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TypeScript library that performs dynamic type checking

TypeScript is great. It warns you of static type errors and hence earns you lots of time and headaches. But your program probably have entrypoints (network requests, file readings, etc.) that can not be trusted completely.

For instance, supose you read some configuration from a JSON file:

import { readFile } from 'fs';

interface AwsConfig {
    // definitions...

readFile('./my-aws-config.json', { encoding: 'utf8' }, (err, awsConfigStr) => {
    if (err) {
    const awsConfig: AwsConfig = JSON.parse(awsConfigStr);

In this example, TypeScript can not prevent errors if the read JSON doesn't have an expected property. These are some cases this library was created for.


  • Infers typings.
  • Very lightweight (under 4kb without minifying).
  • Expressive errors.
  • Works both client and server-side.


npm install --save parmenides


The main concept behind the library is "contracts". A contract is an identity function that throws an error if the parameter doesn't have the expected type.

For example if you pass a string to the str contract, it will return the same value, for other types it will throw an error:

import { str } from 'parmenides';

str('Hello world'); // <- Returns 'Hello world'
str(8 as any); // <- Throws an error

Check for specific values

import { oneOf } from 'parmenides';
import { createInterface } from 'readline';

const isFruit = oneOf('apple', 'banana', 'strawberry', 'orange');
type Fruit = 'apple' | 'banana' | 'strawberry' | 'orange';

const readLine = createInterface({
  input: process.stdin,
  output: process.stdout

const aFruit: Fruit = 'cheese'; // <- static error. It will be warned by TypeScript itself.

readLine.question('Which is your favourite fruit?', (someFruit) => {
    const favouriteFruit: Fruit = isFruit(someFruit); // <- Will throw an error if `someFruit` has any other value than 'apple', 'banana', 'strawberry' or 'orange'. It's a potential dynamic error and TypeScript could not detect it.

It's important to notice that while str is a contract itself, oneOf is not. oneOf is a function that returns a contract. You can think of it like a contract builder.

There are some other functions that help you build contracts. For instance, there is arrOf:

import { arrOf, num, oneOf } from 'parmenides';

const onlyNumbers = arrOf(num);

onlyNumbers([1, 2, 3]); // <- Returns [1, 2, 3]
onlyNumbers(['Hello', 'world', 99] as any); // <- Throws an error

const onlyHobbits = arrOf(oneOf('Frodo', 'Bilbo', 'Merry', 'Pippin', 'Sam', 'Gollum'));

onlyHobbits(['Bilbo', 'Sam']); // <- Returns the array
onlyHobbits(['Frodo', 'Pippin', 'Gandalf']); // <- Throws an error

As you can see, arrOf takes a contract as parameter and returns another contract.

Last, but not least, the objOf function is perhaps the most usefull one:

import { objOf, bool, str, num, arrOf } from 'parmenides';

const personValidator = objOf({
    name: str,
    age: num,
    profession: oneOf('student', 'employed', 'unemployed', 'retired'),
    address: objOf({
        street: str,
        city: str,
        state: str,
        country: str
    driving_license: bool

const peopleValidator = arrOf(personValidator);

// xhr function from any library you like
    .then(people => /* The `people` variable is guaranteed to have the shape you have defined... */);

Notice that the objOf function takes an object that describes the shape of the expected objects as a parameter. That object's properties are contracts.

Type inference

It's important to mention that all the contracts are typed and TypeScript will prevent errors if the parameters are incorrect and will infer the output:

import { str, num, objOf } from 'parmenides';

str(9); // TypeScript will error ("string expected").
const aNumber = num(9); // TypeScript will infere it's a number.

const fooBarContract = objOf({
    foo: str,
    bar: num

    baz: 'Hello'
}); // <- Typescript will error

const fooBar = fooBarContract({
    foo: 'Hello',
    bar: 100
}); // <- TypeScript will infer type {foo: string; bar: number;}

furthermore, you can use the type function ContractOf to put the types of a contract inside a type. For example:

import { ContractOf, num, objOf, str } from 'parmenides';

const personContract = objOf({
    name: str,
    age: num

type Person = ContractOf<typeof personContract>;

const john: Person = {
    name: "John Doe",
    age: 27

This is very useful as you can define the types for your external sources from contracts, and then use it with its name.

Because of the difference between type and interface, when you use the first, the type will be an alias for { name: string, age: number}, and if you use an editor like VSCode, that's what you'll see when you hover over it. If you want to give it a name, you can use the following trick:

import { ContractOf, num, objOf, str } from 'parmenides';

const personContract = objOf({
    name: str,
    age: num

interface Person extends ContractOf<typeof personContract> {}

const john: Person = {
    name: "John Doe",
    age: 27


Built-in contracts

Function Type Example
bool IContract<boolean> bool(true);
num IContract<number> num(89);
str IContract<string> str('Hello world');
undef IContract<undefined> undef(undefined);
nil IContract<null> nil(null);
arr <T> IContract<T[]> arr([1, 2, 3]);
obj <T extends object> IContract<T> bool({foo: 'foo'});
regExp IContract<RegExp> regExp(/^hello/i);
date IContract<Date> date(new Date());
anything <T> IContract<T> anything(4);
never IContract<never> never(4 as never);

A note on anything

anything is just an identity function that will never throw an error. Its static type will be inferred from the value if possible or will default to any. It's useful with another functions like objOf (view below). For instance you can define a contract like:

const objHasFooContract = objOf({
    foo: anything

A note on never

You may think the never contract is useless. But it can be used to do an exhaustive check:

const reactToSemaphore = (semaphoreLight: 'red' | 'yellow' | 'green') => {
    switch (semaphoreLight) {
        case 'red':
            return 'stop';
        case 'yellow':
            return 'hurry';
        case 'green':
            return 'go';

The function reactToSemaphore will fail in runtime if passed another value than 'red' | 'yellow' | 'green', but also with statically check that you aren't missing a case in the switch statement.

You can read more about the use of never here.

contract builders


<T> (IContract<T>) -> IContract<T | undefined>

Takes a contract and returns a new one that matches like the first one but also matches undefined values.

const optionalNumber = optional(num);
// All the following are valid:


<T> (IContract<T>) -> IContract<T | null>

Takes a contract and returns a new one that matches like the first one but also matches null values.

const nullableNumber = nullable(num);
// The following are valid


(...(string | number | boolean)[]) -> IContract<union of valid values>

It is used to validate unum-like values. You specify the valid values and it returns a contract that will check against them. Example:

const osContract = oneOf('Linux', 'Mac OS', 'Windows', 'Other');
const os = osContract('Linux'); // os's type is 'Linux' | 'Mac OS' | 'Windows' | 'Other'

TypeScript will infere the contract's return value as the union of the literal types passed (up to 10 parameters, then behaves like <T extends string | number | boolean> IContract<T>).


...(IContract) _> IContract<union of valid values>

It takes contracts as arguments and returns a new contract that matches if any of the them matches.

const numOrStr = union(num, str);

TypeScript will infere the contract's return value as the union of the return values of the contracts passed (up to 10 parameters, then behaves like IContract<any>).


<T> (IContract<T>) -> IContract<T[]>

It takes a contract "C" as a parameter and returns another contract that expects an array of elements that match C.

const arrOfNumbersContract = arrOf(num);
const numbers = arrOfNumbersContract([1, 2, 3]);


<T> (IMapOfContracts<T>) -> IContract<T>

Takes an object that describes the shape of the objects you want to validate and returns a contract with that validation. That object's values must be contracts.

const petContract = objOf(
    name: str,
    species: oneOf('dog', 'cat', 'golden fish', 'parrot', 'other'),
    age: number,
    gender: oneOf('male', 'female')
// <3
const oddy = petContract({
    name: 'Oddy',
    species: 'dog',
    age: 8,
    gender: 'female'


<T> (IMapOfContracts<T>) -> IContract<T>

It is the same than objOf function, but also checks that the target doesn't have extra properties.

// It only matches empty objects
const emptyObjectContract = strictObjOf({});
const emptyObject = emptyObjectContract({});


<C> (constructor: C) -> IContract<I> (I is instance of C)

It takes a class or a or a constructor function and returns a contract of instances of that class or constructor.

class Foo {}

const instanceOfFooContract = instanceOf(Foo);
const foo = instanceOfFooContract(new Foo());


<T> (IContract<T>) -> IContract<Record<string, T>>

It takes a contract "C" as a parameter and returns another contract that expects a dictionary of elements that match C.

const contactsContract = dictionaryOf(
        name: str,
        age: num

const contacts = contactsContract({
    john: { name: 'john', age: 27},
    jane: { name: 'jane', age: 27}


This library is published through TravisCI when merged to master, the version number is calculated automatically by semantic release.

It is important for the calculation of the release number that the previous npm release is found in the master branch and that the PR are merged without squashing.

The normal flow of development should be to create PR's against the development branch, merge them using the merge commit option. Then, once all the needed features/fixes are merged, we can create a PM from development to master that should be merged with fast-forward. We still need to configure github to disallow other type of merges and ease the release cycle.


Made from the tsall/typescript-library-starter.

Thanks goes to these wonderful people (emoji key):

Gonzalo Gluzman

💻 🎨 📖 💡 🤔 📦 ⚠️

Hernan Rajchert

🎨 🤔 💻

This project follows the all-contributors specification. Contributions of any kind welcome!


Identity functions that checks typings.




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