prop-sets

prop-sets is a test tool to help generate every possible instance of a component in JavaScript + TypeScript.

With prop-sets, you don't need to outsmart your own code when writing tests. Instead of determining fragile points of failure from particular combinations of inputs, simply generate all possible combinations and assert everything.

Works with React, Vue, Jest, Mocha, etc. No dependencies.

Benefits

Let's say you have a React component called Button with the props disabled and color, as well as a test that asserts the button is gray when disabled is true and color otherwise. Here's how to use prop-sets to assert the component renders the correct color:

const Button = props => (
  <button
    disabled={props.disabled}
    style={{
      backgroundColor: props.disabled ? "gray" : props.color
    }}
  />
);

it("is gray when disabled, props.color otherwise", () => {
  propSets({
    color: ["red", "blue"],
    disabled: [true, false]
  }).forEach(props => {
    const component = <Button {...props} />;
    const color = getColor(component);

    expect(color).toBe(props.disabled ? "gray" : props.color);
  });
});

prop-sets helps you easily write tests for assertions that are based on multiple input values (in this case, disabled and color) without greatly increasing the amount of code you have to write.

Without prop-sets, this test will need to be expanded to three assertions:

it("is gray when disabled", () => {
  const component = <Button disabled color="red" />;
  const color = getColor(component);

  expect(color).toBe("gray");
});

it("is red when props.color is red", () => {
  const component = <Button color="red" />;
  const color = getColor(component);

  expect(color).toBe("red");
});

it("is blue when props.color is blue", () => {
  const component = <Button color="blue" />;
  const color = getColor(component);

  expect(color).toBe("blue");
});

Because backgroundColor's value is determined by both the disabled and color prop, we need to have all three assertions to be sure the component behaves as expected. Here are some implementations of Button that will only pass certain tests but fail all others.

// Passes 'is gray when disabled', fails all others
const Button = props => <button style={{ backgroundColor: "gray" }} />;

// Passes 'is red when color is red', fails all others
const Button = props => <button style={{ backgroundColor: "red" }} />;

// Passes 'is blue when color is blue', fails all others
const Button = props => <button style={{ backgroundColor: "blue" }} />;

// Passes 'is gray when disabled', 'is red when color is red', fails all others
const Button = props => (
  <button style={{ backgroundColor: props.disabled ? "gray" : "red" }} />
);

The amount of combinations prop-sets generates is the Cartesian product of all the values passed in (a.length * b.length * c.length * ...), so as the amount of props grows, prop-sets reduces your test's complexity at an exponential rate.

For example, if you have a component that only behaves a certain way if all 5 of its boolean props are true, the amount of tests you would need to write to formally assert that behavior is 32. With prop-sets, just one!:

it("does something if all props are true, false otherwise", () => {
  const tf = [true, false];
  propSets({ a: tf, b: tf, c: tf, d: tf, e: tf }).forEach(props => {
    expect(/* something */).toBe(
      props.a && props.b && props.c && props.d && props.e
    );
  });
});

Use

Install

yarn add prop-sets or npm install prop-sets

Import

import propSets from "prop-sets";

API

propSets(object)

Arguments

Return

TypeScript

prop-sets comes typed but works perfectly fine without TypeScript.

declare const propSets: <
  T extends Readonly<{
    [key: string]: ReadonlyArray<any>;
  }>
>(
  obj: T
) => {
  [key in keyof T]: T[key] extends (infer ElementType)[] ? ElementType : any
}[];

Further reading

Why Don't People Use Formal Methods? by Hillel Wayne

Model checking

Combinatorial explosion (state explosion problem)

License

MIT