dot-on-array.md

'.foo' on arrays

Just as with the .. operator being able to do .foo on an array of objects is useful in cases where an array contains objects and one wants to index into those objects.

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 ...},
                ...
            ]
        }
    ]
}

Getting the IP address of each instance is tricky with the current JSTL 2 (see the .. operator), but simply by allowing dot access on arrays we could solve it with:

.Reservations.Instances.PrivateIpAddress

Here the first step would return a list of objects, .Instances would return the instances inside each, so that the result would be a flat array of instances. Then, the third step would pick the .PrivateIpAddress from each, and skip the ones that didn't have this field.