add support for trailing commas in arrow function parameter lists - fixes #1841

src/acorn/src/expression.js

@@ -432,9 +432,18 @@ pp.parseParenAndDistinguishExpression = function(start, isAsync, canBeArrow) {
     }
 
     let innerStart = this.markPosition(), exprList = [], first = true
-    let refShorthandDefaultPos = {start: 0}, spreadStart, innerParenStart
+    let refShorthandDefaultPos = {start: 0}, spreadStart, innerParenStart, optionalCommaStart
     while (this.type !== tt.parenR) {
-      first ? first = false : this.expect(tt.comma)
+      if (first) {
+        first = false;
+      } else {
+        this.expect(tt.comma)
+        if (this.type === tt.parenR && this.options.features["es7.trailingFunctionCommas"]) {
+          optionalCommaStart = this.start
+          break
+        }
+      }
+
       if (this.type === tt.ellipsis) {
         let spreadNodeStart = this.markPosition()
         spreadStart = this.start
@@ -462,6 +471,7 @@ pp.parseParenAndDistinguishExpression = function(start, isAsync, canBeArrow) {
         this.unexpected(this.lastTokStart)
       }
     }
+    if (optionalCommaStart) this.unexpected(optionalCommaStart)
     if (spreadStart) this.unexpected(spreadStart)
     if (refShorthandDefaultPos.start) this.unexpected(refShorthandDefaultPos.start)
 

test/acorn/tests-babel.js

@@ -3374,6 +3374,49 @@ test("class Foo { bar(a,) { } }", {
   features: { "es7.trailingFunctionCommas": true }
 });
 
+test("(x, y, ) => 1;", {
+  start: 0,
+  body: [{
+    start: 0,
+    expression: {
+      start: 0,
+      id: null,
+      generator: false,
+      expression: true,
+      params: [
+        {
+          start: 1,
+          name: "x",
+          type: "Identifier",
+          end: 2
+        },
+        {
+          start: 4,
+          name: "y",
+          type: "Identifier",
+          end: 5
+        }
+      ],
+      body: {
+        start: 12,
+        value: 1,
+        raw: "1",
+        type: "Literal",
+        end: 13
+      },
+      type: "ArrowFunctionExpression",
+      end: 13
+    },
+    type: "ExpressionStatement",
+    end: 14
+  }],
+  type: "Program",
+  end: 14
+}, {
+  ecmaVersion: 7,
+  features: { "es7.trailingFunctionCommas": true }
+});
+
 testFail("log(,);", "Unexpected token (1:4)", {
   ecmaVersion: 7,
   features: { "es7.trailingFunctionCommas": true }
@@ -3383,3 +3426,8 @@ testFail("function log(,) { }", "Unexpected token (1:13)", {
   ecmaVersion: 7,
   features: { "es7.trailingFunctionCommas": true }
 });
+
+testFail("('foo',)", "Unexpected token (1:7)", {
+  ecmaVersion: 7,
+  features: { "es7.trailingFunctionCommas": true }
+});

test/core/fixtures/transformation/es7.trailing-function-commas/arrow-function/actual.js

@@ -0,0 +1 @@
+(x, y, ) => {};

test/core/fixtures/transformation/es7.trailing-function-commas/arrow-function/expected.js

@@ -0,0 +1 @@
+(function (x, y) {});