Merge pull request #13791 from Microsoft/typeof-string-literal-union-type

`typeof x` now has a string literal union type

src/compiler/checker.ts

@@ -326,7 +326,6 @@ namespace ts {
             "object": TypeFacts.TypeofEQObject,
             "function": TypeFacts.TypeofEQFunction
         });
-
         const typeofNEFacts = createMapFromTemplate({
             "string": TypeFacts.TypeofNEString,
             "number": TypeFacts.TypeofNENumber,
@@ -336,7 +335,6 @@ namespace ts {
             "object": TypeFacts.TypeofNEObject,
             "function": TypeFacts.TypeofNEFunction
         });
-
         const typeofTypesByName = createMapFromTemplate<Type>({
             "string": stringType,
             "number": numberType,
@@ -344,6 +342,7 @@ namespace ts {
             "symbol": esSymbolType,
             "undefined": undefinedType
         });
+        const typeofType = createTypeofType();
 
         let jsxElementType: Type;
         let _jsxNamespace: string;
@@ -1727,6 +1726,10 @@ namespace ts {
             return type;
         }
 
+        function createTypeofType() {
+            return getUnionType(convertToArray(typeofEQFacts.keys(), s => getLiteralTypeForText(TypeFlags.StringLiteral, s)));
+        }
+
         // A reserved member name starts with two underscores, but the third character cannot be an underscore
         // or the @ symbol. A third underscore indicates an escaped form of an identifer that started
         // with at least two underscores. The @ character indicates that the name is denoted by a well known ES
@@ -14626,7 +14629,7 @@ namespace ts {
 
         function checkTypeOfExpression(node: TypeOfExpression): Type {
             checkExpression(node.expression);
-            return stringType;
+            return typeofType;
         }
 
         function checkVoidExpression(node: VoidExpression): Type {

src/compiler/core.ts

@@ -895,6 +895,14 @@ 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.