Funtypes

Safely bring untyped data into the fold

Funtypes allow you to take values about which you have no assurances and check that they conform to some type A. This is done by means of composable type validators of primitives, literals, arrays, tuples, records, unions, intersections and more.

This library is a fork of the excellent runtypes by Tom Crockett

Installation

npm install --save funtypes

Example

Suppose you have objects which represent asteroids, planets, ships and crew members. In TypeScript, you might write their types like so:

type Vector = [number, number, number];

type Asteroid = {
  type: 'asteroid';
  location: Vector;
  mass: number;
};

type Planet = {
  type: 'planet';
  location: Vector;
  mass: number;
  population: number;
  habitable: boolean;
};

type Rank = 'captain' | 'first mate' | 'officer' | 'ensign';

type CrewMember = {
  name: string;
  age: number;
  rank: Rank;
  home: Planet;
};

type Ship = {
  type: 'ship';
  location: Vector;
  mass: number;
  name: string;
  crew: CrewMember[];
};

type SpaceObject = Asteroid | Planet | Ship;

If the objects which are supposed to have these shapes are loaded from some external source, perhaps a JSON file, we need to validate that the objects conform to their specifications. We do so by building corresponding Runtypes in a very straightforward manner:

import { Boolean, Number, String, Literal, Array, Tuple, Object, Union } from 'funtypes';

const Vector = Tuple(Number, Number, Number);

const Asteroid = Object({
  type: Literal('asteroid'),
  location: Vector,
  mass: Number,
});

const Planet = Object({
  type: Literal('planet'),
  location: Vector,
  mass: Number,
  population: Number,
  habitable: Boolean,
});

const Rank = Union(
  Literal('captain'),
  Literal('first mate'),
  Literal('officer'),
  Literal('ensign'),
);

const CrewMember = Object({
  name: String,
  age: Number,
  rank: Rank,
  home: Planet,
});

const Ship = Object({
  type: Literal('ship'),
  location: Vector,
  mass: Number,
  name: String,
  crew: Array(CrewMember),
});

const SpaceObject = Union(Asteroid, Planet, Ship);

(See the examples directory for an expanded version of this.)

Now if we are given a putative SpaceObject we can validate it like so:

// spaceObject: SpaceObject
const spaceObject = SpaceObject.parse(obj);

If the object doesn't conform to the type specification, parse will throw an exception.

Static type inference

In TypeScript, the inferred type of Asteroid in the above example is

Codec<{
  type: 'asteroid'
  location: [number, number, number]
  mass: number
}>

That is, it's a Codec<Asteroid>, and you could annotate it as such. But we don't really have to define the Asteroid type in TypeScript at all now, because the inferred type is correct. Defining each of your types twice, once at the type level and then again at the value level, is a pain and not very DRY. Fortunately you can define a static Asteroid type which is an alias to the Codec-derived type like so:

import { Static } from 'funtypes';

type Asteroid = Static<typeof Asteroid>;

which achieves the same result as

type Asteroid = {
  type: 'asteroid';
  location: [number, number, number];
  mass: number;
};

Type guards

In addition to providing a parse method, funtypes can be used as type guards:

function disembark(obj: {}) {
  if (SpaceObject.test(obj)) {
    // obj: SpaceObject
    if (obj.type === 'ship') {
      // obj: Ship
      obj.crew = [];
    }
  }
}

Constraint checking

Beyond mere type checking, we can add arbitrary runtime constraints to a Codec:

const Positive = Constraint(Number, n => n > 0);

Positive.check(-3); // Throws error: Failed constraint check

You can provide more descriptive error messages for failed constraints by returning a string instead of false:

const Positive = Constraint(Number, n => n > 0 || `${n} is not positive`);

Positive.check(-3); // Throws error: -3 is not positive

You can set a custom name for your runtype, which will be used in default error messages and reflection, by using the name prop on the optional options parameter:

const C = Constraint(Number, n => n > 0, {name: 'PositiveNumber'});

To change the type, you can explicitly specify the underlying type and the constrained type when calling Constrain. Note that TypeScript will not check that your constraint function actually validates that the value is the constrained type.

type Email = `${string}@${string}`
const EmailSchema = Constraint<string, Email>(String, e => e.includes('@'), {name: 'Email'});

Custom Type Validators

The easiest way to add your own custom types is using the Guard function. TypeScript will infer the type for your codec from the function you provide.

// use Buffer.isBuffer, which is typed as: isBuffer(obj: any): obj is Buffer;
const B = Guard(Buffer.isBuffer, { name: "Buffer" });
type T = Static<typeof B>; // T is Buffer

However, if you want to return a custom error message from your constraint function, you can't do this with a type guard because these functions can only return boolean values. Instead, you can roll your own constraint function and use the Constraint<TUnderlying, TParsed>() method. Remember to specify the type parameter for the Constraint because it can't be inferred from your check function!

const check = (o: any) => Buffer.isBuffer(o) || 'Dude, not a Buffer!';
const B = Constraint<unknown, Buffer>(Unknown, check);
type T = Static<typeof B>; // T will have type of `Buffer`

One important choice when changing Constraint static types is choosing the correct underlying type. The implementation of Constraint will validate the underlying type before running your constraint function. So it's important to use a lowest-common-denominator type that will pass validation for all expected inputs of your constraint function or type test. If there's no obvious lowest-common-denominator type, you can always use Unknown as the underlying type, as shown in the Buffer examples above.

Optional values

Funtypes can be used to represent a variable that may be null or undefined as well as representing keys within records that may or may not be present.

// For variables that might be undefined or null
const MyString = String;                        // string             (e.g. 'text')
const MyStringMaybe = Union(String, Undefined); // string | undefined (e.g. 'text', undefined)
const MyStringNullable = Union(String, Null);   // string | null      (e.g. 'text', null)
const MyOtherStringNullable = Nullable(String); // Equivalent to Union(String, Null)

If a Object may or may not have some keys, we can declare the optional keys using myRecord.And(Partial({ ... })). Partial keys validate successfully if they are absent or undefined (but not null) or the type specified (which can be null).

// Using `Ship` from above
const RegisteredShip = Intersect(
  Ship,
  Object({
    // All registered ships must have this flag
    isRegistered: Literal(true),
  }),
  Partial({
    // We may or may not know the ship's classification
    shipClass: Union(Literal('military'), Literal('civilian')),

    // We may not know the ship's rank (so we allow it to be undefined via `Partial`),
    // we may also know that a civilian ship doesn't have a rank (e.g. null)
    rank: Nullable(Rank),
  })
);

If a record has keys which must be present but can be null, then use the Object runtype normally instead.

const MilitaryShip = Intersect(
  Ship,
  Object({
    shipClass: Literal('military'),
    
    // Must NOT be undefined, but can be null
    lastDeployedTimestamp: Nullable(Number),
  })
);

Readonly records and arrays

Arrays, Objects, Tuples and Intersections of Arrays, Objects and Tuples can be made readonly using the Readonly helper:

For example:

const Asteroid = Readonly(
  Object({
    type: Literal('asteroid'),
    location: Vector,
    mass: Number,
  })
)

Static<typeof Asteroid> // { readonly type: 'asteroid', readonly location: Vector, readonly mass: number }

const AsteroidArray = Readonly(Array(Asteroid))

Static<typeof AsteroidArray> // ReadonlyArray<Asteroid>

You can also use ReadonlyArray, ReadonlyObject, ReadonlyPartial, ReadonlyTuple as shorthands if you know you want read only variants, and you can import from funtypes/readonly instead of importing from funtypes to default to read only for all types.

Partial, Pick, Omit

Objects (and intersections of objects) can be manipulated using Partial, Pick and Omit just like they can in TypeScript.

const Asteroid = Object({
  type: Literal('asteroid'),
  location: Vector,
  mass: Number,
})
const PartialAsteroid = Partial(Asteroid) // Codec<{ type?: "asteroid"; location?: Vector; mass?: number }>
const MassObj = Pick(Asteroid, ["mass"]) // Codec<{ mass: number }>
const TypeAndLocObj = Omit(Asteroid, ["mass"]) // Codec<{ type: "asteroid"; location: Vector }>