How to create keys in json file using jq?

I am trying to update a json file using jq. My json file looks like

{
	"A": "123",
	"B": "456",
	"C": "789",
	"D": []
}

Here the value for key D is empty so I am adding some values to it. And this is working

Now, if for some reason the key doesn't exist then I need to first create the key D. And I am not able to achieve this

{
	"A": "123",
	"B": "456",
	"C": "789",
}

cat test.json | jq 'has("D")' = false && cat test.json jq --argjson addobj '{"D": "[]"}'

I am getting the error

jq: error: Could not open file =: No such file or directory
jq: error: Could not open file false: No such file or directory

expected output

{
	"A": "123",
	"B": "456",
	"C": "789",
	"D": []
}

Can anyone please let me know what is the issue here and how to resolve it?

Thanks in advance.

P.S: Please let me know if any info is missing here

Your problem is not (only) with jq, but with shell syntax.

But if all you are trying to do is to update the value of key D whether it exists or not, then you don't need any checks and can simply assign the new value:

$ jq '.D = ["new value"]' test.json 
{
  "A": "123",
  "B": "456",
  "C": "789",
  "D": [
    "new value"
  ]
}

If you want to modify the current value, the operator |= might be helpful.

An alternative, equivalent program would be '. + { D: ["new value"] }'

If you really want to fix your script, here's a working version of it:

if jq -e 'has("D")' test.json >/dev/null; then
  # key exists
  jq --argjson addobj '{"D": "[]"}' 'your program here' test.json
else
  # key doesn't exist
  jq 'your other program here'
fi

But this is arguably easier in jq directly:

jq --argjson addobj '{"D": "[]"}' '
if has("D") then
  # D exists
  . # <- your jq program
else
  # D doesn't exist
  . # <- your other jq program
end
' test.json

If your goal is to simply insert the key with a default value if it doesn't exist, but keep any existing value, the following simple jq program (and nothing else) should take care of that:

jq '{D: []} + .' test.json

(keys in the RHS overwrite keys from the LHS – {a:1}+{a:2} becomes {a:2})

Objects in JavaScript (and by extension JSON), are a bag of unordered key-value pairs and {a:1,b:2} is the same object as {b:2,a:1}.

However, jq mostly keeps order of keys (although I don't think this is specified/guaranteed). So, a slightly more complicated version which puts D at the end of the object, but keeps existing values would be:

jq '.D |= (. // [])' test.json