Pfthb

I am using jsonnet to read in a value which consists of an array. I want to modify the first element in that array to add a value. The data structure looks like this:

 "my_value": [
 
 "env": "something"
 ,
 
 "var": "bar"
 
 ]

I want to add a value to my_value[0]. How can I reference that in jsonnet?

A possible approach using https://jsonnet.org/ref/stdlib.html#mapWithIndex as per below:

$ cat foo.jsonnet 
local my_array = [
 
 env: "something",
 ,
 
 var: "bar",
 ,
];
local add_by_idx(idx) = (
 if idx == 0 then extra: "stuff" else 
);
std.mapWithIndex(function(i, v) v + add_by_idx(i), my_array)

$ jsonnet foo.jsonnet 
[
 
 "env": "something",
 "extra": "stuff"
 ,
 
 "var": "bar"
 
]

A IMO better approach: converting the array to object for easier overloading, then ab-using the fact that jsonnet handles objects by alpha-sorting its keys:

$ cat foo.jsonnet
local array_a = [
 env: "something" ,
 var: "bar" ,
];

local array_b = [
 ,
 extra: "stuff" ,
];

// "Standarize" index key string as "%06d"
local _idx(i) = ("%06d" % [i]);

// Convert array [elem1, elem2, ...] to object as 000000: elem0, 000001: elem1, ...
local _to_obj(a) = std.foldl(
 function(x, y) x + y,
 std.mapWithIndex(function(i, v) [_idx(i)]: v , a),
 
);

local _to_array(obj) = [obj[k] for k in std.objectFields(obj)];

_to_array(std.mergePatch(_to_obj(array_a), _to_obj(array_b)))

$ jsonnet foo.jsonnet 
[
 
 "env": "something"
 ,
 
 "extra": "stuff",
 "var": "bar"
 
]