How to get null as a value in output from jslt

[joel19913]

Expected:
{"data" : "test",
"test" : null
}
Actual:
{"data":"test"
}
When null is returned as value the entire field itself will not be returned while doing jslt transformation. Instead I need to display it as null as given in above expected result. Is it possible?

[catull]

Interesting, this looks like a bug.

In the playground, I used this input

{
  "data" : "It's alway sunny in Philadelpia",
  "type0" : "With type"
}

.... and this transformation

{
  "data" : .data,
  "type" : fallback (.type, null)
  //"type" : if (.type) .type else (null)
}

When the tag in the input is "type0", the result is

{
  "data" : "It's alway sunny in Philadelpia"
}

When the tag is "type" though, we get:

{
  "data" : "It's alway sunny in Philadelpia",
  "type" : "With type"
}

The documentation of fallback states

Returns the first argument that has a value. That is, the first argument that is not null, [], or {}.

It would be great if the fallback also fell back on null as the last resort.
How about a function fallbackWithDefault ?

Btw, using this if-condition-else-default-value trick does not work either, same result.

[anuradharajan]

Please let me know how I can achieve the expected output below.

JSLT
{
"outkey1":.key1,
"outkey2":.key2,
"outkey3":.key3,
}

input
{
"key1": "val1",
"key3": null
}

actual output
{
"outkey1": "val1"
}

expected output
{
"outkey1": "val1",
"outkey3": null
}

I tried object filter but its adding "outkey2": null which I dont want in my output

[larsga]

JSLT will by default leave out object keys whose values are null. If you want to change this behaviour you need to set an object filter, as explained here.

@anuradharajan: You complain about getting "outkey2" ... well, how is JSLT supposed to know that you want outkey2, but not outkey3?

[anuradharajan]

key2 is not present in the input json. so thats why i dont want outkey2 in response

[larsga]

Okay, that makes sense. From JSLT's point of view, however, the output you produced is:

{
"outkey1": "val1",
"outkey2": null,
"outkey3": null
}

So there's no way for JSLT to distinguish the two cases.

Of course, we have methods in JSLT for detecting whether or not a key is present, so with a language extension it would be possible to let you control the inclusion/exclusion of a key explicitly. The question is whether it's worth it.