diff --git a/src/tsconfig.base.json b/src/tsconfig.base.json
index 907c210b530..44595cf5246 100644
--- a/src/tsconfig.base.json
+++ b/src/tsconfig.base.json
@@ -10,6 +10,7 @@
 		"alwaysStrict": true,
 		"strictBindCallApply": true,
 		"strictNullChecks": true,
+		"strictPropertyInitialization": true,
 		"forceConsistentCasingInFileNames": true,
 		"baseUrl": ".",
 		"paths": {
diff --git a/src/vs/base/browser/ui/tree/asyncDataTree.ts b/src/vs/base/browser/ui/tree/asyncDataTree.ts
index 50ab5ce9be3..49cf3251751 100644
--- a/src/vs/base/browser/ui/tree/asyncDataTree.ts
+++ b/src/vs/base/browser/ui/tree/asyncDataTree.ts
@@ -1031,7 +1031,7 @@ export interface ICompressibleAsyncDataTreeOptionsUpdate extends IAsyncDataTreeO
 
 export class CompressibleAsyncDataTree<TInput, T, TFilterData = void> extends AsyncDataTree<TInput, T, TFilterData> {
 
-	protected readonly tree: CompressibleObjectTree<IAsyncDataTreeNode<TInput, T>, TFilterData>;
+	protected readonly tree!: CompressibleObjectTree<IAsyncDataTreeNode<TInput, T>, TFilterData>;
 	protected readonly compressibleNodeMapper: CompressibleAsyncDataTreeNodeMapper<TInput, T, TFilterData> = new WeakMapper(node => new CompressibleAsyncDataTreeNodeWrapper(node));
 
 	constructor(
diff --git a/src/vs/workbench/api/common/extHostTypes.ts b/src/vs/workbench/api/common/extHostTypes.ts
index 158fe11fda7..71bbf302eeb 100644
--- a/src/vs/workbench/api/common/extHostTypes.ts
+++ b/src/vs/workbench/api/common/extHostTypes.ts
@@ -1143,8 +1143,8 @@ export class SelectionRange {
 
 export class CallHierarchyItem {
 
-	_sessionId: string;
-	_itemId: string;
+	_sessionId?: string;
+	_itemId?: string;
 
 	kind: SymbolKind;
 	name: string;
diff --git a/src/vs/workbench/browser/parts/views/viewsViewlet.ts b/src/vs/workbench/browser/parts/views/viewsViewlet.ts
index aa4e7f432ed..cf9681d52e0 100644
--- a/src/vs/workbench/browser/parts/views/viewsViewlet.ts
+++ b/src/vs/workbench/browser/parts/views/viewsViewlet.ts
@@ -328,7 +328,7 @@ export abstract class ViewContainerViewlet extends PanelViewlet implements IView
 export abstract class FilterViewContainerViewlet extends ViewContainerViewlet {
 	private constantViewDescriptors: Map<string, IViewDescriptor> = new Map();
 	private allViews: Map<string, Map<string, IViewDescriptor>> = new Map();
-	private filterValue: string;
+	private filterValue: string | undefined;
 
 	protected onDidChangeFilterValue: Emitter<string> = new Emitter();