Hi, I'm trying to update a my custom package from umbraco v7 to v8

In my controller I have the variable vm.JSONStructure with this content:

{"name":"Configuration Manager","tabs":[{"id":1,"sectionName":"content","label":"CONTENT","alias":"afappContentTab","active":true,"actions":[{"action":"saving","doctypes":[{"doctypeAlias":"simplePage","properties":[{"propertyAlias":"title","config":{"propertyType":"string","propertyAliasToCopyValue":"Name","defaultValue":""}},{"propertyAlias":"date","config":{"propertyType":"datetime","propertyAliasToCopyValue":"CreateDate","defaultValue":""}}]}]}]},{"id":2,"sectionName":"media","label":"MEDIA","alias":"afappMediaTab","active":false,"actions":[{"action":"saving","doctypes":[{"doctypeAlias":null,"properties":null}]}]}]}

In my controller I also have this function that add a new doctype in the vm.JSONStructure variable:

vm.AddNewDocType = function (sectionAlias, actionAlias) {
    vm.sectionPosition = vm.JSONStructure.findIndex(s => s.sectionName === sectionAlias);
    vm.actionPosition = vm.JSONStructure[vm.sectionPosition].actions.findIndex(a => a.action === actionAlias);
    vm.JSONStructure[vm.sectionPosition].actions[vm.actionPosition].doctypes.push({ "doctypeAlias": "", "properties": [{ "propertyAlias": "", "config": { "propertyType": "", "propertyAliasToCopyValue": "", "defaultValue": "" } }] });
    };

With Umbraco v7 the findIndex method worked properly.

With Umbraco v8 instead I receive this error:

TypeError: vm.JSONStructure.findIndex is not a function
at AFAPPJSONConfig.vm.AddNewDocType (afapp.controller.js?cdv=1:61)
at fn (eval at compile (angular.js?cdv=1:16486), <anonymous>:4:409)
at callback (angular.js?cdv=1:29019)
at Scope.$eval (angular.js?cdv=1:19461)
at Scope.$apply (angular.js?cdv=1:19560)
at HTMLAnchorElement.<anonymous> (angular.js?cdv=1:29023)
at HTMLAnchorElement.dispatch (jquery.min.js?cdv=1:2)
at HTMLAnchorElement.v.handle (jquery.min.js?cdv=1:2)
-----------------------------------------------------------

Where line 61 is the following line of my code:

vm.sectionPosition = vm.JSONStructure.findIndex(s => s.sectionName === sectionAlias);

What's changed from Umbraco v7 to v8 and how can I solve the problem?

Thanks for the support Adriano