I’m trying to understand the behavior of jq pipes when using parentheses and commas. Let’s consider this JSON example:

{
  "menu": [
    {
      "id": "file",
      "value": "File",
      "menuitem": [
        {
          "value": "New",
          "onclick": "CreateNewDoc()"
        },
        {
          "value": "Close",
          "onclick": "CloseDoc()"
        }
      ]
    },
    {
      "id": "photo",
      "value": "Photo",
      "menuitem": [
        {
          "value": "New",
          "onclick": "CreateNewDoc()"
        },
        {
          "value": "Open",
          "onclick": "OpenDoc()"
        }
      ]
    }
  ]
}

First, let’s apply a simple filter:

jq '.menu[].menuitem[] | .value'

Output:

"New"
"Close"
"New"
"Open"

This output is as expected.

Now, let’s add a compound filter separated by a comma and group the first filter using parentheses:

jq '(.menu[].menuitem[] | .value), . | {}'

Output:

{}
{}
{}
{}
{}

It looks like second pipe is working with output of first filters

Question: why?

According to the jq documentation:

If two filters are separated by a comma, then the same input will be fed into both and the two filters’ output value streams will be concatenated in order: first, all of the outputs produced by the left expression, and then all of the outputs produced by the right.

Following this logic, I would expect the output to be:

"New"    // output of first filter
"Close"
"New"
"Open"
{}       // output of second filter

However, this is not the case.

Corrected Filter

The filter works as expected if we add parentheses to the second filter:

jq '(.menu[].menuitem[] | .value), (. | {})'

Output:

"New"
"Close"
"New"
"Open"
{}

Main question

Why does the pipe (|) operator seemingly ignore the parentheses and comma in the first compound filter? What is the logic behind this behavior?