of course with numeric enums, the reverse mapping breaks this 🤷

[gcanti]

Of course with numeric enums, the reverse mapping breaks this

Numeric enums shouldn't be used anyway, they are unsafe

export enum E {
  a = 1
}
declare function f(e: E): void
f(666)

[haysmike]

Numeric enums shouldn't be used anyway, they are unsafe

Completely agree!

[webberwang]

I wrote this to check enum types, works for both string or number enums, since enums are just simple objects

update 2020-4-19: add enum reverse mapping check for number enums

should merge this into https://github.com/gcanti/io-ts-types

[mendrik]

I took @samhh's idea, removed the dependency on io-ts-types and turned this into a fromEnum utility function. Works great so far.

// this utility function can be used to turn a TypeScript enum into a io-ts codec.
export function fromEnum<EnumType>(enumName: string, theEnum: Record<string, string | number>) {
  const isEnumValue = (input: unknown): input is EnumType => Object.values<unknown>(theEnum).includes(input);

  return new t.Type<EnumType>(
    enumName,
    isEnumValue,
    (input, context) => (isEnumValue(input) ? t.success(input) : t.failure(input, context)),
    t.identity
  );
}

here's a usage example:

import { PathReporter } from "io-ts/lib/PathReporter";

enum Thing {
  FOO = "foo",
  BAR = "bar"
}

const ThingCodec = fromEnum<Thing>("Thing", Thing);

console.log(PathReporter.report(ThingCodec.decode('invalidvalue')));
// prints [ 'Invalid value "invalidvalue" supplied to : Thing' ]
console.log(PathReporter.report(ThingCodec.decode('foo')));
// prints [ 'No errors!' ]

a bit better version that actually returns typesafe enum values:

export const fromEnum = <T extends string, TEnumValue extends string | number>(
  enumName: string,
  theEnum: { [key in T]: TEnumValue }
): t.Type<TEnumValue> => {
  const isEnumValue = (input: unknown): input is TEnumValue =>
    Object.values(theEnum).includes(input)

  return new t.Type<TEnumValue>(
    enumName,
    isEnumValue,
    (input, context) => (isEnumValue(input) ? t.success(input) : t.failure(input, context)),
    t.identity
  )
}

[mscottx88]

For those that find themselves here, another iteration on the theme.

/**
 * Produce a Map populated with an enumeration's inverted key-value pair(s).
 */
export type ReverseMap<T> = Map<{ [K in keyof T]: T[K] }[keyof T], keyof T>;

/**
 * The Utils service class provides a collection of convenience methods.
 */
export class Utils {
  /**
   * Given an enumeration created in Typescript, produce an io-ts validator schema.
   *
   * @see https://github.com/gcanti/io-ts/issues/216
   *
   * @example
   * ```typescript
   * enum Foo {
   *   bar = 'baz',
   * }
   *
   * const TFizz = t.type({
   *   buzz: Utils.enum<Foo>(Foo),
   * });
   * ```
   *
   * @param {E} enumeration
   * The enumeration object.
   *
   * @param {string} name
   * An optional name of the enumeration type.
   *
   * @returns {t.Type<T>}
   * An io-ts validation schema for the enumeration.
   */
  public static enum<T, E extends object = object>(
    enumeration: E,
    name: string = 'enum',
  ): t.Type<T> {
    return new (class extends t.Type<T> {
      public readonly reverseMap: ReverseMap<E>;

      constructor() {
        super(
          name,
          (value: unknown): value is T => this.reverseMap.has(value as E[keyof E]),
          (value: unknown, context: t.Context): either.Either<t.Errors, T> =>
            this.is(value) ? t.success(value) : t.failure(value, context),
          t.identity,
        );

        this.reverseMap = Utils.enumReverseMap(enumeration);
      }
    })();
  }

  /**
   * Given an arbitrary enumeration, generate the reverse lookup map.
   *
   * @param {object} enumeration
   * The enumeration.
   *
   * @returns {Map}
   * A Mapping where the key is the value and the value is the key.
   */
  public static enumReverseMap<E extends object>(enumeration: E): ReverseMap<E> {
    return new Map(Object.entries(enumeration).map(([key, value]) => [value, key as keyof E]));
  }
}