diff --git a/lib/system/jssys.nim b/lib/system/jssys.nim
index 8065f22555..8c3175eac4 100644
--- a/lib/system/jssys.nim
+++ b/lib/system/jssys.nim
@@ -417,11 +417,11 @@ proc cmpStrings(a, b: string): int {.asmNoStackFrame, compilerProc.} =
     if (`a` == `b`) return 0;
     if (!`a`) return -1;
     if (!`b`) return 1;
-    for (var i = 0; i < `a`.length-1; ++i) {
+    for (var i = 0; i < `a`.length - 1 && i < `b`.length - 1; i++) {
       var result = `a`[i] - `b`[i];
       if (result != 0) return result;
     }
-    return 0;
+    return `a`.length - `b`.length;
   """
 
 proc cmp(x, y: string): int =
diff --git a/tests/js/tstringitems.nim b/tests/js/tstringitems.nim
index 20aed6e8bf..ff016642e3 100644
--- a/tests/js/tstringitems.nim
+++ b/tests/js/tstringitems.nim
@@ -76,3 +76,12 @@ block: # String case of
   case s
   of "Привет!": discard
   else: doAssert(false)
+
+block: # String cmp
+  var a, b: string
+  doAssert(cmp(a, b) == 0)
+  doAssert(cmp("foo", "foo") == 0)
+  doAssert(cmp("foobar", "foo") == 3)
+  doAssert(cmp("foo", "foobar") == -3)
+  doAssert(cmp("fooz", "foog") == 19)
+  doAssert(cmp("foog", "fooz") == -19)