Mocking a callback with Jest

fileUtils.js

const fs = require('fs');
const glob = require('glob');

/**
 * Find files
 * @param {String} globPattern
 * @returns {Promise<String[]>}
 */
function findFiles(globPattern) {
	return new Promise((resolve, reject) => {
		glob(globPattern, {}, (error, files) => {
			if (error) {
				reject(error);
			} else {
				resolve(files);
			}
		});
	});
}

module.exports = { findFiles };

fileUtils.spec.js

jest.mock('glob');

/** @type {jest.Mock} */
const glob = require('glob');
const subject = require('./fileUtils');

describe('lib/fileUtils', () => {
	describe('.findFiles()', () => {
		describe('when it finds files successfully', () => {
			beforeEach(() => {
				const mock = jest.fn((pattern, options, callback) =>
					callback(null, ['file.txt']),
				);

				glob.mockImplementationOnce(mock);
			});

			it('resolves with the returned files', async () => {
				expect(subject.findFiles('*.txt')).resolves.toEqual([
					'file.txt',
				]);
			});
		});

		describe('when it fails for any reason', () => {
			beforeEach(() => {
				const mock = jest.fn((pattern, options, callback) =>
					callback(new Error('Oh no!'), null),
				);

				glob.mockImplementationOnce(mock);
			});

			it('rejects with the returned error', async () => {
				expect(subject.findFiles('*.txt')).rejects.toThrowError(
					'Oh no!',
				);
			});
		});
	});
});