🐛 Fix instrumention of null function with 3rd party wrapper (#1570) (#…

…1697)

* Wrap function to protect against nulls/undefined

A simple solution for #1570. It feels like we could also just skip wrapping anything that isn't a function on L11, and immediately returning a no-op stop, but I was less comfortable making that change.

* add unit tests for instrument method when null

* fix type to allow null return

* change to undefined and a clearer order

* move test and rename

packages/core/src/tools/instrumentMethod.spec.ts

@@ -99,12 +99,26 @@ describe('instrumentMethod', () => {
 
         expect(instrumentationSpy).not.toHaveBeenCalled()
       })
+
+      it('should not throw errors if original method was undefined', () => {
+        const object: { method?: () => number } = {}
+        const instrumentationStub = () => 2
+        const { stop } = instrumentMethod(object, 'method', () => instrumentationStub)
+
+        thirdPartyInstrumentation(object)
+
+        stop()
+
+        expect(object.method).not.toThrow()
+      })
     })
   })
 
-  function thirdPartyInstrumentation(object: { method: () => number }) {
+  function thirdPartyInstrumentation(object: { method?: () => number }) {
     const originalMethod = object.method
-    object.method = () => originalMethod() + 2
+    if (typeof originalMethod === 'function') {
+      object.method = () => originalMethod() + 2
+    }
   }
 })
 

packages/core/src/tools/instrumentMethod.ts

@@ -12,7 +12,10 @@ export function instrumentMethod<OBJECT extends { [key: string]: any }, METHOD e
 
   let instrumentation = instrumentationFactory(original)
 
-  const instrumentationWrapper = function (this: OBJECT): ReturnType<OBJECT[METHOD]> {
+  const instrumentationWrapper = function (this: OBJECT): ReturnType<OBJECT[METHOD]> | undefined {
+    if (typeof instrumentation !== 'function') {
+      return undefined
+    }
     // eslint-disable-next-line @typescript-eslint/no-unsafe-return
     return instrumentation.apply(this, arguments as unknown as Parameters<OBJECT[METHOD]>)
   }