sinon

Spies

Spies are functions that record their usage, such as how many times they were called, with what arguments, and what was returned. They can be used to wrap existing functions to add this tracking ability.

const sinon = require('sinon');
const myFunction = sinon.spy();
myFunction('Hello', 'World');
console.log(myFunction.calledOnce); // true

Stubs

Stubs are like spies, but they can replace the target function's behavior, either by returning a specific value or by throwing an exception. They are useful for controlling a function's behavior in a test.

const sinon = require('sinon');
const myObj = { myMethod: () => 'original' };
const stub = sinon.stub(myObj, 'myMethod').returns('stubbed');
console.log(myObj.myMethod()); // 'stubbed'

Mocks

Mocks are fake methods (like stubs) with pre-programmed behavior and expectations. They are used to assert that certain methods are called in certain ways.

const sinon = require('sinon');
const myObj = { myMethod: () => 'original' };
const mock = sinon.mock(myObj);
mock.expects('myMethod').once().returns('mocked');
console.log(myObj.myMethod()); // 'mocked'
mock.verify();