sebald
jest-mock-proxy
Mock classes and objects with the power of proxies!
Creates a Proxy that will dynamically create spies when a property is accessed the first time.
Every subsequent access will use the same spy. In combination with TypeScript this allows us to create a mock for any class/object without having to specify all its properties and methods.
tl;dr;
- The
Proxymakes any property and method available on the mock at runtime. - TypeScript limits access to properties and methods to the specified generic.
Install
Requires node 8+.
$ yarn add -D jest-mock-proxyor
$ npm install -D jest-mock-proxyUsage
Mock objects and instances
// service.ts
export class Service {
foo() {
console.log('hello');
}
bar(s: string) {
return s;
}
}
// some.test.ts
import { createMockProxy } from 'jest-mock-proxy';
import { service } from './service';
const mock = createMockProxy<typeof Service>();
mock.foo();
mock.bar.mockReturnValue('some string');
mock.bar('test'); // 'some string'Example: Mock an elastic search client.
import { Client } from 'elasticsearch';
import { createMockProxy } from 'jest-mock-proxy';
import fixture from './__fixtures__/elastic-response.json';
// This is an imaginary service that depends on the elastic search client.
import createService from './createService';
const client = createMockProxy<Client>();
const service = createService(client);
beforeEach(() => {
client.mockClear();
client.search.mockResolvedValue(fixture);
});
test('use service to query', async () => {
await service.query('https://example.com?q=hello');
expect(client.search.mock.calls).toMatchSnapshot();
});Mock a class and use jest's automock
When you need to mock a dependency via jest.mock, because you have no access to the module.
// query.ts
import { Pool, PoolConfig } from 'pg';
// 😨 This makes testing hard...
const pool = new Pool();
export const query = async (q: string, values?: any[]) => {
// ...because how to mock this?
const { rows } = pool.query(q, values);
return rows;
};
// query.test.ts
import { Pool } from 'pg';
import { createProxyFromMock } from 'jest-mock-proxy';
import { query } from './query';
jest.mock('pg');
const mockedPool = createProxyFromMock(Pool);
test('you can now mock the pool.query', async () => {
// Use mockedPool so you get good type inference from TS
mockedPool.query.mockResolvedValue({ rows: [{ id: 1, data: 'data' }] });
await query('SELECT * FROM table1'); // returns `[{ id: 1, data: 'data' }]`
});