diff --git a/clang/lib/Format/TokenAnnotator.cpp b/clang/lib/Format/TokenAnnotator.cpp
--- a/clang/lib/Format/TokenAnnotator.cpp
+++ b/clang/lib/Format/TokenAnnotator.cpp
@@ -3370,6 +3370,12 @@
              Style.BitFieldColonSpacing == FormatStyle::BFCS_Before;
     return true;
   }
+  // Do not merge "- -" into "--".
+  if ((Left.isOneOf(tok::minus, tok::minusminus) &&
+       Right.isOneOf(tok::minus, tok::minusminus)) ||
+      (Left.isOneOf(tok::plus, tok::plusplus) &&
+       Right.isOneOf(tok::plus, tok::plusplus)))
+    return true;
   if (Left.is(TT_UnaryOperator)) {
     if (!Right.is(tok::l_paren)) {
       // The alternative operators for ~ and ! are "compl" and "not".
diff --git a/clang/unittests/Format/FormatTestJS.cpp b/clang/unittests/Format/FormatTestJS.cpp
--- a/clang/unittests/Format/FormatTestJS.cpp
+++ b/clang/unittests/Format/FormatTestJS.cpp
@@ -276,6 +276,12 @@
   // ES6 spread operator.
   verifyFormat("someFunction(...a);");
   verifyFormat("var x = [1, ...a, 2];");
+
+  // "- -1" is legal JS syntax, but must not collapse into "--".
+  verifyFormat("- -1;", " - -1;");
+  verifyFormat("-- -1;", " -- -1;");
+  verifyFormat("+ +1;", " + +1;");
+  verifyFormat("++ +1;", " ++ +1;");
 }
 
 TEST_F(FormatTestJS, UnderstandsAmpAmp) {