def showpage():
      ...
      test = [1,2,3,4,5,6]
      return render_template("sample.html",test=test)

<script> var counts = {{test}}; </script>

def showpage():
    ...
    test = [1,2,3,4,5,6]
    test = json.dumps(test)
    return render_template("sample.html",test=test)

<script> var counts = JSON.parse("{{ test }}"); </script>

<script> var counts = {{ test|tojson }}; </script>

from flask import json
return render_template("sample.html",test=json.dumps(test))