Runtypes

Safely bring untyped data into the fold

Runtypes 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.

Installation

npm install --save runtypes

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, Record, Union } from 'runtypes';

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

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

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

const Ship = Record({
  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.check(obj);

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

Static type inference

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

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

That is, it's a Runtype<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 Runtype-derived type like so:

import { Static } from 'runtypes';

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 check method, runtypes can be used as type guards:

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

Pattern matching

The Union runtype offers the ability to do type-safe, exhaustive case analysis across its variants using the match method:

const isHabitable = SpaceObject.match(
  asteroid => false,
  planet => planet.habitable,
  ship => true,
);

if (isHabitable(spaceObject)) {
  // ...
}

There's also a top-level match function which allows testing an ad-hoc sequence of runtypes:

const makeANumber = match(
  [Number, n => n * 3],
  [Boolean, b => b ? 1 : 0],
  [String, s => s.length],
);

makeANumber(9); // = 27

To allow the function to be applied to anything and then handle match failures, simply use an Unknown case at the end:

const makeANumber = match(
  [Number, n => n * 3],
  [Boolean, b => b ? 1 : 0],
  [String, s => s.length],
  [Unknown, () => 42]
);

Constraint checking

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

const Positive = Number.withConstraint(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 = Number.withConstraint(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 = Number.withConstraint(n => n > 0, {name: 'PositiveNumber'});

To change the type, there are two ways to do it: passing a type guard function to a new Runtype.withGuard() method, or using the familiar Runtype.withConstraint() method. (Both methods also accept an options parameter to optionally set the name.)

Using a type guard function is the easiest option to change the static type, because TS will infer the desired type from the return type of the guard function.

// use Buffer.isBuffer, which is typed as: isBuffer(obj: any): obj is Buffer;
const B = Unknown.withGuard(Buffer.isBuffer);
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 withConstraint<T>() 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 = Unknown.withConstraint<Buffer>(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 guard. 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.

Speaking of base types, if you're using a type guard function and your base type is Unknown, then there's a convenience runtype Guard available, which is a shorthand for Unknown.withGuard.

// use Buffer.isBuffer, which is typed as: isBuffer(obj: any): obj is Buffer;
const B = Guard(Buffer.isBuffer);
type T = Static<typeof B>; // T will have type of `Buffer`

Function contracts

Runtypes along with constraint checking are a natural fit for enforcing function contracts. You can construct a contract from Runtypes for the parameters and return type of the function:

const divide = Contract(
  // Parameters:
  Number,
  Number.withConstraint(n => n !== 0 || 'division by zero'),
  // Return type:
  Number,
).enforce((n, m) => n / m);

divide(10, 2); // 5

divide(10, 0); // Throws error: division by zero

Optional values

Runtypes 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 = String.Or(Undefined); // string | undefined (e.g. 'text', undefined)
const MyStringNullable = String.Or(Null);   // string | null      (e.g. 'text', null)

If a Record 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 = Ship.And(Record({
  // All registered ships must have this flag
  isRegistered: Literal(true),
})).And(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: Rank.Or(Null),
}));

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

const MilitaryShip = Ship.And(Record({
  shipClass: Literal('military'),
  
  // Must NOT be undefined, but can be null
  lastDeployedTimestamp: Number.Or(Null),
}));

Readonly records and arrays

Array and Record runtypes have a special function .asReadonly(), that creates a new runtype where the values are readonly.

For example:

const Asteroid = Record({
  type: Literal('asteroid'),
  location: Vector,
  mass: Number,
}).asReadonly()

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

const AsteroidArray = Array(Asteroid).asReadonly()

Static<typeof AsteroidArray> // ReadonlyArray<Asteroid>

Related libraries

• runtypes-generate Generates random data by Runtype for property-based testing
• rest.ts Allows building type safe and runtime-checked APIs