Fakes

Introduction

sinon.fake allows creation of a fake Function with the ability to set a default behavior.

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

A fake is an immutable object: once created, its behavior will not change.

Basic usage

import tap from "tap";
import sinon from "sinon";

tap.test("basic fake usage - creates a fake that returns a value", (t) => {
  // Create a fake that returns a value
  const fake = sinon.fake.returns(42);

  // Call it like any function
  const result = fake();
  t.equal(result, 42, "fake returns the configured value");

  // Fakes record all calls
  t.ok(fake.calledOnce, "fake.calledOnce is true");
  t.equal(
    fake.firstArg,
    undefined,
    "fake.firstArg is undefined when no arguments passed"
  );

  t.end();
});

Prefer fakes over spies and stubs

Fakes are alternatives to the older spies and stubs, and can replace them in all use cases.

They are designed to be simpler and easier to use, while avoiding confusion by being immutable.

All fakes have the same API as spies. This includes access to call information through the spy call API, such as firstArg, lastArg, and callback properties.

Using fakes instead of spies

import tap from "tap";
import * as sinon from "sinon";

tap.test("Using fakes instead of spies", (t) => {
  const foo = {
    bar: () => "baz"
  };

  // wrap existing method without changing its behavior
  const fake = sinon.replace(foo, "bar", sinon.fake(foo.bar));

  // behavior is the same
  t.equal(fake(), "baz", "fake returns original behavior");

  // records information about calls
  t.equal(fake.callCount, 1, "callCount is tracked");

  sinon.restore();
  t.end();
});

Using fakes instead of stubs

import tap from "tap";
import * as sinon from "sinon";

tap.test("Using fakes instead of stubs", (t) => {
  const foo = {
    bar: () => "baz"
  };

  // replace method with a fake one
  const fake = sinon.replace(foo, "bar", sinon.fake.returns("fake value"));

  // returns fake value
  t.equal(foo.bar(), "fake value", "fake returns fake value");

  // records information about calls
  t.equal(fake.callCount, 1, "callCount is tracked");

  sinon.restore();
  t.end();
});