$ cat > foo.jsonnet
local applyModifications(obj, f) =
    obj + {
        [x] : f(obj[x]) for x in std.objectFieldsAll(obj)
    }
;

applyModifications({
    visible: "foo",
    hidden:: "bar",
}, function(x) x + " modified")

$ jsonnet foo.jsonnet
{
   "hidden": "bar modified",
   "visible": "foo modified"
}

$ jsonnet --version
Jsonnet commandline interpreter v0.17.0