ref(utils): Use type predicates in is utility functions (#4124)

We have a number of `is` utility functions (`isString`, `isError`, and so on), and while they tell the human reading the code something about the value in question, they don't tell TypeScript anything, because it doesn't see them as type checks. As a result, we end up with a lot of code like this:

```
if( isString(msg) ) {
  const words = (msg as string).split(' ');
}
```

where we have to typecast our variables, despite their type already having been checked immediately beforehand.

In order to fix this, TS lets you return a type predicate[1] from a type-checking function rather than a simple boolean, and that lets it know what the human reader knows - if `isString(msg)` is true, then `msg` must be a string.

This switches all of our `is` functions to use type predicates, and removes all of the typecasts which are no longer necessary as a result of this change.

[1] https://www.typescriptlang.org/docs/handbook/2/narrowing.html#using-type-predicates

Author: lobsterkatie

packages/browser/src/eventbuilder.ts

@@ -65,15 +65,14 @@ export function eventFromUnknownInput(
 ): Event {
   let event: Event;
 
-  if (isErrorEvent(exception as ErrorEvent) && (exception as ErrorEvent).error) {
+  if (isErrorEvent(exception)) {
     // If it is an ErrorEvent with `error` property, extract it to get actual Error
-    const errorEvent = exception as ErrorEvent;
     // eslint-disable-next-line no-param-reassign
-    exception = errorEvent.error;
-    event = eventFromStacktrace(computeStackTrace(exception as Error));
+    exception = exception.error;
+    event = eventFromStacktrace(computeStackTrace(exception));
     return event;
   }
-  if (isDOMError(exception as DOMError) || isDOMException(exception as DOMException)) {
+  if (isDOMError(exception) || isDOMException(exception)) {
     // If it is a DOMError or DOMException (which are legacy APIs, but still supported in some browsers)
     // then we just extract the name, code, and message, as they don't provide anything else
     // https://developer.mozilla.org/en-US/docs/Web/API/DOMError
@@ -90,9 +89,9 @@ export function eventFromUnknownInput(
 
     return event;
   }
-  if (isError(exception as Error)) {
+  if (isError(exception)) {
     // we have a real Error object, do nothing
-    event = eventFromStacktrace(computeStackTrace(exception as Error));
+    event = eventFromStacktrace(computeStackTrace(exception));
     return event;
   }
   if (isPlainObject(exception) || isEvent(exception)) {

packages/integrations/src/ember.ts

@@ -58,7 +58,7 @@ export class Ember implements Integration {
         getCurrentHub().withScope(scope => {
           if (isInstanceOf(reason, Error)) {
             scope.setExtra('context', 'Unhandled Promise error detected');
-            getCurrentHub().captureException(reason, { originalException: reason as Error });
+            getCurrentHub().captureException(reason, { originalException: reason });
           } else {
             scope.setExtra('reason', reason);
             getCurrentHub().captureMessage('Unhandled Promise error detected');

packages/integrations/src/extraerrordata.ts

@@ -97,7 +97,7 @@ export class ExtraErrorData implements Integration {
           continue;
         }
         const value = error[key];
-        extraErrorInfo[key] = isError(value) ? (value as Error).toString() : value;
+        extraErrorInfo[key] = isError(value) ? value.toString() : value;
       }
 
       // Check if someone attached `toJSON` method to grab even more properties (eg. axios is doing that)
@@ -106,7 +106,7 @@ export class ExtraErrorData implements Integration {
 
         for (const key of Object.keys(serializedError)) {
           const value = serializedError[key];
-          extraErrorInfo[key] = isError(value) ? (value as Error).toString() : value;
+          extraErrorInfo[key] = isError(value) ? value.toString() : value;
         }
       }
 

packages/nextjs/src/utils/instrumentServer.ts

@@ -227,7 +227,7 @@ function makeWrappedReqHandler(origReqHandler: ReqHandler): WrappedReqHandler {
           // If there is a trace header set, extract the data from it (parentSpanId, traceId, and sampling decision)
           let traceparentData;
           if (req.headers && isString(req.headers['sentry-trace'])) {
-            traceparentData = extractTraceparentData(req.headers['sentry-trace'] as string);
+            traceparentData = extractTraceparentData(req.headers['sentry-trace']);
             logger.log(`[Tracing] Continuing trace ${traceparentData?.traceId}.`);
           }
 

packages/nextjs/src/utils/withSentry.ts

@@ -39,7 +39,7 @@ export const withSentry = (origHandler: NextApiHandler): WrappedNextApiHandler =
           // If there is a trace header set, extract the data from it (parentSpanId, traceId, and sampling decision)
           let traceparentData;
           if (req.headers && isString(req.headers['sentry-trace'])) {
-            traceparentData = extractTraceparentData(req.headers['sentry-trace'] as string);
+            traceparentData = extractTraceparentData(req.headers['sentry-trace']);
             logger.log(`[Tracing] Continuing trace ${traceparentData?.traceId}.`);
           }
 

packages/node/src/backend.ts

@@ -41,7 +41,7 @@ export class NodeBackend extends BaseBackend<NodeOptions> {
         const message = `Non-Error exception captured with keys: ${extractExceptionKeysForMessage(exception)}`;
 
         getCurrentHub().configureScope(scope => {
-          scope.setExtra('__serialized__', normalizeToSize(exception as Record<string, unknown>));
+          scope.setExtra('__serialized__', normalizeToSize(exception));
         });
 
         ex = (hint && hint.syntheticException) || new Error(message);

packages/node/src/handlers.ts

@@ -56,7 +56,7 @@ export function tracingHandler(): (
     // If there is a trace header set, we extract the data from it (parentSpanId, traceId, and sampling decision)
     let traceparentData;
     if (req.headers && isString(req.headers['sentry-trace'])) {
-      traceparentData = extractTraceparentData(req.headers['sentry-trace'] as string);
+      traceparentData = extractTraceparentData(req.headers['sentry-trace']);
     }
 
     const transaction = startTransaction(

packages/serverless/src/awslambda.ts

@@ -256,7 +256,7 @@ export function wrapHandler<TEvent, TResult>(
     let traceparentData;
     const eventWithHeaders = event as { headers?: { [key: string]: string } };
     if (eventWithHeaders.headers && isString(eventWithHeaders.headers['sentry-trace'])) {
-      traceparentData = extractTraceparentData(eventWithHeaders.headers['sentry-trace'] as string);
+      traceparentData = extractTraceparentData(eventWithHeaders.headers['sentry-trace']);
     }
     const transaction = startTransaction({
       name: context.functionName,

packages/serverless/src/gcpfunction/http.ts

@@ -53,7 +53,7 @@ function _wrapHttpFunction(fn: HttpFunction, wrapOptions: Partial<HttpFunctionWr
     let traceparentData;
     const reqWithHeaders = req as { headers?: { [key: string]: string } };
     if (reqWithHeaders.headers && isString(reqWithHeaders.headers['sentry-trace'])) {
-      traceparentData = extractTraceparentData(reqWithHeaders.headers['sentry-trace'] as string);
+      traceparentData = extractTraceparentData(reqWithHeaders.headers['sentry-trace']);
     }
     const transaction = startTransaction({
       name: `${reqMethod} ${reqUrl}`,

packages/tracing/src/browser/request.ts

@@ -185,7 +185,7 @@ export function fetchCallback(
     const options = (handlerData.args[1] = (handlerData.args[1] as { [key: string]: any }) || {});
     let headers = options.headers;
     if (isInstanceOf(request, Request)) {
-      headers = (request as Request).headers;
+      headers = request.headers;
     }
     if (headers) {
       // eslint-disable-next-line @typescript-eslint/no-unsafe-member-access