fix: action transparently forwards toString of underlying function (#…

…3654)

* fix: action transparently forwards toString of underlying function

* changeset

.changeset/sweet-experts-allow.md

@@ -0,0 +1,5 @@
+---
+"mobx": patch
+---
+
+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)