Throw error in development when @observable is used on getters

src/api/computed.ts

@@ -24,7 +24,7 @@ export const computedDecorator = createPropDecorator(
     ) => {
         const { get, set } = descriptor // initialValue is the descriptor for get / set props
         // Optimization: faster on decorator target or instance? Assuming target
-        // Optimiziation: find out if declaring on instance isn't just faster. (also makes the property descriptor simpler). But, more memory usage..
+        // Optimization: find out if declaring on instance isn't just faster. (also makes the property descriptor simpler). But, more memory usage..
         const options = decoratorArgs[0] || {}
         defineComputedProperty(instance, propertyName, { ...options, get, set })
     }

src/api/observabledecorator.ts

@@ -1,5 +1,5 @@
 import { defineObservableProperty } from "../types/observableobject"
-import { fail } from "../utils/utils"
+import { fail, invariant } from "../utils/utils"
 import { IEnhancer } from "../types/modifiers"
 import { createPropDecorator, BabelDescriptor } from "../utils/decorators2"
 
@@ -18,6 +18,12 @@ export function createDecoratorForEnhancer(enhancer: IEnhancer<any>): IObservabl
             _decoratorTarget,
             decoratorArgs: any[]
         ) => {
+            if (process.env.NODE_ENV !== "production") {
+                invariant(
+                    !descriptor || !descriptor.get,
+                    `@observable cannot be used on getter (property "${propertyName}"), use @computed instead.`
+                )
+            }
             const initialValue = descriptor
                 ? descriptor.initializer ? descriptor.initializer.call(target) : descriptor.value
                 : undefined

src/utils/decorators2.ts

@@ -74,7 +74,7 @@ export function createPropDecorator(
             descriptor: BabelDescriptor | undefined,
             applyImmediately?: any
             // This is a special parameter to signal the direct application of a decorator, allow extendObservable to skip the entire type decoration part,
-            // as the instance to apply the deorator to equals the target
+            // as the instance to apply the decorator to equals the target
         ) {
             if (applyImmediately === true) {
                 propertyCreator(target, prop, descriptor, target, decoratorArguments)

test/base/decorate.js

@@ -351,3 +351,20 @@ test("decorate should work with ES6 constructor", () => {
         title: observable
     })
 })
+
+test("decorate should not allow @observable on getter", function() {
+    const obj = {
+        x: 0,
+        get y() {
+            return 0
+        }
+    }
+
+    decorate(obj, {
+        x: observable,
+        y: observable
+    })
+
+    expect(() => obj.x).toThrow(/"y"/)
+    expect(() => obj.y).toThrow()
+})