Merge pull request #1081 from mobxjs/fix-1077-observe

Make sure fields are initialized before observing them, fixes #1072

CHANGELOG.md

@@ -1,6 +1,7 @@
 # 3.1.17
 
 * Improved typings of `IObservableArray.intercept`: use more restrictive types for `change` parameter of `handler`, by @bvanreeven
+* Fixed [#1072](https://github.com/mobxjs/mobx/issues/1072), fields without a default value could not be observed yet when using TypeScript
 
 # 3.1.16
 

src/types/type-utils.ts

@@ -26,6 +26,8 @@ export function getAtom(thing: any, property?: string): IDepTreeNode {
 		}
 		// Initializers run lazily when transpiling to babel, so make sure they are run...
 		runLazyInitializers(thing);
+		if (property && !thing.$mobx)
+			thing[property]; // See #1072 // TODO: remove in 4.0
 		if (isObservableObject(thing)) {
 			if (!property)
 				return fail(`please specify a property`);

test/typescript/typescript-tests.ts

@@ -1133,3 +1133,13 @@ test("803 - action.bound and action preserve type info", t => {
 	const bound2 = action.bound(function () {}) as (() => void)
 	t.end()
 })
+
+test("1072 - @observable without initial value and observe before first access", t => {
+	class User {
+		@observable loginCount: number;
+	}
+
+	const user = new User();
+	observe(user, 'loginCount', () => {});
+	t.end()
+})