The -R causes jq to read each line without parsing it. Then the fromjson? filter attempts to parse each line and the ? appended to the end causes jq to skip further parsing if the line is invalid JSON. Nothing will be printed to the terminal for lines with invalid JSON.

jq -R 'fromjson? | .' file.json

Removing the ? will cause each line that isn’t valid JSON to produce an error, which can be redirected to a file to collect the filename and line number of all invalid JSON.

$ cat file.json
{"a": 1}
[1, 2, 3]
heello
{"else": "for"}
$ jq -R 'fromjson | .' file.json 2>errors.txt
$ cat errors.txt
jq: error (at file.json:3): Invalid numeric literal at EOF at line 1, column 6 (while parsing 'heello')