No, while you can use Jinja to create JavaScript embedded in <script> tags, you cannot use it the other way around. When Flask renders this page it's static. The page does not still have blocks of code inside {{ }} or {% %}, it holds the result of those operations. However, there are other options. You could put the contents of current_data into hidden input with the keys as the id attributes.
View
{% for key in current_data %}
<input id="{{ key }}" type="hidden" value="{{ current_data[key] }}"/>
{% endfor %}
JavaScript
var num_key = "4";
alert(document.getElementById(num_key).value);
One extra piece of advice, it's not good practice to embed JavaScript into your html. You should have a separate .js file containing this and then include it in your html.