how to cancel promises with bluebird

Question

I think I misunderstand how promise cancellation with bluebird works. I wrote a test that demonstrates this. How do I make it green? Thanks:

describe('cancellation tests', () => {
  function fakeFetch() {
    return new Promise((resolve, reject) => {
      setTimeout(() => {
        resolve(1);
      }, 100);
    });
  }

  function awaitAndAddOne(p1) {
    return p1.then(res => res + 1);
  }

  it('`cancel` cancels promises higher up the chain', () => {
    const p1 = fakeFetch();
    const p2 = awaitAndAddOne(p1);

    expect(p2.isCancellable()).toBeTruthy();
    p2.cancel();

    return p2
      .then(() => {
        console.error('then');
      })
      .catch(err => {
        console.error(err);
      })
      .finally(() => {
        expect(p2.isCancelled()).toBeTruthy(); // Expected value to be truthy, instead received false
        expect(p1.isCancelled()).toBeTruthy(); 
      });
  });
});

Answer 1

@Karen if correct. But the issue is that your test is also a bit wrong

If you look at the isCancellable method

Promise.prototype.isCancellable = function() {
    return this.isPending() && !this.isCancelled();
};

This is just checking if the promise is pending and is not already cancelled. This doesn't mean then cancellation is enabled.

http://bluebirdjs.com/docs/api/cancellation.html

If you see the above url, it quotes below

The cancellation feature is by default turned off, you can enable it using Promise.config.

And if you look at the cancel method

Promise.prototype["break"] = Promise.prototype.cancel = function() {
    if (!debug.cancellation()) return this._warn("cancellation is disabled");

Now if I update your test correct like below

var Promise = require("bluebird");
var expect = require("expect");

describe('cancellation tests', () => {
    function fakeFetch() {
      return new Promise((resolve, reject) => {
        setTimeout(() => {
          resolve(1);
        }, 100);
      });
    }

    function awaitAndAddOne(p1) {
      return p1.then(res => res + 1);
    }

    it('`cancel` cancels promises higher up the chain', () => {
      const p1 = fakeFetch();
      const p2 = awaitAndAddOne(p1);

      value = p2.isCancellable();
      expect(p2.isCancellable()).toBeTruthy();
      p2.cancel();

      expect(p2.isCancelled()).toBeTruthy(); // Expected value to be truthy, instead received false
      expect(p1.isCancelled()).toBeTruthy(); 
    });
  });

You can see that cancellation is not enable and it executes the warning code

The execution results fail as expected

spec.js:46
  cancellation tests
spec.js:46
    1) `cancel` cancels promises higher up the chain
spec.js:78
  0 passing (3m)
base.js:354
  1 failing
base.js:370
  1) cancellation tests
base.js:257
       `cancel` cancels promises higher up the chain:
     Error: expect(received).toBeTruthy()
Expected value to be truthy, instead received
  false
      at Context.it (test/index.test.js:37:36)

Now if you update the code to enable cancellation

var Promise = require("bluebird");
var expect = require("expect");

Promise.config({
  cancellation: true
});

describe('cancellation tests', () => {
    function fakeFetch() {
      return new Promise((resolve, reject) => {
        setTimeout(() => {
          resolve(1);
        }, 100);
      });
    }

    function awaitAndAddOne(p1) {
      return p1.then(res => res + 1);
    }

    it('`cancel` cancels promises higher up the chain', () => {
      const p1 = fakeFetch();
      const p2 = awaitAndAddOne(p1);

      value = p2.isCancellable();
      expect(p2.isCancellable()).toBeTruthy();
      p2.cancel();

      expect(p2.isCancelled()).toBeTruthy(); // Expected value to be truthy, instead received false
      expect(p1.isCancelled()).toBeTruthy(); 
    });
  });

It works!