JSON migrations with `jq`

expected.json

{
  "A": "1!",
  "b": "2",
  "Z": "spam!",
  "?": [
    83,
    65,
    84,
    85,
    82,
    68,
    65,
    89
  ],
  "_": "*__*",
  "&": "eggs",
  "falsy": false,
  "": "wait, really?",
  "no": null,
  "hello": "world",
  "waldo": "there",
  "here": "reset",
  "dontchangefalse": false
}

input.json

{
  "a": "1",
  "b": "2",
  "z": "spam",
  "?": "SATURDAY",
  "_": "*__*",
  "&": "eggs",
  "falsy": false,
  "": "wait, really?",
  "yo": "hey!",
  "no": null,
  "rm": true,
  "hello": "world",
  "here": "hear",
  "dontchangefalse": false
}

migration.jq

def default($checkKey; expr):
  if has($checkKey) then . else expr end;

def _translate($DEATH_MAP):
  if .key == "a" or .key == "z" then
    {"key": (.key | ascii_upcase), "value": "\(.value)!"}
  elif .key == "?" then
    .value |= explode
  elif .key == "here" then
    # since we set .waldo when we see .here
    # we should *not* see "oswald" as the value
    {"key": "waldo", "value": "there"}
  elif $DEATH_MAP[.key] then
    empty
  else
    .
  end;

def translate:
  {"yo": true, "rm": true} as $DEATH_MAP |
  with_entries(_translate($DEATH_MAP))
  # Should not change existing non-null values
  | default("dontchangefalse"; .dontchangeme = "wrong")
  # Set defaults if they didn't get fixed during migration above
  | default("hello"; .hello = "world")
  | default("waldo"; .waldo = "oswald")
  # You could even re-set .here
  | default("here"; .here = "reset")
;

test.sh

#!/usr/bin/env bash

set -euo pipefail

jq 'include "migration"; translate' input.json | tee output.json

printf 'Comparing to expected.json \n'

cmp <(jq -cS . output.json) <(jq -cS . expected.json) || true

diff <(jq -S . output.json) <(jq -S . expected.json)

printf '✅\n'