Adding values to dynamic keys with jq - json

Question

I try to construct a json object with jq. I start with an empty object and want to add keys and values dynamically.
This works but the key is not variable. It's fixed to "foo":
echo '{"foo": ["baz"]}' | jq --arg value "bar" '.foo += [$value]'
output as expected:
{"foo": ["baz", "bar"]}
What I actually want do do is something like this:
echo '{"foo": ["baz"]}' | jq --arg key "foo" --arg value "bar" '.($key) += [$value]'
Unfortunately this does not work. Here is the output:
jq: error: syntax error, unexpected '(' (Unix shell quoting issues?) at <top-level>, line 1:
.($key) += [$value]
jq: error: try .["field"] instead of .field for unusually named fields at <top-level>, line 1:
.($key) += [$value]
jq: 2 compile errors
I couldn't find a solution or figure it out.
I know that this works: jq --null-input --arg key foo '{($key): "bar"}' but it doesn't solve my problem since I want to append values to existing lists as you can see in the examples.

Answer 1

You need to use square parens [..] instead of (..) as reported in the error message. Just do
jq --arg key "foo" --arg value "bar" '.[$key] += [$value]'
This error line is quite verbose to recommend you the right syntax to use. The emphasis with # is mine
jq: error: try .["field"] instead of .field for unusually named fields at <top-level>, line 1
# ^^^^^^^^^^^^^