When a cloud function has no result the response is malformed

[SebC99]

Issue Description

Cloud functions return a json response formatted as {result: ... } which is then interpreted by the client SDKs
But when it should return a undefined result, instead of returning {result: undefined}, it returns {} which is really not a valid json object, and which causes an error in the JS SDK for example.

To ensure the JSON response can be stringify we could use a replacer method to replace all the undefined by null values...?
Or we need to ensure all SDKs can cope with this kind of invalid json result...

[joshkopecek]

Under what circumstances should it return undefined?
Can you give an example of a successful query which returns undefined?

[flovilmart]

undefined is not serializable, this is why when your result is ‘undefined’ the response payload is empty.

If you attempt to return an empty result, use false, null, true etc...

[SebC99]

@joshkopecek it's not a query, it's a cloud function. So you could have a cloud function to retrieve a value if exists and nothing if not (i.e. undefined)
So as @flovilmart said, the best way to do it is to force a null return value

[JacobJT]

@SebC99 I appreciate the issue as this was not functionality I was aware of! We just happen to not have any cloud functions that have no response run from the JS SDK.

@flovilmart Would a PR adding a warning when a cloud code function returns no value be approved? I suddenly feel my cloud code is a bit more full of holes than I had realized, and personally I would have heeded such a warning from the start if it had been there.

[flovilmart]

Perhaps we should return an empty object when response.success() is called without a parameters or true. Whatever would be less breaking

[SebC99]

One way to do it would be to include a status field in the response object, which would be ok when no error, even without a result in case of no result.
It would then allow to pass some cloud functions errors in the status and not in the http headers (for example when you detect a parameter is missing)

[flovilmart]

The problem at hand is that the response body is entirely an empty object. Status is not a regular field, and should not be added at that time, as it is inconsistent with the rest of the platform.

[SebC99]

true
And when we stringify an empty result it removes the key, so we end up with an empty (i.e. invalid) json being sent to the client.
We could replace undefined values by null values before stringify it to avoid that, or add a warning to avoid undefined values, or add another field indicating a NO_VALUE / EMPTY result as in usual rest api like json-api

[flovilmart]

Again, I’m usually against adding new magic constants like ‘NO_VALUE’ which bears as much consistency as 42 or null.

I’ll look into the original docs and iOS / JS Sdk’s , but I believe the original parse.com implementation used true to indicate a successful Cloud code call that was completed without a return value.

[JacobJT]

@flovilmart I believe forcing node 8+ is coming in a 3.0.0 release, right?

The only non-breaking change I can think of is that warning. Even passing an empty object as the result I believe could potentially break some people's code, even if it wasn't intended behavior.

But, if a 3.0.0 is on the horizon, I can't think of a better time to update the result to at least force an empty object being returned. I think it's the least damaging in terms of breaking changes, and thus probably the most appropriate.

I would be wary of straight up returning true for missing values, as it's very likely many people look for a truthy / falsey value, so this would cause more widespread breakages.

[flovilmart]

Node 8+ has been the target for the latests releases already as 6 is not supported anymore.

So no breaking changes here just pure evolution

Also, i’m Curious of the cases where JSON decoding fails or other issues. @SebC99 do you have an example of failing code?

[SebC99]

Well, every cloud function where you respond a response.success() without any result fails in the web JS SDK

[flovilmart]

Uhm... ok!

[JacobJT]

@flovilmart Calling callEmptyResponse returned ParseError {code: 107, message: "The server returned an invalid response."}

Parse.Cloud.define('emptyResponse', (req, res) => {
    return res.success();
});

Parse.Cloud.define('callEmptyResponse', (req, res) => {
    Parse.Cloud.run('emptyResponse', null, null).then(
        success => {
            res.success();
        },
        error => {
            res.erorr(error);
        }
    );
});