Given input.json, I'd like to extract all the variable names and any default values

input.json

{
  "variable": {
    "cloud": {
      "description": "The cloud for this service"
    },
    "cluster": {
      "default": "default",
      "description": "The cluster name to use for this service"
    },
    "cost_center": {
      "default": "unknown",
      "description": "Used to set Cost Center tag, for accounting purposes"
    },
    "owner": {
      "default": "unknown",
      "description": "Used to set Owner tag, for accounting purposes"
    },
    "region": {
      "description": "The region for this service"
    },
    "service": {
      "default": "netbox",
      "description": "The service name"
    },
    "site": {
      "description": "The site for this service"
    },
    "ssm_region": {
      "description": "The region to use for the SSM parameter store"
    },
    "stage": {
      "description": "The stage for this service"
    }
  }
}

output.json

{
  "cloud": "",
  "cluster": "default",
  "cost_center": "unknown",
  "owner": "unknown",
  "region": "",
  "service": "netbox",
  "site": "",
  "ssm_region": "",
  "stage": ""
}

Actually, the input file is terraform code. I first converted to JSON using hcltool.

This is variables.tf:

variable "cloud" {
  description = "The cloud for this service"
}

variable "cluster" {
  default     = "default"
  description = "The cluster name to use for this service"
}

variable "cost_center" {
  default     = "unknown"
  description = "Used to set Cost Center tag, for accounting purposes"
}

variable "owner" {
  default     = "unknown"
  description = "Used to set Owner tag, for accounting purposes"
}

variable "region" {
  description = "The region for this service"
}

variable "service" {
  default     = "netbox"
  description = "The service name"
}

variable "site" {
  description = "The site for this service"
}

variable "ssm_region" {
  description = "The region to use for the SSM parameter store"
}

variable "stage" {
  description = "The stage for this service"
}

This is how I converted it:

jq '.variable | with_entries(.value |= if .default then .default else "" end)' < <(hcltool variables.tf)

Output:

{
  "cloud": "",
  "cluster": "default",
  "cost_center": "unknown",
  "owner": "unknown",
  "region": "",
  "service": "netbox",
  "site": "",
  "ssm_region": "",
  "stage": ""
}