Convert bash array to json array and insert to file using jq - arrays

Question

Given a bash array, how to convert it to a JSON array in order to output to a file with jq?
Additionnally: is there a way to keep the server_nohup array unchanged instead of re-writing the whole json file each time?
newArray=(100 200 300)
jq -n --arg newArray $newArray '{
client_nohup: [
$newArray
],
server_nohup: [
]
}' > $projectDir/.watch.json
Current output:
{
"client_nohup": [
"100"
],
"server_nohup": []
}
Desired output:
{
"client_nohup": [
100,
200,
300
],
"server_nohup": []
}

Answer 1

(1) If all the values in newArray are valid as JSON values without spaces, then you could get away with piping the values as a stream, e.g.
newArray=(100 200 300)
echo "${newArray[#]}" |
jq -s '{client_nohup: ., server_nohup: []}'
(2)
Now let's suppose you merely wish to update the "nohup" object in a file, say nohup.json:
{ "client_nohup": [], "server_nohup": [ "keep me" ] }
Since you are using bash, you can then write:
echo "${newArray[#]}" |
jq -s --argjson nohup "$(cat nohup.json)" '
. as $newArray | $nohup | .client_nohup = $newArray
'
Output
(1)
{
"client_nohup": [
100,
200,
300
],
"server_nohup": []
}
(2)
{
"client_nohup": [
100,
200,
300
],
"server_nohup": [
"keep me"
]
}
Other cases
Where there's a will, there's a jq way :-)
See for example the accepted answer at How to format a bash array as a JSON array (though this is not a completely generic solution).
For a generic solution, see 𝑸: How can a variable number of arguments be passed to jq? How can a bash array of values be passed in to jq as a single argument? at the jq FAQ https://github.com/stedolan/jq/wiki/FAQ
Generic Solutions
To be clear, if the array values are known to be valid JSON, there are several good options; if the array values are arbitrary bash strings, then the only efficient, generic way to handle them with jq is by using the -R jq option (e.g. in conjunction with -s), but then the (bash) strings will all be read in as JSON strings, so any intended type information will be lost. (The point here hinges on the technicality that bash strings cannot CONTAIN NUL characters.)
Often, to alleviate the latter concern, one can convert numeric strings to JSON numbers, e.g. using the jq idiom: (tonumber? // .).

Answer 2

In general, the only truly safe way to do this is with multiple invocations of jq, adding each element to the output of the previous command.
arr='[]' # Empty JSON array
for x in "${newArray[#]}"; do
arr=$(jq -n --arg x "$x" --argjson arr "$arr" '$arr + [$x]')
done
This ensures that each element x of your bash array is properly encoded prior to adding it to the JSON array.
This is complicated, though, by the fact that bash doesn't not distinguish between numbers and strings. This encodes your array as ["100", "200", "300"], not [100, 200, 300]. In the end, you need to have some awareness of what your array contains, and preprocess it accordingly.