Promises puzzle cheat sheet

promises_answer_sheet.md

Promises puzzle answer sheet

Solution to https://twitter.com/nolanlawson/status/578948854411878400.

Puzzle #1

doSomething().then(function () {
  return doSomethingElse();
}).then(finalHandler);

Answer:

doSomething
|-----------------|
                  doSomethingElse(undefined)
                  |------------------|
                                     finalHandler(resultOfDoSomethingElse)
                                     |------------------|

Puzzle #2

doSomething().then(function () {
  doSomethingElse();
}).then(finalHandler);

Answer:

doSomething
|-----------------|
                  doSomethingElse(undefined)
                  |------------------|
                  finalHandler(undefined)
                  |------------------|

Puzzle #3

doSomething().then(doSomethingElse())
  .then(finalHandler);

Answer:

doSomething
|-----------------|
doSomethingElse(undefined)
|---------------------------------|
                  finalHandler(resultOfDoSomething)
                  |------------------|

Puzzle #4

doSomething().then(doSomethingElse)
  .then(finalHandler);

Answer:

doSomething
|-----------------|
                  doSomethingElse(resultOfDoSomething)
                  |------------------|
                                     finalHandler(resultOfDoSomethingElse)
                                     |------------------|

Understanding these puzzles correctly requires an assumption that doSomething() and doSomethingElse() return promises. @scottnonnenberg is correct that if doSomethingElse() returned a function then the results listed for puzzle #4 would be expected for #3. Since doSomethingElse() returns a promise and not a function that returns a promise, however, the then() function treats the promise parameter as a null and the value therefore falls through to the finalHandler. This threw me as well given I arrived at this gist prior to reading the associated blog article: https://pouchdb.com/2015/05/18/we-have-a-problem-with-promises.html.

it's called doSomething not doSomethingAndReturnPromise. duuuh!