I'm new to unit tests using Jest but basically I have a file client.js looking like this:

function add(a, b) {
    return a + b;
}
const fetcher = () => {

    return fetch('http://localhost:3000/').then(res => res.json);
}


module.exports = {add, fetcher};

The client.test.js is as follows:

const client = require('../client');

test('Testing addition', () => {
    expect(client.add(1, 2)).toBe(3);
});

test('Yet another addition', () => {
    expect(client.add(3, 10)).toBe(13);
});



test('Testing a promise', () => {
    return client.fetcher().then(data => {
        expect(data).toBe(1824);
    })
});

When I try to run tests it throws an error ReferenceError: fetch is not defined. How do I fix this?