How to debug in `jq`

jq comes with a handy debug filter. Use it while you are developing your exercise solutions to inspect the data that is currently in the jq pipline.

debug prints a "debug array" to stderr. It outputs the input unchanged. The first element of the array is the string "DEBUG:". The second element depends on how you invoke debug.

1. the zero-arity debug function puts a compact representation of the input into the debug array:

   jq -n '[11, 22, 33] | debug | map(. * 2)'
   

   outputs

    ["DEBUG:",[11,22,33]]
    [
      22,
      44,
      66
    ]
   

2. the one-arity version pipes the input through the given filter:

   jq -n '[11, 22, 33] | debug("length: \(length), last: \(.[-1])") | map(. * 2)'
   

    ["DEBUG:","length: 3, last: 33"]
    [
      22,
      44,
      66
    ]
   

   The filter doesn't need to be a string. It can be anything, including multiple comma-separated expressions for "multi-line" debug output:

   jq -n '[11, 22, 33] as $a | 44 | debug("I am here:", $a, .)'
   

   ["DEBUG:","I am here:"]
   ["DEBUG:",[11,22,33]]
   ["DEBUG:",44]
   44