Replace isNaN usage with the more robust Number.isNaN

Also, refactor the conversion into a helper function. Don't define and
export this from `core/index.ts`, since we don't want to make this
irrelevant helper part of the public API.

Author: ruohola

packages/express/__tests__/index.test.ts

@@ -241,7 +241,10 @@ describe('apilyticsMiddleware()', () => {
     const agent = createAgent({ apiKey });
     const numberSpy = jest
       .spyOn(global, 'Number')
-      .mockImplementation(() => NaN);
+      .mockImplementationOnce(() => {
+        numberSpy.mockRestore();
+        return NaN;
+      });
     const response = await agent.get('/empty');
     numberSpy.mockRestore();
     expect(response.status).toEqual(200);

packages/express/src/index.ts

@@ -42,15 +42,13 @@ export const apilyticsMiddleware = (
         'http://_', // Cannot parse a relative URL, so make it absolute.
       );
 
-      const _requestSize = Number(req.headers['content-length']);
-      const requestSize = isNaN(_requestSize) ? undefined : _requestSize;
+      const requestSize = numberOrUndefined(req.headers['content-length']);
 
-      const _responseSize = Number(
+      const responseSize = numberOrUndefined(
         // @ts-ignore: `_contentLength` is not typed, but it does exist sometimes
         // when the header doesn't. Even if it doesn't this won't fail at runtime.
         res.getHeader('content-length') ?? res._contentLength,
       );
-      const responseSize = isNaN(_responseSize) ? undefined : _responseSize;
 
       sendApilyticsMetrics({
         apiKey,
@@ -71,3 +69,8 @@ export const apilyticsMiddleware = (
     next();
   };
 };
+
+const numberOrUndefined = (value: unknown): number | undefined => {
+  const converted = Number(value);
+  return Number.isNaN(converted) ? undefined : converted;
+};

packages/next/__tests__/index.test.ts

@@ -280,7 +280,10 @@ describe('withApilytics()', () => {
     const agent = createAgent({ apiKey });
     const numberSpy = jest
       .spyOn(global, 'Number')
-      .mockImplementation(() => NaN);
+      .mockImplementationOnce(() => {
+        numberSpy.mockRestore();
+        return NaN;
+      });
     const response = await agent.get('/empty');
     numberSpy.mockRestore();
     expect(response.status).toEqual(200);

packages/next/src/index.ts

@@ -40,7 +40,7 @@ export const withApilytics = <T>(
     res: NextApiResponse<T>,
   ): Promise<void> => {
     let statusCode: number | undefined;
-    let responseSize: number | undefined;
+
     const timer = milliSecondTimer();
 
     try {
@@ -57,15 +57,13 @@ export const withApilytics = <T>(
         ));
       }
 
-      const _requestSize = Number(req.headers['content-length']);
-      const requestSize = isNaN(_requestSize) ? undefined : _requestSize;
+      const requestSize = numberOrUndefined(req.headers['content-length']);
 
-      const _responseSize = Number(
+      const responseSize = numberOrUndefined(
         // @ts-ignore: `_contentLength` is not typed, but it does exist sometimes
         // when the header doesn't. Even if it doesn't this won't fail at runtime.
         res.getHeader('content-length') ?? res._contentLength,
       );
-      responseSize = isNaN(_responseSize) ? undefined : _responseSize;
 
       sendApilyticsMetrics({
         apiKey,
@@ -83,3 +81,8 @@ export const withApilytics = <T>(
     }
   };
 };
+
+const numberOrUndefined = (value: unknown): number | undefined => {
+  const converted = Number(value);
+  return Number.isNaN(converted) ? undefined : converted;
+};