Sinon.js is aJavaScriptlibrary that includes standalone fakes, spies, and mocks that can be used in any unit testing framework. TheSinon spies documentationhighlights the different information that can be pulled from spies.

How to Spy?

There are multiple ways to use a spy, they can be anonymous functions or they can wrap around existing methods in a program. You can see all the many ways to use spies in theSinon spies documentation, but we’ll focus on one type in particular: wrapping methods.

Our spy will be wrapped around a method and, as we use the method in our tests, will observe everything that happens involving that method. We can think of a spy with a wiretap, taking notes on every interaction that method has.

The following example demonstrates some basics of using thesinon.spy()methodto test the method of an object.

const robot = {  greet(name){  // Unit being tested    return 'Hello ' + name;  }};
test('greet should return hello codey', () => {  sinon.spy(robot, 'greet'); // Initialize the spy  robot.greet('codey'); // Call the method  expect(robot.greet.called).to.be.true;  expect(robot.greet.calledWith('codey')).to.be.true;  expect(robot.greet.returned('Hello codey')).to.be.true;  robot.greet.restore(); // Remove spy from wrapped method});
Copy to clipboard

const robot = {  greet(name){  // Unit being tested    return 'Hello ' + name;  }};
test('greet should return hello codey', () => {  sinon.spy(robot, 'greet'); // Initialize the spy  robot.greet('codey'); // Call the method  expect(robot.greet.called).to.be.true;  expect(robot.greet.calledWith('codey')).to.be.true;  expect(robot.greet.returned('Hello codey')).to.be.true;  robot.greet.restore(); // Remove spy from wrapped method});

In the example above, thesinon.spy()method is used to wrap therobotobject’s.greet()method. Doing so adds a number of useful properties and methods that allow us to monitor its behavior after we callrobot.greet('codey'). Using a few of these methods, we test if the.greet()method…

• has been called using the.calledproperty
• is called with the correct argument using the.calledWith()method
• returns what we expect it to return using the.returned()property

Finally, to ensure that the method returns to its original, un-spied-on state, we use the.restore()method.

There are too many properties and methods available to spied-on functions to cover in one article and even to commit to memory, so we recommend that you check out thedocumentationto find the right tools for your needs.

Summary

Spies are an incredibly useful tool that serves as a means for observing and recording information about how our methods are used in tests. Though there are a number of JavaScript libraries that provide spying functionality, Sinon.js is well known and used widely, including at Codecademy!

'The Codecademy Team, composed of experienced educators and tech experts, is dedicated to making tech skills accessible to all. We empower learners worldwide with expert-reviewed content that develops and enhances the technical skills needed to advance and succeed in their careers.'