diff --git a/lib/compress.js b/lib/compress.js
index 992d78f8..c994a3ab 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -1751,14 +1751,14 @@ merge(Compressor.prototype, {
             // XXX: intentionally falling down to the next case
           case "==":
           case "!=":
-            if (self.left instanceof AST_String
-                && self.left.value == "undefined"
-                && self.right instanceof AST_UnaryPrefix
-                && self.right.operator == "typeof"
-                && compressor.option("unsafe")) {
-                if (!(self.right.expression instanceof AST_SymbolRef)
-                    || !self.right.expression.undeclared()) {
-                    self.left = self.right.expression;
+            if (compressor.option("unsafe")
+                && self.left instanceof AST_UnaryPrefix
+                && self.left.operator == "typeof"
+                && self.right instanceof AST_String
+                && self.right.value == "undefined") {
+                if (!(self.left.expression instanceof AST_SymbolRef)
+                    || !self.left.expression.undeclared()) {
+                    self.left = self.left.expression;
                     self.right = make_node(AST_Undefined, self.left).optimize(compressor);
                     if (self.operator.length == 2) self.operator += "=";
                 }
diff --git a/test/compress/issue-105.js b/test/compress/issue-105.js
index 349d732d..0c37eb82 100644
--- a/test/compress/issue-105.js
+++ b/test/compress/issue-105.js
@@ -1,10 +1,9 @@
 typeof_eq_undefined: {
     options = {
-        comparisons: true,
-        unsafe: false
+        comparisons: true
     };
     input: { a = typeof b.c != "undefined" }
-    expect: { a = "undefined" != typeof b.c }
+    expect: { a = typeof b.c != "undefined" }
 }
 
 typeof_eq_undefined_unsafe: {
diff --git a/test/compress/issue-143.js b/test/compress/issue-143.js
new file mode 100644
index 00000000..4c79790b
--- /dev/null
+++ b/test/compress/issue-143.js
@@ -0,0 +1,48 @@
+/**
+ * There was an incorrect sort behaviour documented in issue #143:
+ * (x = f(…)) <= x → x >= (x = f(…))
+ *
+ * For example, let the equation be:
+ * (a = parseInt('100')) <= a
+ *
+ * If a was an integer and has the value of 99,
+ * (a = parseInt('100')) <= a → 100 <= 100 → true
+ *
+ * When transformed incorrectly:
+ * a >= (a = parseInt('100')) → 99 >= 100 → false
+ */
+
+tranformation_sort_order_equal: {
+    options = {
+        comparisons: true,
+    };
+
+    input: { (a = parseInt('100')) == a }
+    expect: { (a = parseInt('100')) == a }
+}
+
+tranformation_sort_order_unequal: {
+    options = {
+        comparisons: true,
+    };
+
+    input: { (a = parseInt('100')) != a }
+    expect: { (a = parseInt('100')) != a }
+}
+
+tranformation_sort_order_lesser_or_equal: {
+    options = {
+        comparisons: true,
+    };
+
+    input: { (a = parseInt('100')) <= a }
+    expect: { (a = parseInt('100')) <= a }
+}
+tranformation_sort_order_greater_or_equal: {
+    options = {
+        comparisons: true,
+    };
+
+    input: { (a = parseInt('100')) >= a }
+    expect: { (a = parseInt('100')) >= a }
+}
\ No newline at end of file