If running a jq filter produces an empty result, the result is an empty string ('') even if the output option is set to json.

It may be a bit confusing because an empty string is also a different valid result (see examples below).

// this filter yields no results
jq.run('select(.foo == "bar")', {foo: ""}, {input: "json", output: "json"}).then(result => {
  // result is an empty string ''
});

// this filter yields an empty string
jq.run('.foo', {foo: ""}, {input: "json", output: "json"}).then(result => {
  // result is an empty string ''
});

I believe that undefined would be a better option because it is used neither in JSON nor in JQ.