Let me also provide just what the title asks for, because I’m sure a lot of people that stepped on this question look for something simpler.
Any of the following (added math2001 and pmf answers):
echo -e '["a","b"]\n["c","d"]' | jq -s 'add'
echo -e '["a","b"]\n["c","d"]' | jq -s 'flatten(1)'
echo -e '["a","b"]\n["c","d"]' | jq -s 'map(.[])'
echo -e '["a","b"]\n["c","d"]' | jq -s '[.[][]]'
echo -e '["a","b"]\n["c","d"]' | jq '.[]' | jq -s
results in:
[
“a”,
“b”,
“c”,
“d”
]
Note: Also any of the above can apply to arrays of objects.