Welcome to the ultimate Sinon.js and see how it can help us create mocks in our JavaScript code.

Getting Started

But before we dive in, let's make sure you have Sinon.js installed and ready to go.

To install Sinon.js using yarn, another popular package manager for JavaScript:

yarn add sinon

If you prefer not to use a package manager, you can also download the Sinon.js is its ability to create "spies." A spy is a function that records arguments, return value, the value of this, and exception thrown (if any) for all of its calls. Spies are useful for verifying the behavior of a function that is called indirectly by the code under test, or for stubbing a function that has side effects.

To create a spy with Sinon.js, you can use the sinon.stub function. Here's an example:

const stub = sinon.stub().returns(10);

Once you have a stub, you can use it to replace the behavior of a function. For example, you can use the returns method to specify the return value of the stub, or the throws method to throw an exception when the stub is called.

const stub = sinon.stub().returns(10);

function foo() {
  return stub();
}

console.log(foo()); // 10


const stub = sinon.stub().throws(new Error('Boom!'));

function foo() {
  return stub();
}

try {
  foo();
} catch (error) {
  console.log(error)
}

Mocks

In addition to creating spies and stubs, Sinon.js also provides a way to create "mocks." A mock is an object that combines the behavior of a spy and a stub, and it's useful for verifying the interaction between different units of code.

To create a mock with Sinon.js, you can use the sinon.mock function. Here's an example:

const mock = sinon.mock({ foo: () => 10 });

Once you have a mock, you can use it to verify the interaction between different units of code. For example, you can use the expects method to specify the expected behavior of the mock, and the verify method to check if the mock was called as expected.

const mock = sinon.mock({ foo: () => 10 });

mock.expects('foo').once();

function foo() {
  mock.foo();
}

foo();

mock.verify(); // passes

Faking Time

In addition to creating mocks, Sinon.js also provides a way to create "fake timers." Fake timers are useful for testing code that depends on the current time, and they allow you to control the flow of time in your tests.

To create a fake timer with Sinon.js, you can use the sinon.useFakeTimers function. Here's an example:

sinon.useFakeTimers();

Once you have a fake timer, you can use it to control the flow of time in your tests. For example, you can use the tick method to advance the timer by a specific amount of time, or the reset method to reset the timer to its initial value.

sinon.useFakeTimers();

setTimeout(() => console.log('Hello, world!'), 1000);

sinon.tick(1000);


sinon.useFakeTimers();

setTimeout(() => console.log('Hello, world!'), 1000);

sinon.tick(1000);
sinon.reset();

Conclusion

That's it! You now know the basics of how to use Sinon.js to create mocks, spies, stubs, and fake timers in your JavaScript code. With these powerful tools at your disposal, you'll be able to write more robust and reliable tests for your code, and you'll be well on your way to becoming a JavaScript testing pro. Happy mocking!