dotdot.md

The .. operator

The use case is for JSON files that contain lists where one wants to extract the same property from all the objects in the list. For example, aws ec2 describe-instances returns the structure:

{
    "Reservations": [
        {
            "Instances": [
                { ... description of instance ...},
                { ... description of instance ...},
                { ... description of instance ...},
                { ... description of instance ...},
                { ... description of instance ...},
                ...
            ]
        }
    ]
}

So if you want to find the IP address of all instances, you currently have to write:

for (.Reservations)
  for (.Instances)
    .PrivateIpAddress

which will give you something like:

[ [ "10.0.0.90", "10.0.0.91", "10.0.0.92" ], [ "10.6.16.69" ], [ "10.0.0.177" ],
  [ "10.0.0.57" ], [ null ], [ "172.31.16.62" ], [ "172.31.7.19" ],
  [ "172.31.38.173" ], [ "10.6.16.122" ], [ null ], [ "172.31.23.235" ],
  [ "172.31.37.247" ], [ "10.0.1.20" ], [ "10.0.1.213" ], [ "172.31.17.39" ],
  [ "10.0.5.203" ], [ null, null, null, null, null, null, null ], ...
]

There are several issues with this:

• Having to use for.
• Having to nest the fors.
• Ending up with nested arrays.
• Getting null values.

The .. operator, which is like ., except it also recurses through sub-objects, would solve this. With that operator we could solve the above with ..PrivateIpAddress. Done.

The exact rules would be:

• Can appear anywhere .foo can appear today.
• Returns a list of all the matches.
• For objects, if the key is there, add the value to the matches.
• For objects, traverse all values that are objects or arrays.
• For arrays, same.

Related ideas

• Using .foo on arrays

Schibsted relevance

We haven't seen any need for this crop up just yet.

Other languages

JsonPath has this exact operator.

jsonata doesn't seem to have any equivalent.

jq has something similar, but it's not exactly equivalent.