1.6.4 is polluting arrays with a 'sequence' property

[meganwalker-ibm]

PR #292 included in JSONata 1.6.4 started polluting data returned to consumers with a 'sequence' property attached onto an array.

I don't think this internal property should be externalised to consumers; especially in a way that means it can't be serialised losslessly into a string.

The sequence property here is added by JSONata, not from the data we are working with, and it cannot be serialised correctly into JSON (Nor can the equivalent JSON string be parsed into javascript)

{
  "foo": {
    "bar": [
      {
        "name": "freddy",
        "price": 6
      },
      {
        "name": "jsonata",
        "price": 1000000
      },
      sequence: true
    ]
  }
}

Represents the returned data structure, except it's not valid JSON.

[andrew-coleman]

How are you serializing to JSON? The following Javascript code:

const arr = [1,2,3];
arr.sequence=true;
JSON.stringify(arr);

produces
"[1,2,3]"

[mattbaileyuk]

I believe the concern can be represented with the following:
If you have

const jsonata = require('jsonata')
let input = {
  "units": [
    {
      "name": "bob",
      "price": 20
    },
    {
      "name": "jessica",
      "price": 500
    },
    {
      "name": "may",
      "price": 40
    }
  ]
}
console.log(jsonata('units[price > 20]').evaluate(input))

then the output to the console (also seen in a Javascript debugger) is

[
  {
    name: 'jessica', price: 500
  },
  {
    name: 'may', price: 40
  },
  sequence: true
]

where sequence: true wasn't present in 1.6.3 and below. This does seem to go with JSON.stringify(), but I guess the discussion is around whether evaluate() should be returning this property in the object.

I guess the wider question is, should evaluate() be returning valid JSON? I would have thought so, which suggests this should be cleaned up.

[andrew-coleman]

The sequence property was always there, but it used to be created using Object.defineProperty with enumerable set to false. However, this had a significant performance hit, and rewriting it this way gave a 3x performance improvement on the expression $sum(Account.Order.Product.(Price*Quantity)). If you can come up with an alternative solution that doesn't degrade performance, I'd welcome it.

[mattbaileyuk]

I was aware that the sequence property was there internally before as processing is taking place, and hence didn't feel concerned by the change in the PR, I didn't realise that this would have an impact on what was returned from an evaluate() call, which is an external change. Having evaluate() return a sequence property is new behaviour in 1.6.4.

[mtiller]

FYI, in my (TypeScript) implementation, I always "wrapped" the values in a wrapper where the sequence and a few other properties were properties of the wrapper, not of the actual value. This allowed me to include any such properties pretty easily. I would unwrap such values before calling functions, for example.

Now, that approach may have had a significant impact on performance, I have no idea. I didn't do any performance benchmarking.

[edbrannin]

Possibly related: I'm seeing keepSingleton: true instead of sequence: true in arrays of Strings.

EDIT: I'm noticing this because it's throwing off ava's t.deepEqual() in my unit tests.

[cherukurisurendra2010]

Hi Team,

We are facing the same issue, we are getting extra sequence key.

Could you please provide any updated status on this bug.

Thanks.

[andrew-coleman]

The 'status' is as discussed above. Any changes in this area could have significant performance impact. Before touching this, we need a framework for measuring performance so that we can be certain of no regressions.

These properties are only added to the Array object. When you JSON.stringify() the structure, they don't appear in the output.

[loganvolkers]

The other option is to have a final "sanitization" step that makes sure that a clean array is returned.

function cleanOutput(raw) {
  if (Array.isArray(raw)) {
    return [...raw];
  } else {
    return raw;
  }
}

This could be done as a final step in evaluate, but it can also be done externally.

const jsonata = require("jsonata");
let input = {
  units: [
    {
      name: "bob",
      price: 20
    },
    {
      name: "jessica",
      price: 500
    },
    {
      name: "may",
      price: 40
    }
  ]
};
const output = jsonata("units[price > 20]").evaluate(input);
output.keepSingleton = "bar" // For illustration only
console.log(output, Object.keys(output));
const clean = cleanOutput(output);
console.log(clean, Object.keys(clean));

The above code outputs

["0", "1", "sequence", "keepSingleton"]
["0", "1"]

This could provide an immediate workaround for:

• @edbrannin concern about keepSingleton
• @cherukurisurendra2010 concern about sequence

I'll leave the performance fix and potential for integration into core up to @andrew-coleman. If performance is a big concern, maybe we can get a jsonata->webassembly compiler written ;)

[ameinhardt]

@loganvolkers : that doens't work for nested arrays, does it?
How about:

cleanOutput = JSON.parse(JSON.stringify(raw))

[knolleary]

I have been looking at this issue in related to #473.

I've got a simple performance test that runs the $sum(Account.Order.Product.(Price*Quantity)) expression 100000 times, across 10 separate runs, and then averages the time taken and number of evals/sec.

I have used that to compare some possible approaches to resolving this issue:

1. Object.defineProperty to make the properties non-enumerable. As @andrew-coleman said, this is known to be slower, but I tried it out to get a sense of how much slower given the current code.

2. WeakMap - this uses a WeakMap in util.js to track the 'known' sequence arrays, rather than having to flag the array itself

3. Subclass Array - create a custom class that extends Array, so we can use instanceof to check its type, but still looks just like an Array.

   class Sequence extends Array {}
   

Code	evals/sec
existing	40891
Object.defineProperty	29178
WeakMap	23581
subclass Array	30491

So yeah... no quick win with those options.

Also, having got this far, I realise there's more to it than just being able to identify it as a sequence - there is also the keepSingleton property used in some places that gets added to the object. So I'm going to post this comment as a record of my failure to find a performant fix so far.

[andrew-coleman]

Thanks Nick! :)

[agarwalamit662]

Any update on this issue ..? I am getting sequence property in my arrays.