Hey đź‘‹ new reader,
Pluralsight gave me some free1 month
, get them before
subscriptions
they are gone!
Stubs, Spies and Mocks in JavaScript
11 Apr 2018 · 4 mins read Edit PostI have put together a GitHub repo as an example of how to use Stubs, Spies and Mocks for JavaScript tests. This demo project shows how you would use a popular mocking library called SinonJS (Pronounced “sigh-non”) to Mock a simple in-memory store.
Technologies
Install
To install the mocking library Sinon, to give you spies, stubs and mocks:
$ yarn add sinon
To have the describe("...") and it("..."):
$ yarn add mocha
Basic In-memory storage
To follow along with the example repo, clone it, then call the following command to run the mocking tests.
$ git clone https://github.com/harrymt/js-mocking-demo.git .
$ yarn test
Stubs
Replace calls to databases or network requests. Stubbing (replaces) a function.
Pros:
- Replace calls to db
- Replace network requests with this
- Force an error to test error handling
Unit Test
// index.js
const { stub, assert } = require("sinon");
const database = require("./src/db");
const store = require("./src/store")(database);
it("basic stub", () => {
// Stub database method
const getUsers = stub(database, "getUsers");
// Overrides database.getUsers() implementation
getUsers.onFirstCall().returns([harry, john]);
// `store.database.getUsers()` returns `[harry, john]`
assert.match(store.database.getUsers().length, 2)
// Put method back to normal 'remove stub'
getUsers.restore();
});
Spies
Spy on functions and see when or if they are called.
Pros:
- Check if a method has been called with certain variables
- Check if callbacks have been called, and how many times
Unit Test
// index.js
const { spy, assert } = require("sinon");
const database = require("./src/db");
const store = require("./src/store")(database);
it("basic spy", () => {
// Spy on a variable
// Sinon is now watching this variable
const callback = spy();
// Use the variable in our function
store.addUser(john, callback);
// Now check that `callback([harry, john])` was called inside of `database.addUser()`
assert.calledWith(callback, [harry, john]);
});
Mocks
Use mocks to stub out whole objects.
// index.js
const { mock, assert } = require("sinon");
const database = require("./src/db");
const store = require("./src/store")(database);
it("mocks", () => {
// Mock our whole database object
var db = mock(store.database);
// Setup our mock to replace functionality,
// if all we care about is that these methods
// were called once, we replace them with checks.
db.expects("setUsers").once();
// This needs to return something to continue with the method
// so we stub out the return value of `getUsers()`
db.expects("getUsers").once().returns([]);
store.addUser(harry, () => {});
// Verify that the mocked expects happened
// setUsers() was called once
// getUsers() was called once
db.verify();
});
Further Reading
Suggested
- Solving Google Interview Questions — 27 November 2017
- S.O.L.I.D. Principles — 16 May 2018
Code examples of the SOLID principles.