Replace as assertions with redundant type guards

Sometimes TypeScript's type narrowing and inference doesn't fully work quite like it should. Even in these cases, using `as` assertions should be avoided.

It may seem like we know for certain what the type should be and there's no risk of it breaking, but there's always the possibility that some combination of changes elsewhere might affect the typing, in which case `as` will suppress the alerts TypeScript would otherwise provide us with. This may lead to code breaking silently.

In this case, adding type guards and null checks -- even if they're redundant -- is preferable to the above scenario.

- TODO: document and expound in MetaMask/contributor-docs#47

Author: MajorLift

src/update-changelog.ts

@@ -217,7 +217,7 @@ export async function updateChangelog({
 
   if (
     isReleaseCandidate &&
-    mostRecentTag === `${tagPrefixes[0]}${currentVersion || ''}`
+    mostRecentTag === `${tagPrefixes[0]}${currentVersion ?? ''}`
   ) {
     throw new Error(
       `Current version already has tag, which is unexpected for a release candidate.`,
@@ -250,19 +250,16 @@ export async function updateChangelog({
   // Ensure release header exists, if necessary
   if (
     isReleaseCandidate &&
+    currentVersion &&
     !changelog
       .getReleases()
       .find((release) => release.version === currentVersion)
   ) {
-    // Typecast: currentVersion will be defined here due to type guard at the
-    // top of this function.
-    changelog.addRelease({ version: currentVersion as Version });
+    changelog.addRelease({ version: currentVersion });
   }
 
-  if (isReleaseCandidate && hasUnreleasedChanges) {
-    // Typecast: currentVersion will be defined here due to type guard at the
-    // top of this function.
-    changelog.migrateUnreleasedChangesToRelease(currentVersion as Version);
+  if (isReleaseCandidate && currentVersion && hasUnreleasedChanges) {
+    changelog.migrateUnreleasedChangesToRelease(currentVersion);
   }
 
   const newChangeEntries = newCommits.map(({ prNumber, description }) => {