[[FEAT]] Implement support for ES8 trailing commas

src/jshint.js

@@ -1062,12 +1062,11 @@ var JSHINT = (function() {
       case "}":
       case "]":
       case ",":
+      case ")":
         if (opts.allowTrailing) {
           return true;
         }
 
-        /* falls through */
-      case ")":
         error("E024", state.tokens.next, state.tokens.next.value);
         return false;
       }
@@ -2952,7 +2951,15 @@ var JSHINT = (function() {
         if (state.tokens.next.id !== ",") {
           break;
         }
-        parseComma();
+        parseComma({ allowTrailing: true });
+
+        if (state.tokens.next.id === ")") {
+          if (!state.inES8()) {
+            warning("W119", state.tokens.curr, "Trailing comma in arguments lists", "8");
+          }
+
+          break;
+        }
       }
     }
 
@@ -3429,13 +3436,18 @@ var JSHINT = (function() {
 
       // now we have evaluated the default expression, add the variable to the param scope
       currentParams.forEach(addParam);
-
       if (state.tokens.next.id === ",") {
         if (pastRest) {
           warning("W131", state.tokens.next);
         }
-        parseComma();
-      } else {
+        parseComma({ allowTrailing: true });
+      }
+
+      if (state.tokens.next.id === ")") {
+        if (state.tokens.curr.id === "," && !state.inES8()) {
+          warning("W119", state.tokens.curr, "Trailing comma in function parameters", "8");
+        }
+
         advance(")", next);
         return {
           arity: arity,

src/state.js

@@ -65,6 +65,15 @@ var state = {
     return this.option.moz;
   },
 
+  /**
+   * Determine if constructs introduced in ECMAScript 8 should be accepted.
+   *
+   * @returns {boolean}
+   */
+  inES8: function() {
+    return this.esVersion >= 8;
+  },
+
   /**
    * Determine if constructs introduced in ECMAScript 7 should be accepted.
    *