TypeScriptToLua
diff --git a/‎src/LuaTransformer.ts‎
Lines changed: 8 additions & 0 deletions b/‎src/LuaTransformer.ts‎
Lines changed: 8 additions & 0 deletions
diff --git a/‎src/TSHelper.ts‎
Lines changed: 8 additions & 0 deletions b/‎src/TSHelper.ts‎
Lines changed: 8 additions & 0 deletions
diff --git a/‎test/unit/string.spec.ts‎
Lines changed: 11 additions & 0 deletions b/‎test/unit/string.spec.ts‎
Lines changed: 11 additions & 0 deletions
@@ -3895,6 +3895,14 @@ export class LuaTransformer {
         }
 
         switch (ownerType.flags) {
+            case ts.TypeFlags.Union:
+                const unionOwnerType = ownerType as ts.UnionOrIntersectionType;
+                if (
+                    !tsHelper.isUnionAllOfType(unionOwnerType, ts.TypeFlags.String) &&
+                    !tsHelper.isUnionAllOfType(unionOwnerType, ts.TypeFlags.StringLiteral)
+                ) {
+                    break;
+                }
             case ts.TypeFlags.String:
             case ts.TypeFlags.StringLiteral:
                 return this.transformStringCallExpression(node);
 
@@ -176,6 +176,14 @@ export function isArrayType(type: ts.Type, checker: ts.TypeChecker, program: ts.
     return forTypeOrAnySupertype(type, checker, t => isExplicitArrayType(t, checker, program));
 }
 
+export function isUnionAllOfType(ownerType: ts.UnionOrIntersectionType, type: ts.TypeFlags): boolean {
+    return !ownerType.types.some(subType => {
+        const flags = subType.flags;
+
+        return flags !== type;
+    });
+}
+
 export function isLuaIteratorType(node: ts.Node, checker: ts.TypeChecker): boolean {
     const type = checker.getTypeAtLocation(node);
     return getCustomDecorators(type, checker).has(DecoratorKind.LuaIterator);
 
@@ -233,6 +233,17 @@ test.each(["hello test"])("string.toUpperCase (%p)", inp => {
     expect(result).toBe(inp.toUpperCase());
 });
 
+test.each(["hello", "test", "hello test"])("string.toUpperCase (union) (%p)", inp => {
+    const result = util.transpileAndExecute(`
+      const _union: string = "${inp}";
+      if (_union === 'hello' || _union === 'test' || _union === 'hello test') {
+        return _union.toUpperCase();
+      }
+    `);
+
+    expect(result).toBe(inp.toUpperCase());
+});
+
 test.each([
     { inp: "hello test", separator: "" },
     { inp: "hello test", separator: " " },