jq: Append JSON object via shell variable - macos

Question

I have a JSON file and I'm trying to add another field to it.
Example JSON File:
{"data":{}}
Looking at other answers the += seems to work:
objectName="objName"
cat $jsonFile | jq --arg objName $objectName '.data[$objName] += {"test": "json"}'
Outputs
{
"data": {
"objName": {
"test": "json"
}
}
}
just as expected.
The problem is that I can't hardcode the JSON so I'm inputting the string as a variable. But I can't get the syntax to work:
objectName="objName"
objJSON='{"test": "json"}'
cat $jsonFile | jq -r --arg objName $objectName --arg jsonString $objJSON '.data[$objName] += $jsonString'
I'm getting the error jq: error: syntax error, unexpected INVALID_CHARACTER, expecting $end (Unix shell quoting issues?) at <top-level>, line 1: "json"}

Answer 1

Use --argjson rather than --arg when passing JSON rather than strings.
When in doubt, quote your strings; that way if the shell executing your code isn't the one you think it is, you won't have your values being munged. (It's also helpful to stay in the habits necessary to write portable code; the whole reason I dropped zsh after spending 6 months learning it back in the mid-2000s is that my code quality when writing for other shells suffered).
objName="objName"
objJSON='{"test": "json"}'
echo '{"data":{}}' |
jq --arg objectName "$objName" \
--argjson jsonString "$objJSON" \
'.data[$objectName] += $jsonString'
...properly emits:
{
"data": {
"objName": {
"test": "json"
}
}
}