[[FIX]] Correct invalid function invocation

The second parameter of the `expression` function accepts a value that
modifies how the next token is parsed. When set to `true`, the token is
interpreted as the beginning of a statement; otherwise, it is
interpreted as the beginning of an expression.

Two call sites within the `for` statement's parsing logic specified the
string value `"for"`. Due to inconsistencies in how the parameter was
interpreted internally, this causes `expression` to mix parsing
strategies. Specifically:

- It used the "statement" parsing strategy to mark the token value as
  beginning a statement. This could be observed in the application of
  the `singleGroups` linting option. Further, it disqualify it for
  consideration as a Mozilla "let expression"
- It used the "expression" parsing strategy to ignore the token's `fud`
  method

Since the grammar does not allow a statement in either location, the
"expression" parsing strategy should be used consistently.

Achieve this by removing the argument from the call site, thereby
falling back to the default behavior of the `expression` function.
Update `expression` to consistently interpret the parameter for
"truthiness" in order to discourage similar ambiguity in the future.

src/jshint.js

@@ -924,7 +924,7 @@ var JSHINT = (function() {
       state.tokens.curr.beginsStmt = true;
     }
 
-    if (initial === true && state.tokens.curr.fud) {
+    if (initial && state.tokens.curr.fud) {
       left = state.tokens.curr.fud();
     } else {
       if (state.tokens.curr.nud) {
@@ -4514,7 +4514,7 @@ var JSHINT = (function() {
           state.tokens.curr.fud();
         } else {
           for (;;) {
-            expression(0, "for");
+            expression(0);
             if (state.tokens.next.id !== ",") {
               break;
             }
@@ -4538,7 +4538,7 @@ var JSHINT = (function() {
       }
       if (state.tokens.next.id !== ")") {
         for (;;) {
-          expression(0, "for");
+          expression(0);
           if (state.tokens.next.id !== ",") {
             break;
           }

tests/unit/options.js

@@ -3330,13 +3330,17 @@ singleGroups.objectLiterals = function (test) {
     // Invalid forms
     "var a = ({}).method();",
     "if (({}).method()) {}",
-    "var b = { a: ({}).method() };"
+    "var b = { a: ({}).method() };",
+    "for (({}); ;) {}",
+    "for (; ;({})) {}"
   ];
 
   TestRun(test, "grouping operator not required")
     .addError(4, 9, "Unnecessary grouping operator.")
     .addError(5, 5, "Unnecessary grouping operator.")
     .addError(6, 14, "Unnecessary grouping operator.")
+    .addError(7, 6, "Unnecessary grouping operator.")
+    .addError(8, 9, "Unnecessary grouping operator.")
     .test(code, { singleGroups: true });
 
   test.done();
@@ -3458,13 +3462,17 @@ singleGroups.destructuringAssign = function (test) {
     "([ x ] = [ 1 ]);",
     // expressions
     "1, ({ x } = { x : 1 });",
-    "1, ([ x ] = [ 1 ]);"
+    "1, ([ x ] = [ 1 ]);",
+    "for (({ x } = { X: 1 }); ;) {}",
+    "for (; ;({ x } = { X: 1 })) {}"
   ];
 
   TestRun(test)
     .addError(2, 1, "Unnecessary grouping operator.")
     .addError(3, 4, "Unnecessary grouping operator.")
     .addError(4, 4, "Unnecessary grouping operator.")
+    .addError(5, 6, "Unnecessary grouping operator.")
+    .addError(6, 9, "Unnecessary grouping operator.")
     .test(code, { esversion: 6, singleGroups: true, expr: true });
 
   test.done();

tests/unit/parser.js

@@ -3148,7 +3148,8 @@ exports["let block and let expression"] = function (test) {
     "{",
     "  let(t=4) print(x, y, z, t);",
     "  print(let(u=4) u,x);",
-    "}"
+    "}",
+    "for (; ; let (x = 1) x) {}"
   ];
 
   TestRun(test)
@@ -3163,14 +3164,17 @@ exports["let block and let expression as esnext"] = function (test) {
     "{",
     "  let(t=4) print(x, y, z, t);",
     "  print(let(u=4) u,x);",
-    "}"
+    "}",
+    "for (; ; let (x = 1) x) {}"
   ];
 
   TestRun(test)
     .addError(1, 5, "'let block' is only available in Mozilla JavaScript extensions (use moz option).")
     .addError(3, 6, "'let block' is only available in Mozilla JavaScript extensions (use moz option).")
     .addError(4, 9, "'let expressions' is only available in Mozilla JavaScript extensions " +
       "(use moz option).")
+    .addError(6, 10, "'let expressions' is only available in Mozilla JavaScript extensions " +
+      "(use moz option).")
     .test(code, {esnext: true, unused: true, undef: true, predef: ["print"]});
   test.done();
 };