diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 514e8f401a9f6..bc2f859f8ecb9 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -1930,7 +1930,7 @@ namespace ts {
         }
 
         function createTypeofType() {
-            return getUnionType(convertToArray(typeofEQFacts.keys(), getLiteralType));
+            return getUnionType(arrayFrom(typeofEQFacts.keys(), getLiteralType));
         }
 
         // A reserved member name starts with two underscores, but the third character cannot be an underscore
@@ -10531,12 +10531,8 @@ namespace ts {
                 }
             }
 
-            function inferFromParameterTypes(source: Type, target: Type) {
-                return inferFromTypes(source, target);
-            }
-
             function inferFromSignature(source: Signature, target: Signature) {
-                forEachMatchingParameterType(source, target, inferFromParameterTypes);
+                forEachMatchingParameterType(source, target, inferFromTypes);
 
                 if (source.typePredicate && target.typePredicate && source.typePredicate.kind === target.typePredicate.kind) {
                     inferFromTypes(source.typePredicate.type, target.typePredicate.type);
diff --git a/src/compiler/commandLineParser.ts b/src/compiler/commandLineParser.ts
index 178578ec4521b..5da7e83655fe5 100644
--- a/src/compiler/commandLineParser.ts
+++ b/src/compiler/commandLineParser.ts
@@ -1290,7 +1290,7 @@ namespace ts {
                 case "object":
                     return {};
                 default:
-                    return arrayFrom((<CommandLineOptionOfCustomType>option).type.keys())[0];
+                    return (option as CommandLineOptionOfCustomType).type.keys().next().value;
             }
         }
 
diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index 2eae414db66e9..5c7536a825ed6 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -996,14 +996,6 @@ namespace ts {
         return result;
     }
 
-    export function convertToArray<T, U>(iterator: Iterator<T>, f: (value: T) => U) {
-        const result: U[] = [];
-        for (let { value, done } = iterator.next(); !done; { value, done } = iterator.next()) {
-            result.push(f(value));
-        }
-        return result;
-    }
-
     /**
      * Calls `callback` for each entry in the map, returning the first truthy result.
      * Use `map.forEach` instead for normal iteration.
@@ -2527,4 +2519,4 @@ namespace ts {
     export function isCheckJsEnabledForFile(sourceFile: SourceFile, compilerOptions: CompilerOptions) {
         return sourceFile.checkJsDirective ? sourceFile.checkJsDirective.enabled : compilerOptions.checkJs;
     }
-}
\ No newline at end of file
+}