jest installation

babel with jest

yarn add --dev babel-jest @babel/core @babel/preset-env

babel.config.js

module.exports = {
    presets: [
        ['@babel/preset-env', {targets: {node: 'current'}}],
    ],
};

calculator.test.js

class Calculator {
    sum(first, second) {
        return first+second;
    }
}

describe('calculator add', function () {
    it('positive add positive', () => {
        let calculator = new Calculator();
        let sum = calculator.sum(1,2);
        expect(sum).toBe(3);
    });
});

install jest by yarn

yarn add --dev jest