diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 47811d6ccaf..ba4e8c6993d 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -14609,9 +14609,7 @@ namespace ts {
 
         function checkTypeOfExpression(node: TypeOfExpression): Type {
             checkExpression(node.expression);
-            const types: Type[] = [];
-            typeofEQFacts.forEach((_, s) => types.push(getLiteralTypeForText(TypeFlags.StringLiteral, s)));
-            return getUnionType(types);
+            return getUnionType(arrayFromMap(typeofEQFacts.keys(), s => getLiteralTypeForText(TypeFlags.StringLiteral, s)));
         }
 
         function checkVoidExpression(node: VoidExpression): Type {
diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index de3472afd88..486dd5b42c8 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -895,6 +895,14 @@ namespace ts {
         return result;
     }
 
+    export function arrayFromMap<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.