在“ Promise.then（）”中返回带有“ then”功能的对象

Question

Within my Node.JS application I have written a function ( findByReference ) that goes to a database and asynchronously yields a fetched database row. I have written this function using Promises. Additionally, I have written an implementation of the Maybe monad and want my findByReference function to yield an instance of Maybe .

My code looks like the below:

findByReference(r)
  .then(raw => raw ? Just(raw) : Nothing())
  .then(row => {
    (row instanceof Maybe) === true;
  });

Without going into what Just and Nothing mean, the implication of this (because of how I've written Maybe ) is that the row variable in the above code has a function on it called "then" . To cut a long story short, it appears that Javascript is getting confused and is for some reason automatically calling MY "then" and instead of passing the Maybe is actually passing to the callback whatever MY "then" returns as the value of row . This is obviously leading to all manner of weird behaviour. If I simply remove the "then" function from my object then it all works as expected.

I am aware that if a Promise.then returns another Promise, then execution will pause until that promise is resolved. I have been unable to find any official documentation to back this up, but is it the case that this decision is simply based on the existence of a "then" function (the closest I have found is this https://developers.google.com/web/fundamentals/primers/promises which refers to the return value as "something Promise-like"). If this is the case, it would be my understanding that "then" as a function name is basically a reserved word in Javascript? I have seen other implementations of Maybe (such as this one https://www.npmjs.com/package/data.maybe ) that use the word "chain" for a similar thing - I wondered if this is why?

Can anyone shed any light on if my deduction here is correct and if so is there any workaround I can use other than renaming my function?

FYI the only other SO question I've found that touches this problem is this one - Resolve promise with an object with a "then" function - but since that is angular-specific I don't believe this is a duplication.

Thanks in advance!

Answer 1

...the row variable in the above code has a function on it called "then". To cut a long story short, it appears that Javascript is getting confused and is for some reason automatically calling MY "then"...

It's not confused. :-) This is the definition of how promises work. JavaScript's promises work according to the Promises/A+ specification , which uses this terminology:

1.1 “promise” is an object or function with a then method whose behavior conforms to this specification.

1.2 “thenable” is an object or function that defines a then method.

If you have an object passing through a promise chain that's a thenable but not a promise , it's incompatible with promises.

So yes, in a sense, the then property of objects passing through promise chains is "reserved" by the Promises/A+ spec. You'll need to wrap your raw value in an object that doesn't have a then (and then unwrap it later). Or if you can, rename then in your design to remove the conflict.