clang-format: [JS] support new assignment operators.

[mprobst]

Summary

Before:

a && = b;

After:

a &&= b;

These operators are new additions in ES2021.

[mprobst]

• fix comment

clang/lib/Format/FormatToken.h

#define LIST_TOKEN_TYPES                                                       \
   TYPE(JsFatArrow)                                                             \
   TYPE(JsNonNullAssertion)                                                     \
   TYPE(JsNullishCoalescingOperator)                                            \
   TYPE(JsNullPropagatingOperator)                                              \
   TYPE(JsPrivateIdentifier)                                                    \
   TYPE(JsTypeColon)                                                            \
   TYPE(JsTypeOperator)                                                         \
   TYPE(JsTypeOptionalQuestion)                                                 \
+  TYPE(JsAndAndEqual)                                                          \
+  TYPE(JsPipePipeEqual)                                                        \
+  TYPE(JsNullishCoalescingEqual)                                               \
   TYPE(LambdaArrow)                                                            \
   TYPE(LambdaLBrace)                                                           \
   TYPE(LambdaLSquare)                                                          \
   TYPE(LeadingJavaAnnotation)                                                  \
   TYPE(LineComment)                                                            \
   TYPE(MacroBlockBegin)                                                        \
   TYPE(MacroBlockEnd)                                                          \
   TYPE(NamespaceMacro)                                                         \

clang/lib/Format/FormatTokenLexer.cpp

if (Style.Language == FormatStyle::LK_JavaScript) {
     static const tok::TokenKind JSRightArrow[] = {tok::equal, tok::greater};
     static const tok::TokenKind JSExponentiation[] = {tok::star, tok::star};
     static const tok::TokenKind JSExponentiationEqual[] = {tok::star,
                                                            tok::starequal};
     static const tok::TokenKind JSNullPropagatingOperator[] = {tok::question,
                                                                tok::period};
     static const tok::TokenKind JSNullishOperator[] = {tok::question,
                                                        tok::question};
+    static const tok::TokenKind JSNullishEqual[] = {tok::question,
+                                                    tok::question, tok::equal};
+    static const tok::TokenKind JSPipePipeEqual[] = {tok::pipepipe, tok::equal};
+    static const tok::TokenKind JSAndAndEqual[] = {tok::ampamp, tok::equal};
 
     // FIXME: Investigate what token type gives the correct operator priority.
     if (tryMergeTokens(JSIdentity, TT_BinaryOperator))
       return;
     if (tryMergeTokens(JSNotIdentity, TT_BinaryOperator))
       return;
     if (tryMergeTokens(JSShiftEqual, TT_BinaryOperator))
       return;
if (tryMergeTokens(JSNullishOperator, TT_JsNullishCoalescingOperator)) {
       return;
     }
     if (tryMergeTokens(JSNullPropagatingOperator,
                        TT_JsNullPropagatingOperator)) {
       // Treat like a regular "." access.
       Tokens.back()->Tok.setKind(tok::period);
       return;
     }
+    if (tryMergeTokens(JSAndAndEqual, TT_JsAndAndEqual) ||
+        tryMergeTokens(JSPipePipeEqual, TT_JsPipePipeEqual) ||
+        tryMergeTokens(JSNullishEqual, TT_JsNullishCoalescingEqual)) {
+      // Treat like the "=" assignment operator.
+      Tokens.back()->Tok.setKind(tok::equal);
+      return;
+    }
     if (tryMergeJSPrivateIdentifier())
       return;
   }
 
   if (Style.Language == FormatStyle::LK_Java) {
     static const tok::TokenKind JavaRightLogicalShiftAssign[] = {
         tok::greater, tok::greater, tok::greaterequal};
     if (tryMergeTokens(JavaRightLogicalShiftAssign, TT_BinaryOperator))

clang/unittests/Format/FormatTestJS.cpp

TEST_F(FormatTestJS, NullishCoalescingOperator) {
   verifyFormat("const val = something ?? 'some other default';\n");
   verifyFormat(
       "const val = something ?? otherDefault ??\n"
       "    evenMore ?? evenMore;\n",
       "const val = something ?? otherDefault ?? evenMore ?? evenMore;\n",
       getGoogleJSStyleWithColumns(40));
 }
 
+TEST_F(FormatTestJS, AssignmentOperators) {
+  verifyFormat("a &&= b;\n");
+  verifyFormat("a ||= b;\n");
+  // NB: need to split ? ?= to avoid it being interpreted by C++ as a trigraph
+  // for #.
+  verifyFormat("a ?"
+               "?= b;\n");
+}
+
 TEST_F(FormatTestJS, Conditional) {
   verifyFormat("y = x ? 1 : 2;");
   verifyFormat("x ? 1 : 2;");
   verifyFormat("class Foo {\n"
                "  field = true ? 1 : 2;\n"
                "  method(a = true ? 1 : 2) {}\n"
                "}");
 }