Format number in thousands separators with jq json cli

Given {"a": 1234567890}, I want 1,234,567,890 in the result, how this can be done with jq

echo '{"a": 1234567890}' | jq '.a | FORMAT?'

Thanks for @peak's answer, the solution is

echo '{"a": 1234567890}' | jq -r 'def h: [while(length>0; .[:-3]) | .[-3:]] | reverse | join(","); .a | tostring | h'//-> 1,234,567,890