How calling an external function

Hello, i want to call a function, which is outside of the vue object. it works with variables, but it can’t call functions.: error: ReferenceError: change is not defined

<script>
//vanilla js
let value = true;
function change() {value = false}

// i try to call it in vue
var  vm = new Vue({

   el: '#vue',

   data: {
   },
   methods: {

      hz : function() {

        change();

      },

   },

});

Your code works fine for me:

https://jsfiddle.net/skirtle/gdv9h6tf/

Thanks, the fiddle helped me a lot. It was just a scope problem.