JS: fix error in generation of if/else statements in some cases

See KT-19495

js/js.ast/src/org/jetbrains/kotlin/js/backend/JsToStringGenerationVisitor.java

@@ -644,7 +644,7 @@ public class JsToStringGenerationVisitor extends JsVisitor {
 
         JsStatement thenStmt = x.getThenStatement();
         JsStatement elseStatement = x.getElseStatement();
-        if (elseStatement != null && thenStmt instanceof JsIf && ((JsIf)thenStmt).getElseStatement() == null) {
+        if (elseStatement != null && isIfWithoutElse(thenStmt)) {
             thenStmt = new JsBlock(thenStmt);
         }
         nestedPush(thenStmt);
@@ -680,6 +680,18 @@ public class JsToStringGenerationVisitor extends JsVisitor {
         }
     }
 
+    private static boolean isIfWithoutElse(@NotNull JsStatement statement) {
+        while (statement instanceof JsIf) {
+            JsIf ifStatement = (JsIf) statement;
+            if (ifStatement.getElseStatement() == null) {
+                return true;
+            }
+            statement = ifStatement.getElseStatement();
+        }
+
+        return false;
+    }
+
     @Override
     public void visitInvocation(@NotNull JsInvocation invocation) {
         pushSourceInfo(invocation.getSource());

js/js.tests/test/org/jetbrains/kotlin/js/test/semantics/BoxJsTestGenerated.java

@@ -2578,6 +2578,12 @@ public class BoxJsTestGenerated extends AbstractBoxJsTest {
                 doTest(fileName);
             }
 
+            @TestMetadata("ifElseCurlyBraces.kt")
+            public void testIfElseCurlyBraces() throws Exception {
+                String fileName = KotlinTestUtils.navigationMetadata("js/js.translator/testData/box/expression/if/ifElseCurlyBraces.kt");
+                doTest(fileName);
+            }
+
             @TestMetadata("ifInsideLambda.kt")
             public void testIfInsideLambda() throws Exception {
                 String fileName = KotlinTestUtils.navigationMetadata("js/js.translator/testData/box/expression/if/ifInsideLambda.kt");

js/js.translator/testData/box/expression/if/ifElseCurlyBraces.kt

@@ -0,0 +1,24 @@
+// EXPECTED_REACHABLE_NODES: 996
+var log = ""
+
+fun foo(s: String, a: Int, b: Int) {
+    when (s) {
+        "A" -> if (a > b) {
+            log += "1"
+        }
+        else if (b > a) {
+            log += "2"
+        }
+        "B" -> log += "3"
+    }
+}
+
+fun box(): String {
+    foo("A", 3, 2)
+    foo("A", 2, 3)
+    foo("B", 2, 3)
+
+    if (log != "123") return "fail: $log"
+
+    return "OK"
+}