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cunningBear/force-app/main/default/classes/AccountProcessor.cls

@@ -0,0 +1,15 @@
+public class AccountProcessor
+{
+  @future
+  public static void countContacts(Set<id> setId)
+  {
+      List<Account> lstAccount = [select id,Number_of_Contacts__c , (select id from contacts ) from account where id in :setId ];
+      for( Account acc : lstAccount )
+      {
+          List<Contact> lstCont = acc.contacts ;
+
+          acc.Number_of_Contacts__c = lstCont.size();
+      }
+      update lstAccount;
+  }
+}

cunningBear/force-app/main/default/classes/AccountProcessor.cls-meta.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ApexClass xmlns="http://soap.sforce.com/2006/04/metadata">
+    <apiVersion>47.0</apiVersion>
+    <status>Active</status>
+</ApexClass>

cunningBear/force-app/main/default/classes/AccountProcessorTest.cls

@@ -0,0 +1,26 @@
+@isTest
+public class AccountProcessorTest {
+    @isTest
+    public static void TestAccountProcessorTest(){
+        Account a = new Account();
+        a.Name = 'Test Account';
+        Insert a;
+
+        Contact cont = New Contact();
+        cont.FirstName ='Bob';
+        cont.LastName ='Masters';
+        cont.AccountId = a.Id;
+        Insert cont;
+
+        set<Id> setAccId = new Set<ID>();
+        setAccId.add(a.id);
+
+        Test.startTest();
+            AccountProcessor.countContacts(setAccId);
+        Test.stopTest();
+
+        Account ACC = [select Number_of_Contacts__c from Account where id = :a.id LIMIT 1];
+        System.assertEquals ( Integer.valueOf(ACC.Number_of_Contacts__c) ,1);
+  }
+
+}

cunningBear/force-app/main/default/classes/AccountProcessorTest.cls-meta.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ApexClass xmlns="http://soap.sforce.com/2006/04/metadata">
+    <apiVersion>47.0</apiVersion>
+    <status>Active</status>
+</ApexClass>

cunningBear/force-app/main/default/lwc/jsconfig.json

@@ -2,7 +2,11 @@
     "compilerOptions": {
         "experimentalDecorators": true,
         "baseUrl": ".",
-        "paths": {}
+        "paths": {
+            "c/lW_comp": [
+                "lW_comp/lW_comp.js"
+            ]
+        }
     },
     "include": [
         "**/*",