fix: action transparently forwards toString of underlying function

packages/mobx/__tests__/v5/base/action.js

@@ -646,3 +646,18 @@ test("auto action should not update state from inside a derivation", async () =>
  })
  d()
 })
+
+test("action forwards toString of underlying function", async () => {
+ const fn = () => {
+ /* not actually doing anything */
+ }
+ fn.a = 42
+ fn.toString = function () {
+ return `toString referencing this, a=${this.a}`
+ }
+
+ const act = mobx.action(fn)
+
+ expect(fn.toString()).toBe("toString referencing this, a=42")
+ expect(act.toString()).toBe("toString referencing this, a=42")
+})

packages/mobx/src/core/action.ts

@@ -50,6 +50,7 @@ export function createAction(
  return executeAction(actionName, autoAction, fn, ref || this, arguments)
  }
  res.isMobxAction = true
+ res.toString = () => fn.toString()
  if (isFunctionNameConfigurable) {
  tmpNameDescriptor.value = actionName
  defineProperty(res, "name", tmpNameDescriptor)