how to handle async callbacks

[jsardev]

Description

For some reason td.verify does not work well with subjects that use async/await callbacks.

Take a look at this example:

const t = require('tap');
const td = require('testdouble');

const printBalance = (id, fetchBalance, getMessage, print) => {
  fetchBalance(id, async (error, balance) => {
    const message = await getMessage(balance);
    print(message);
  });
};

t.test('should print 1337', async () => {
  const fetchBalance = td.function('.fetchBalance');
  const getMessage = td.function('.getMessage');
  const print = td.function('.print');
  td.when(fetchBalance(42)).thenCallback(null, 1337);
  td.when(getMessage(1337)).thenResolve('Your balance is 1337');

  printBalance(42, fetchBalance, getMessage, print);

  td.verify(print('Your balance is 1337'));
});

This test doesn't work throwing there were no invocations of the test double.. This totally doesn't make sense to me as the callback thanks to async/await is being processed synchronously which should make the print function invoke immediately.

Any ideas what's wrong in here?

Environment

• node: 9.11.2
• npm: 5.6.0
• testdouble: 3.8.1

[jsardev]

I came to a solution. But is this really the only way to handle this kind of situation?

const t = require('tap');
const td = require('testdouble');

const printBalance = (id, fetchBalance, getMessage, print) => {
  fetchBalance(id, async (error, balance) => {
    const message = await getMessage(balance);
    print(message);
  });
};

t.test('should print 1337', t => {
  const fetchBalance = td.function('.fetchBalance');
  const getMessage = td.function('.getMessage');
  const print = td.function('.print');
  td.when(fetchBalance(42)).thenCallback(null, 1337);
  td.when(getMessage(1337)).thenResolve('Your balance is 1337');

  td.when(print('Your balance is 1337')).thenDo(() => {
    t.end();
  });

  printBalance(42, fetchBalance, getMessage, print);
});

[searls]

I read your first example, but I'm struggling to understand how it would work. When you resolve a promise, even if no network activity occurs, it will defer a stack and so won't trigger synchronously.

That means, if I'm reading you correctly, this code:

const printBalance = (id, fetchBalance, getMessage, print) => {  // sync - stack 1
  fetchBalance(id, async (error, balance) => { // sync - stack 1
    const message = await getMessage(balance); // async! - message will be assigned on stack 2
    print(message); // stack 2
  });
};

As a result, in your first text example:

  td.when(getMessage(1337)).thenResolve('Your balance is 1337'); // stack 1

  printBalance(42, fetchBalance, getMessage, print); // stack 1 but schedules resolution of `getMessage` to stack 2

  td.verify(print('Your balance is 1337')); // stack 1 -- message will not have been printed yet.

Therefore, if you make printBalance an async function and then await its result, as done here, your test will pass:

const t = require('tap');
const td = require('testdouble');

const printBalance = async (id, fetchBalance, getMessage, print) => {
  fetchBalance(id, async (error, balance) => {
    const message = await getMessage(balance);
    print(message);
  });
};

t.test('should print 1337', async () => {
  const fetchBalance = td.function('.fetchBalance');
  const getMessage = td.function('.getMessage');
  const print = td.function('.print');
  td.when(fetchBalance(42)).thenCallback(null, 1337);
  td.when(getMessage(1337)).thenResolve('Your balance is 1337');

  await printBalance(42, fetchBalance, getMessage, print);

  td.verify(print('Your balance is 1337'));
});

[jsardev]

@searls Damn, you're right 😄 I guess I've just got lost in the async world and didn't see the thing here. Thanks and sorry for the problem! 👍