@netatwork/spy

Mock the dependencies in unit tests with this proxy-based spy implementation. This is easy to use because of the straight forward and simple API. Additionally it provides better development experience with great IntelliSense, also for the object being spied, when used with TypeScript.

import { Spy, AssertionFactory } from '@netatwork/spy';

class Service {
  public getData(): Promise<any> {
    // here we make HTTP requests
  }
}

class Sut {
  public constructor(
    public readonly service: Service
  ) { }
}

const spy = Spy.create(
  /* object to mock       */ new Service(),
  /* call through         */ false,
  /* mock implementations */ {
    getData() {
      return Promise.resolve([]);
    }
  }
);

// Do it once
await AssertionFactory.configureDefault();

const sut = new Sut(spy.proxy); //<-- !Important: inject the mock service
await sut.service.getData();
// assert method call
serviceSpy.isCalled('getData');

API

For this section of documentation let us consider the following classes as the target for mocking and test.

class Dependency {
  public readonly member: number = 42;

  public get prop(): number { return 42; }

  public foo() {
    return 'real foo';
  }

  public bar() {
    return 'real bar';
  }

  public add(num1: number, num2: number) {
    return num1 + num2;
  }
}

class Sut {
  public constructor(
    private readonly dep: Dependency,
  ) { }

  public doSomething() {
    const dep = this.dep;
    return `${dep.foo()} - ${dep.bar()}`;
  }
  public add(num1: number | NumWrapper, num2: number | NumWrapper) {
    return this.dep.add(num1, num2);
  }
}

Spy.create and basic usage

Creates the spy.

// with call-through
const spy = Spy.create(new Dependency(), true);
const proxy = spy.proxy;
proxy.foo() === 'real foo'; // true

// without call-through
Spy.create(new Dependency(), false);
const proxy = spy.proxy;
proxy.foo() === 'real foo'; // false
proxy.foo() === undefined;  // true

Mock implementations can be provided.

// with call-through
const spy = Spy.create(new Dependency(), true, { foo() { return '42'; }, member: 43 });
const proxy = spy.proxy;
proxy.foo()  === '42';       // true
proxy.bar()  === 'real bar'; // true
proxy.member === 43;         // true
proxy.prop   === 42;         // true

// without call-through
const spy = Spy.create(new Dependency(), false, { foo() { return '42'; }, prop: 43 });
const proxy = spy.proxy;
proxy.foo()  === '42';       // true
proxy.bar()  === undefined;  // true
proxy.member === undefined;  // true
proxy.prop   === 43;         // true

Mocking a single function can be less verbose.

// with call-through
const spy = Spy.create(new Dependency(), 'foo', true, () => '42');
const proxy = spy.proxy;
proxy.foo()  === '42';       // true
proxy.bar()  === 'real bar'; // true
proxy.member === 42;         // true
proxy.prop   === 42;         // true

// without call-through
const spy = Spy.create(new Dependency(), 'foo', false, () => '42');
const proxy = spy.proxy;
proxy.foo()  === '42';       // true
proxy.bar()  === undefined;  // true
proxy.member === undefined;  // true
proxy.prop   === undefined;  // true

The package also exposes a createSpy method which is just an alias of Spy.create. Feel free to use whatever suits your esthetics.

callThrough

The original method can be called from the mock implementation using callThrough.

let fooCounter = 0, addCounter = 0;
const spy: Spy<Dependency> = Spy.create(new Dependency(), false, {
  foo() {
    fooCounter++;
    return fooCounter % 2 !== 0
      ? `fake foo#${fooCounter}`
      : spy.callThrough('foo');
  },
  add(num1, num2) {
    addCounter++;
    return addCounter % 2 !== 0
      ? num1 * num2
      : spy.callThrough('add', num1, num2);
  }
});
const sut = new Sut(spy.proxy);
sut.doSomething() === 'fake foo#1 - undefined' // true
sut.doSomething() === 'real foo - undefined'   // true
sut.doSomething() === 'fake foo#3 - undefined' // true
sut.add(2, 40)    === 80                       // true
sut.add(40, 2)    === 42                       // true

Note that from the mock implementation of foo, we can also call through bar.

let fooCounter = 0;
const spy: Spy<Dependency> = Spy.create(new Dependency(), false, {
  foo() {
    fooCounter++;
    return fooCounter % 2 !== 0
      ? `fake foo#${fooCounter}`
      : spy.callThrough('bar');
  }
});
const sut = new Sut(spy.proxy);
sut.doSomething() === 'fake foo#1 - undefined' // true
sut.doSomething() === 'real bar - undefined'   // true
sut.doSomething() === 'fake foo#3 - undefined' // true

Note that the examples above are without call-through (false as the 2nd argument in Spy.create). callThrough also works with call-through for the whole object. Why don't you try that on your own!

getCallCount

As the name suggests it returns the number of times a method is called.

const spy: Spy<Dependency> = Spy.create(new Dependency(), false);
const sut = new Sut(spy.proxy);
sut.doSomething();
spy.getCallCount('foo') === 1 // true
spy.getCallCount('bar') === 1 // true

clearCallRecords

Clears all the captured call records.

const spy = Spy.create(new Dependency(), true);
const sut = new Sut(spy.proxy);
sut.add(1, 2);
sut.add(2, 3);
sut.doSomething();
spy.getCallCount('add') === 2 // true
spy.getCallCount('foo') === 1 // true
spy.getCallCount('bar') === 1 // true

spy.clearCallRecords('foo');
spy.getCallCount('add') === 2 // true
spy.getCallCount('foo') === 0 // true
spy.getCallCount('bar') === 1 // true

spy.clearCallRecords();
spy.getCallCount('add') === 0 // true
spy.getCallCount('foo') === 0 // true
spy.getCallCount('bar') === 0 // true

sut.add(40, 2);
spy.getCallCount('add') === 1 // true

Assertion

By default this package uses chai to provide the assertions. The assertions should be configured once before starting the tests.

import { AssertionFactory } from '@netatwork/spy';

// do this in the bootstrapping code for you test
await AssertionFactory.configureDefault();

If you are not using chai as assertion library, you can customize the AssertionFactory with any other assertion implementation, using configureDefault. The implementation needs to agree the following interface.

interface Assert {
  strictEqual<TValue>(actual: TValue, expected: TValue, errorMessage: string): void;
  isAbove(actual: number, expected: number, errorMessage: string): void;
  deepStrictEqual<TValue>(actual: TValue, expected: TValue, errorMessage: string): void;
}

Below is an example of such customization.

import { Assert, AssertionFactory } from '@netatwork/spy';
import { strict, AssertionError } from 'assert';

const assert: Assert = {
  strictEqual: strict.strictEqual,
  deepStrictEqual: strict.deepStrictEqual,
  isAbove(actual, expected, message) {
    if (actual <= expected) {
      throw new AssertionError({ message, actual, expected });
    }
  }
}

AssertionFactory.configure(assert);

isCalled

Asserts if a method is called and how many times.

const spy = Spy.create(new Dependency(), true);
const sut = new Sut(spy.proxy);
sut.add(1, 2);
sut.add(2, 3);

spy.isCalled('add');    // works
spy.isCalled('add', 2); // works
spy.isCalled('add', 3); // throws

spy.isCalled('foo', 0); // works
spy.isCalled('foo');    // throws
spy.isCalled('foo', 1); // throws

isCalledWith

Asserts the arguments for a method call.

const spy = Spy.create(new Dependency(), true);
const sut = new Sut(spy.proxy);
sut.add(1, 2);
sut.add(2, 3);

spy.isCalledWith('add', [[1, 2], [2, 3]]);    // works
spy.isCalledWith('add', [1, 2], 0);           // works
spy.isCalledWith('add', [2, 3], 1);           // works
spy.isCalledWith('add', [2, 5], 1);           // throws

When dealing with complex argument list, a transformation function also be used.

const spy = Spy.create(new Dependency(), true);
const sut = new Sut(spy.proxy);
sut.add(1, 2);
sut.add(2, 3);

spy.isCalledWith(
  'add',
  '1 + 2',
  0,
  (args) => {
    const [arg0, arg2] = args as [number, number];
    return `${arg0} + ${arg2}`
  });

spy.isCalledWith(
  'add',
  '1 + 2 | 2 + 3',
  undefined,
  (args) => {
    return (args as [number, number][]).map(([arg0, arg1, arg2]) => `${arg0} + ${arg2}`).join(' | ');
  });

For more examples refer the tests.

Acknowledgements

The original work is highly influenced by the work done for Aurelia2.