makejs.sh

#!/bin/sh

makejs_arg () {
  if [ "$1" = true ] || [ "$1" = false ] || [ "$1" = null ]; then
    # TODO: ints/numbers
    # non-quoted value
    echo "$1"
  else
    # quoted value
    #strip |tostring, wrap in quotes
    echo "\"${1%|tostring}\""
  fi
}

makejs () {
  jqprog=""

  while [ -n "$1" ]; do
    if [ "$(echo "$1" | cut -c 2-)" = "[]" ]; then
      # append to array
      # key |= . + [value] |
      jqprog="$(printf '%s\n%s |= . + [%s] |' "$jqprog" "$1" "$(makejs_arg $2)")"
      shift
      shift
    else
      #strip |tostring
      #wrap in quotes
      jqprog="$(printf '%s\n%s = %s |' "$jqprog" "$1" "$(makejs_arg $2)")"
      shift
      shift
    fi
  done

  jqprog="$(printf '%s\ntostring' "$jqprog")"
  
  echo "jq program is:"
  echo "$jqprog"

  echo
  echo "Output is:"
  echo
  echo null | jq -r "$jqprog"
}

makejs .baz qux \
  .foo[3] three \
  .foo[2] null \
  .one two \
  .foo[] bar \
  .foo2[0] f2

output

$ ./genjs.sh 
jq program is:

.baz = "qux" |
.foo[3] = "three" |
.foo[2] = null |
.one = "two" |
.foo[] = "bar" |
.foo2[0] = "f2" |
tostring

Output is:

{"baz":"qux","foo":["bar","bar","bar","bar"],"one":"two","foo2":["f2"]}