What methods can be used to determine if a bound function is equal when performing unit testing?

Question

What methods can be used to determine if a bound function is equal when performing unit testing?

I am looking to verify that when I pass an argument to a function, it is indeed a function reference even though the function reference is passed using the bind() method.

Take a look at this shortened code snippet that needs to be tested:

initialize: function () {
    this.register(this.handler.bind(this));
}

Here is the unit test designed to confirm if register() was called with handler():

it('register handler', function () {
    spyOn(bar, 'register');
    bar.initialize();
    expect(bar.register.calls.argsFor(0)[0]).toEqual(bar.handler);
});

It seems like the argument does not match the function reference possibly due to the use of bind() - how can I ensure that the correct function reference is being passed while still utilizing the bind() method?

Disclaimer: Although I mentioned Jasmine, this issue is not exclusive to that framework and is more related to the methods being employed.

javascript unit-testing jasmine

Answer 1

Answer №1

Instead of

make sure to check if the registration for the handler is correct:
expect(bar.register.calls.argsFor(0)[0]).toEqual(bar.handler);

another approach is to use:

verify that the handler is an instance of the registered function:
expect(Object.create(bar.handler.prototype) instanceof bar.register.calls.argsFor(0)[0])
  .toBe(true);

or try:

confirm that the prototype matches the registered function:
expect(Object.create(bar.handler.prototype)).
  toEqual(jasmine.any(bar.register.calls.argsFor(0)[0]));

This method works because the internal [[HasInstance]] function of the bound function redirects to the [[HasInstance]] function of the original function.

For further information, check out this blog post.

Answer 2

Instead of

make sure to check if the registration for the handler is correct:
expect(bar.register.calls.argsFor(0)[0]).toEqual(bar.handler);

another approach is to use:

verify that the handler is an instance of the registered function:
expect(Object.create(bar.handler.prototype) instanceof bar.register.calls.argsFor(0)[0])
  .toBe(true);

or try:

confirm that the prototype matches the registered function:
expect(Object.create(bar.handler.prototype)).
  toEqual(jasmine.any(bar.register.calls.argsFor(0)[0]));

This method works because the internal [[HasInstance]] function of the bound function redirects to the [[HasInstance]] function of the original function.

For further information, check out this blog post.

Answer 3

Answer №2

When using this.handler.bind(this)

, a completely new function is created, making it different from bar.handler. For more information, visit Function.prototype.bind().

One way to test this is by passing the bound function as an argument to your initialize function, like so:

var newHandler = bar.handler.bind(bar);
bar.initialize(newHandler);
expect(bar.register.calls.argsFor(0)[0]).toEqual(newHandler);

Answer 4

When using this.handler.bind(this)

, a completely new function is created, making it different from bar.handler. For more information, visit Function.prototype.bind().

One way to test this is by passing the bound function as an argument to your initialize function, like so:

var newHandler = bar.handler.bind(bar);
bar.initialize(newHandler);
expect(bar.register.calls.argsFor(0)[0]).toEqual(newHandler);

Answer 5

Answer №3

Successfully navigating through the test and code, I was able to find a solution.

To verify the function reference, I used an empty anonymous function to spy on it. Then, I triggered the function by spying on the register method - if the spy was called, I could confirm it was the correct reference.

it('register handler', function () {
    spyOn(foo, 'handler').and.callFake(function(){}); // do nothing
    spyOn(foo, 'register').and.callFake(function(callback){
        callback();
        expect(foo.handler).toHaveBeenCalled();
    });
    foo.initialize();
});

Answer 6

Successfully navigating through the test and code, I was able to find a solution.

To verify the function reference, I used an empty anonymous function to spy on it. Then, I triggered the function by spying on the register method - if the spy was called, I could confirm it was the correct reference.

it('register handler', function () {
    spyOn(foo, 'handler').and.callFake(function(){}); // do nothing
    spyOn(foo, 'register').and.callFake(function(callback){
        callback();
        expect(foo.handler).toHaveBeenCalled();
    });
    foo.initialize();
});

Answer 7

Answer №4

Here is an alternative approach that I find to be more streamlined and elegant.

Consider a class structure like this:

class Bar {
  public initialize() {
    this.register(this.handler.bind(this));
  }
  private register(callback) {}
  private handler() {}
}

The detailed specification could be as follows:

describe('Bar', () => {
  let bar;

  beforeEach(() => {
    bar = new Bar();
  });

  describe('initialize', () => {
    let handlerContext;

    beforeEach(() => {
      bar.handler = function() {
        handlerContext = this;
      };
      bar.register = jest.fn(callback => {
        callback();
      });
      bar.initialize();
    });

    it('calls register with the handler', () => {
      expect(bar.register).toHaveBeenCalledWith(expect.any(Function));
    });

    it('handler is context bound', () => {
      expect(handlerContext).toEqual(bar);
    });
  });
});

Answer 8

Here is an alternative approach that I find to be more streamlined and elegant.

Consider a class structure like this:

class Bar {
  public initialize() {
    this.register(this.handler.bind(this));
  }
  private register(callback) {}
  private handler() {}
}

The detailed specification could be as follows:

describe('Bar', () => {
  let bar;

  beforeEach(() => {
    bar = new Bar();
  });

  describe('initialize', () => {
    let handlerContext;

    beforeEach(() => {
      bar.handler = function() {
        handlerContext = this;
      };
      bar.register = jest.fn(callback => {
        callback();
      });
      bar.initialize();
    });

    it('calls register with the handler', () => {
      expect(bar.register).toHaveBeenCalledWith(expect.any(Function));
    });

    it('handler is context bound', () => {
      expect(handlerContext).toEqual(bar);
    });
  });
});

Answer 9

Answer №5

During my experience with testing (using jest), I implemented a mock for the bind function to ensure I was getting the original function rather than a bound copy.

Here is the specific approach that I took:

Code snippet under test:

// module test.js

export const funcsToExecute = [];
function foo(func) {
    funcsToExecute.push(func);
}

export function bar(someArg) {
    // body of bar function
}

export function run(someArg) {
    foo(bar.bind(null, someArg));
}

My goal was to verify that when the run function is invoked, funcsToExecute includes bar

Therefore, I crafted the test in this manner:

import * as test from 'test';

it('should validate that "funcsToExecute" only contains "bar"', () => {
    jest.spyOn(test.bar, 'bind').mockImplementation((thisVal, ...args) => test.bar);

    test.run(5);

    expect(test.funcsToExecute.length).toBe(1);
    expect(test.funcsToExecute[0]).toBe(test.bar);
});

If we were to apply this to another scenario, it might look something like this:

it('register handler', function () {
    spyOn(bar, 'register');
    spyOn(bar.handler, 'bind').mockImplementation((thisVal, ...args) => bar.handler);
    bar.initialize();
    expect(bar.register.calls.argsFor(0)[0]).toBe(bar.handler);
});

However, this is theoretical and has not been tested yet.

Answer 10

During my experience with testing (using jest), I implemented a mock for the bind function to ensure I was getting the original function rather than a bound copy.

Here is the specific approach that I took:

Code snippet under test:

// module test.js

export const funcsToExecute = [];
function foo(func) {
    funcsToExecute.push(func);
}

export function bar(someArg) {
    // body of bar function
}

export function run(someArg) {
    foo(bar.bind(null, someArg));
}

My goal was to verify that when the run function is invoked, funcsToExecute includes bar

Therefore, I crafted the test in this manner:

import * as test from 'test';

it('should validate that "funcsToExecute" only contains "bar"', () => {
    jest.spyOn(test.bar, 'bind').mockImplementation((thisVal, ...args) => test.bar);

    test.run(5);

    expect(test.funcsToExecute.length).toBe(1);
    expect(test.funcsToExecute[0]).toBe(test.bar);
});

If we were to apply this to another scenario, it might look something like this:

it('register handler', function () {
    spyOn(bar, 'register');
    spyOn(bar.handler, 'bind').mockImplementation((thisVal, ...args) => bar.handler);
    bar.initialize();
    expect(bar.register.calls.argsFor(0)[0]).toBe(bar.handler);
});

However, this is theoretical and has not been tested yet.

What methods can be used to determine if a bound function is equal when performing unit testing?

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

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