Adds computedConfigurable (#2014)

Author: jillesme

src/api/configure.ts

@@ -9,13 +9,15 @@ import {
 export function configure(options: {
     enforceActions?: boolean | "strict" | "never" | "always" | "observed"
     computedRequiresReaction?: boolean
+    computedConfigurable?: boolean
     isolateGlobalState?: boolean
     disableErrorBoundaries?: boolean
     reactionScheduler?: (f: () => void) => void
 }): void {
     const {
         enforceActions,
         computedRequiresReaction,
+        computedConfigurable,
         disableErrorBoundaries,
         reactionScheduler
     } = options
@@ -52,6 +54,9 @@ export function configure(options: {
     if (computedRequiresReaction !== undefined) {
         globalState.computedRequiresReaction = !!computedRequiresReaction
     }
+    if (computedConfigurable !== undefined) {
+        globalState.computedConfigurable = !!computedConfigurable
+    }
     if (disableErrorBoundaries !== undefined) {
         if (disableErrorBoundaries === true)
             console.warn(

src/core/globalstate.ts

@@ -101,14 +101,20 @@ export class MobXGlobals {
      */
     computedRequiresReaction = false
 
+    /**
+     * Allows overwriting of computed properties, useful in tests but not prod as it can cause
+     * memory leaks. See https://github.com/mobxjs/mobx/issues/1867
+     */
+    computedConfigurable = false
+
     /*
      * Don't catch and rethrow exceptions. This is useful for inspecting the state of
      * the stack when an exception occurs while debugging.
      */
     disableErrorBoundaries = false
 
     /*
-     * If true, we are already handling an exception in an action. Any errors in reactions should be supressed, as 
+     * If true, we are already handling an exception in an action. Any errors in reactions should be supressed, as
      * they are not the cause, see: https://github.com/mobxjs/mobx/issues/1836
      */
     suppressReactionErrors = false

src/types/observableobject.ts

@@ -385,7 +385,7 @@ export function generateComputedPropConfig(propName) {
     return (
         computedPropertyConfigs[propName] ||
         (computedPropertyConfigs[propName] = {
-            configurable: false, // See https://github.com/mobxjs/mobx/issues/1867, for computeds, we don't want reconfiguration, as this will potentially leak memory!
+            configurable: globalState.computedConfigurable,
             enumerable: false,
             get() {
                 return getAdministrationForComputedPropOwner(this).read(propName)

test/base/strict-mode.js

@@ -249,3 +249,25 @@ test("#1869", function() {
     }).toThrow("Since strict-mode is enabled")
     mobx._resetGlobalState() // should preserve strict mode
 })
+
+test("allow overwriting computed if configured", function() {
+    try {
+        mobx.configure({ computedConfigurable: true })
+        const x = mobx.observable({
+            v: 2,
+            get multiplied() {
+                return x * 2
+            }
+        })
+        mobx.decorate(x, { multiplied: mobx.computed })
+
+        expect(() => {
+            Object.defineProperty(x, "multiplied", {
+                value: 12
+            })
+        }).not.toThrow()
+        expect(x.multiplied).toBe(12)
+    } finally {
+        mobx.configure({ computedConfigurable: false })
+    }
+})