Introduction

fake was introduced with Sinon with v5. It simplifies and merges concepts from spies and stubs.

In Sinon, a fake is a Function that records arguments, return value, the value of this and exception thrown (if any) for all of its calls.

It can be created with or without behavior; it can wrap an existing function.

A fake is immutable: once created, the behavior will not change.

Unlike sinon.spy and sinon.stub methods, the sinon.fake API knows only how to create fakes, and doesn’t concern itself with plugging them into the system under test. To plug the fakes into the system under test, you can use the sinon.replace* methods.

Creating a fake

// create a basic fake, with no behavior
var fake = sinon.fake();

fake();

console.log(fake.callCount);
// 1

Fakes with behavior

Fakes can be created with behavior, which cannot be changed once the fake has been created.

sinon.fake.returns(value);

Creates a fake that returns the value argument

var fake = sinon.fake.returns("apple pie");

fake();
// apple pie

sinon.fake.throws(value);

Creates a fake that throws an Error with the provided value as the message property.

If an Error is passed as the value argument, then that will be the thrown value. If any other value is passed, then that will be used for the message property of the thrown Error.

var fake = sinon.fake.throws(new Error("not apple pie"));

fake();
// Error: not apple pie

sinon.fake.resolves(value);

Creates a fake that returns a resolved Promise for the passed value.

sinon.fake.rejects(value);

Creates a fake that returns a rejected Promise for the passed value.

If an Error is passed as the value argument, then that will be the value of the promise. If any other value is passed, then that will be used for the message property of the Error returned by the promise.

sinon.fake.yields([value1, ..., valueN]);

sinon.fake.yields takes some values, and returns a function that when being called, expects the last argument to be a callback and invokes that callback with the same previously given values. The returned function is normally used to fake a service function that takes a callback as the last argument.

In code example below, the ‘readFile’ function of the ‘fs’ module is replaced with a fake function created by sinon.fake.yields. When the fake function is called, it always calls the last argument it received, which is expected to be a callback, with the values that the yields function previously took.

var fake = sinon.fake.yields(null, "file content");
sinon.replace(fs, "readFile", fake);
fs.readFile("somefile", (err, data) => {
  console.log(data);
});
console.log("end of this event loop");
// file content
// end of this event loop

sinon.fake.yieldsAsync([value1, ..., valueN]);

Similar to yields, yieldsAsync also returns a function that when invoked, the function expects the last argument to be a callback and invokes that callback with the same previously given values. However, the returned function invokes that callback asynchronously rather than immediately, i.e. in the next event loop.

Compare the output of the code example below with the output of the code example above for yields to see the difference.

var fakeAsync = sinon.fake.yieldsAsync(null, "file content");
sinon.replace(fs, "readFile", fakeAsync);
fs.readFile("somefile", (err, data) => {
  console.log(data);
});
console.log("end of this event loop");
// end of this event loop
// file content

sinon.fake(func);

Wraps an existing Function to record all interactions, while leaving it up to the func to provide the behavior.

This is useful when complex behavior not covered by the sinon.fake.* methods is required or when wrapping an existing function or method.

Instance properties

f.callback

This property is a convenience to easily get a reference to the last callback passed in the last to the fake.

var f = sinon.fake();
var cb1 = function () {};
var cb2 = function () {};

f(1, 2, 3, cb1);
f(1, 2, 3, cb2);

f.callback === cb2;
// true

The same convenience has been added to spy calls:

f.getCall(1).callback === cb2;
// true
//
f.lastCall.callback === cb2;
// true

f.lastArg

This property is a convenient way to get a reference to the last argument passed in the last call to the fake.

var f = sinon.fake();
var date1 = new Date();
var date2 = new Date();

f(1, 2, date1);
f(1, 2, date2);

f.lastArg === date2;
// true

The same convenience has been added to spy calls:

f.getCall(0).lastArg === date1;
// true
f.getCall(1).lastArg === date2;
// true

f.lastCall.lastArg === date2;
// true

Adding the fake to the system under test

Unlike sinon.spy and sinon.stub, sinon.fake only knows about creating fakes, not about replacing properties in the system under test.

To replace a property, you can use the sinon.replace method.

var fake = sinon.fake.returns("42");

sinon.replace(console, "log", fake);

console.log("apple pie");
// 42

When you want to restore the replaced properties, simply call the sinon.restore method.

// restores all replaced properties set by sinon methods (replace, spy, stub)
sinon.restore();