jq argjson does not work with string from variable

I am trying to create a json with jq

f=1
jq -n --argjson a ${f} '{"test":$a}'

or

f="1"
jq -n --argjson a ${f} '{"test":$a}'

or

jq -n --argjson a "teststring" '{"test":$a}'

work fine

but

f="teststring"
jq -n --argjson a ${f} '{"test":$a}'

yields
jq: invalid JSON text passed to –argjson

What am I doing wrong?

Cheers
Gordon

>Solution :

Your shell is interpreting the double quotes and passing bare strings to jq.

The correct way to define your input is to add an extra level of quoting to make the double quotes part of the value:

f='"teststring"'
jq -n --argjson a ${f} '{"test":$a}'