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+# Tree Shaking Principles
+
+When designing code please keep these principles in mind
+
+## Enums for features are not-tree-shakable
+
+Here is an example of code which is not tree shakable.
+
+```
+export function query<T>(
+    predicate: Type<any>| string[], descend?: boolean,
+    read?: QueryReadType | Type<T>): QueryList<T> {
+  ngDevMode && assertPreviousIsParent();
+  const queryList = new QueryList<T>();
+  const query = currentQuery || (currentQuery = new LQuery_());
+  query.track(queryList, predicate, descend, read);
+  return queryList;
+}
+```
+
+Notice that `query()` takes the `QueryReadType` as enumeration. 
+
+```
+function readFromNodeInjector(
+    nodeInjector: LInjector, node: LNode, read: QueryReadType | Type<any>): any {
+  if (read === QueryReadType.ElementRef) {
+    return getOrCreateElementRef(nodeInjector);
+  } 
+  if (read === QueryReadType.ViewContainerRef) {
+    return getOrCreateContainerRef(nodeInjector);
+  }
+  if (read === QueryReadType.TemplateRef) {
+    return getOrCreateTemplateRef(nodeInjector);
+  } 
+  const matchingIdx = geIdxOfMatchingDirective(node, read);
+  if (matchingIdx !== null) {
+    return node.view.data[matchingIdx];
+  }
+  return null;
+}
+```
+
+Sometimes later in the above code the `readFromNodeInjector` takes the `QueryReadType` enumeration and performs specific behavior.
+
+The issue is that once the `query` instruction is pulled in it will pull in `ElementRef`, `ContainerRef`, and `TemplateRef` regardless if the `query` instruction queries for them.
+
+A better way to do this is to encapsulate the work into an object or function which will then be passed into the `query` instead of the enumeration. 
+
+```
+function queryElementRefFeature() {...}
+function queryContainerRefFeature() {...}
+function queryTemplateRefFeature() {...}
+
+query(predicate, descend, queryElementRefFeature) {...}
+```
+
+this would allow the `readFromNodeInjector` to simply call the `read` function (or object) like so.
+
+```
+function readFromNodeInjector(
+    nodeInjector: LInjector, node: LNode, readFn: (injector: Injector) => any) | Type<any>): any {
+  if (isFeature(readFn)) {
+    return readFn(nodeInjector);
+  } 
+  const matchingIdx = geIdxOfMatchingDirective(node, readFn);
+  if (matchingIdx !== null) {
+    return node.view.data[matchingIdx];
+  }
+  return null;
+}
+```
+
+This approach allows us to preserve the tree-shaking. In essence the if statement has moved from runtime (non-tree-shakable) to compile time (tree-shakable) position.
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