Usage with Jest

Let’s set up a project with Jest and assertive-ts to write our first assertion.

First, let’s install the dependencies:

npm install --save-dev typescript ts-jest jest @types/jest @assertive-ts/core

This library is meant to be used with TypeScript, so to have better results we encourage you to use TypeScript in your project.

Let’s create a tsconfig.json file:

npx tsc -init

Let’s also create a jest configuration file at the root directory by running the following command:

npx ts-jest config:init

For more information about Jest and its configuration please refer to the official documentation.

We are going to test a simple function:

src/mathUtils.ts

export const sum = (a: number, b: number): number => {
  return a + b;
}

Now let’s write a test for that function. Make sure to import expect from the @assertive-ts/core module:

tests/mathUtils.test.ts

import { expect } from "@assertive-ts/core";
import { sum } from "../src/mathUtils";

describe("sum", () => {
  it("adds two numbers", () => {
    expect(sum(1, 2)).toBeEqual(3);
  })
});

And that’s it! Now you can run your tests as usual with your test script.

Keeping Jest’s expect function

You might want to use the expect function from Jest along with assertive-ts assertions. There are a couple ways to achieve this:

• Instead of importing expect from the assertive-ts module, you can import one of the available aliases: assert or assertThat:

    import { expect } from "@jest/globals";
    import { assert } from "@assertive-ts/core";

• Or use an explicit import to rename the expect function from Jest to something like jestExpect:

    import { expect as jestExpect } from "@jest/globals";
    import { expect } from "@assertive-ts/core";