JS Fetch JSON data not aviable

Question

first I'm kind of new to JS so I hope my question is not trivial.

I' creating a dashboard for GPU Cluster Monitoring. The data is stored via JSON and I want to fetch the file and assign the dtata to a variable as JSObject, so I can use it further in the code.

Here a chunk of my data.json:

{
  "time": [
    "16:50:07",
    "16:55:07"],
  "node01": {
    "cpu": [
      "5",
      "8"],
    "mem": [
      "7",
      "5"],
    "gpu": {
      "g1": {
        "gpu": [
          "0",
          "5"],
        "mem": [
          "2",
          "8"]
        }
      }
    }
  }

And here is my JS Code using fetch:

var obj;
    fetch('http://.../data.json')
        .then(res => res.json())
        .then(res => {obj = res})

My Problem is that if I want to access for example obj.time[0] I get the data in the browser console but if I try to access it in the script I get a Uncaught TypeError: obj is undefined error. I hope for me some enlightenment!