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    Lawful Type Classes

    lawful-typeclasses is a library designed to provide a way of asserting the behavior of your JavaScript classes.

    "Lawful" here refers to a characteristic of principled type classes.

    What it does

    This library allows you to define two things: classes and instances. Perhaps a bit confusedly, classes are JavaScript objects and instances are JavaScript classes.

    We'll be referring to the JavaScript classes that implement the behavior of a type class (and are thus instances of that class) as constructors and to the instances of those JavaScript classes as instance values.

    What this library then allows you to do is to check if every constructor follows the rules defined in their classes, so that you to modularize your tests in a neat way.


    A class is what defines the behavior that you want your instances to conform to.

    For example, let's say that you want to define a class of things that can be added:

    // our class is going to be an instance of Class
    const addable = new Class({
      // this is what I've decided to name my class
      // this option is not necessary, but it helps to improve error messages
      name: 'Addable',
      // next, we define the properties we expect our instances to have.
      // we'll start out by using the `all` function to say that, in order to
      // be an Addable, the constructor must obey all of the following laws
      // (not just any)
      laws: all(
        // using named functions is not necessary, but it helps to improve error
        // messages as well
        // the first parameter is the instance constructor itself, the others are
        // random values of that constructor
        obey(function commutativity(Instance, x, y) {
          const a = x.add(y)
          const b = y.add(x)
          return a.equals(b)
        obey(function associativity(Instance, x, y, z) {
          const a = x.add(y.add(z))
          const b = x.add(y).add(z)
          return a.equals(b)

    But, as you might have seen, we also expect our instances to implement an #equals method.

    That could be another class:

    const eq = new Class({
      name: 'Eq',
      laws: all(
        obey(function reflexivity(Instance, x) {
          return x.equals(x)

    And then the Addable class may extend Eq, meaning that, in order to be an instance of Addable, the constructor must also be an instance of Eq:

    const addable = new Class({
      name: 'Addable',
      extends: [eq],
      laws: // ...


    Instances are JavaScript classes that behave according to some (type) class.

    Using the Addable example above, one could almost define an instance as:

    // you may use this decorator as many times as needed
    class Number {
      constructor(n) {
        this.n = n
      equals(other) {
        return this.n === other.n
      add(other) {
        return new Number(this.n + other.n)

    The only extra step is to define a static method called generateData, that will take any number of random numbers in the range [0, 1] as parameters and should return a random instance value.

    class Number {
      // ...
      static generateData(n) {
        // this is quite a trivial example, we just wrap the n.
        // in case you need more random values, just add them as parameters and they
        // will be provided
        return new Number(n)

    With that done, you need only to call validate on the constructor and the validators will run. You should call this at some point in your tests.

    // will throw an Error if it fails


    You may also check whether a value is of an instance of a class using the isInstance function:

    const n = new Number(50)
    isInstance(n, addable) // true, because Numbers are addable

    How it works

    When you define your constructor using the @instance decorator, some metadata will be injected into it and into every value it produces. That metadata will be used to both run the proper validations during validate and also to allow the isInstance to work.

    When validate is called, for each class that the constructor should be an instance of, a sample of random instance values will be generated using your constructor's generateData, and each class property will be tested using those. If any of the laws fails to be asserted, an error is thrown, and you may be sure that the constructor in question is not an instance of the class you declared in the decorator.

    In case it passes, you may have a high confidence that it is.

    What if I can't use decorators?

    An approach would be to use the instance(...) decorator as a regular function:

    class Number {
      // ...
    // new instances shall be instantiated using the returned constructor
    const VNumber = instance(addable)(Number)


    npm i lawful-typeclasses

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