# Add field
echo '{"hello": "world"}' | jq --arg foo bar '. + {foo: $foo}'
# {
#   "hello": "world",
#   "foo": "bar"
# }

# Override field value
echo '{"hello": "world"}' | jq --arg foo bar '. + {hello: $foo}'
{
  "hello": "bar"
}
# {
#   "hello": "bar"
# }

# Concat and add
echo '{"hello": "world"}' | jq --arg foo bar '. + {hello: ("not" + $foo)}'
# {
#   "hello": "world",
#   "foo": "notbar"
# }