Allow primitive type guards with typeof on right

sandersn · sandersn · commit 7c1b28f2cbb0 · 2016-06-08T15:55:08.000-07:00
Previously, only type guards of the form `typeof x === 'string'` were
allowed. Now you can write `'string' === typeof x`.
diff --git a/src/compiler/binder.ts b/src/compiler/binder.ts
@@ -612,7 +612,7 @@ namespace ts {
                     if (isNarrowingExpression(expr.left) && (expr.right.kind === SyntaxKind.NullKeyword || expr.right.kind === SyntaxKind.Identifier)) {
                         return true;
                     }
-                    if (expr.left.kind === SyntaxKind.TypeOfExpression && isNarrowingExpression((<TypeOfExpression>expr.left).expression) && expr.right.kind === SyntaxKind.StringLiteral) {
+                    if (isTypeOfNarrowingBinaryExpression(expr)) {
                         return true;
                     }
                     return false;
@@ -624,6 +624,20 @@ namespace ts {
             return false;
         }
 
+        function isTypeOfNarrowingBinaryExpression(expr: BinaryExpression) {
+            let typeOf: Expression;
+            if (expr.left.kind === SyntaxKind.StringLiteral) {
+                typeOf = expr.right;
+            }
+            else if (expr.right.kind === SyntaxKind.StringLiteral) {
+                typeOf = expr.left;
+            }
+            else {
+                typeOf = undefined;;
+            }
+            return typeOf && typeOf.kind === SyntaxKind.TypeOfExpression && isNarrowingExpression((<TypeOfExpression>typeOf).expression);
+        }
+
         function createBranchLabel(): FlowLabel {
             return {
                 flags: FlowFlags.BranchLabel,
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
@@ -7864,7 +7864,8 @@ namespace ts {
                         if (isNullOrUndefinedLiteral(expr.right)) {
                             return narrowTypeByNullCheck(type, expr, assumeTrue);
                         }
-                        if (expr.left.kind === SyntaxKind.TypeOfExpression && expr.right.kind === SyntaxKind.StringLiteral) {
+                        if (expr.left.kind === SyntaxKind.TypeOfExpression && expr.right.kind === SyntaxKind.StringLiteral ||
+                            expr.left.kind === SyntaxKind.StringLiteral && expr.right.kind === SyntaxKind.TypeOfExpression) {
                             return narrowTypeByTypeof(type, expr, assumeTrue);
                         }
                         break;
@@ -7897,12 +7898,12 @@ namespace ts {
             function narrowTypeByTypeof(type: Type, expr: BinaryExpression, assumeTrue: boolean): Type {
                 // We have '==', '!=', '====', or !==' operator with 'typeof xxx' on the left
                 // and string literal on the right
-                const left = <TypeOfExpression>expr.left;
-                const right = <LiteralExpression>expr.right;
-                if (!isMatchingReference(reference, left.expression)) {
+                const typeOf = <TypeOfExpression>(expr.left.kind === SyntaxKind.TypeOfExpression ? expr.left : expr.right);
+                const literal = <LiteralExpression>(expr.right.kind === SyntaxKind.StringLiteral ? expr.right : expr.left);
+                if (!isMatchingReference(reference, typeOf.expression)) {
                     // For a reference of the form 'x.y', a 'typeof x === ...' type guard resets the
                     // narrowed type of 'y' to its declared type.
-                    if (containsMatchingReference(reference, left.expression)) {
+                    if (containsMatchingReference(reference, typeOf.expression)) {
                         return declaredType;
                     }
                     return type;
@@ -7915,14 +7916,14 @@ namespace ts {
                     // We narrow a non-union type to an exact primitive type if the non-union type
                     // is a supertype of that primtive type. For example, type 'any' can be narrowed
                     // to one of the primitive types.
-                    const targetType = getProperty(typeofTypesByName, right.text);
+                    const targetType = getProperty(typeofTypesByName, literal.text);
                     if (targetType && isTypeSubtypeOf(targetType, type)) {
                         return targetType;
                     }
                 }
                 const facts = assumeTrue ?
-                    getProperty(typeofEQFacts, right.text) || TypeFacts.TypeofEQHostObject :
-                    getProperty(typeofNEFacts, right.text) || TypeFacts.TypeofNEHostObject;
+                    getProperty(typeofEQFacts, literal.text) || TypeFacts.TypeofEQHostObject :
+                    getProperty(typeofNEFacts, literal.text) || TypeFacts.TypeofNEHostObject;
                 return getTypeWithFacts(type, facts);
             }
 
