What is Jest?

Jest is test runner for JavaScript and TypeScript. You can use Jest to write the unit test, spec file or integration test for your JavaScript or TypeScript program.

Suppose you have the following TypeScript module:

// add.ts
function add(a: number, b: number): number {
    return a + b;
}

export default add;

You can write the unit test to make sure that your function returns correctly:

// add.spec.ts
import add from "./add";

describe("Given a equal to b", () => {
    it("Should return correct values", () => {
        const res = add(1, 1);
        expect(res).toBe(2);
    });
});

describe("Given a less than b", () => {
    it("Should return correct values", () => {
        const res = add(2, 3);
        expect(res).toBe(5);
    });
});

describe("Given a greater than b", () => {
    it("Should return correct values", () => {
        const res = add(3, 2);
        expect(res).toBe(5);
    });
});

Then you can use jest to run the unit test.