ensure consistent expression value for post-assignment unary expressions

packages/babel-plugin-transform-es2015-modules-systemjs/src/index.js

@@ -43,10 +43,26 @@ export default function ({ types: t }) {
 
       let node = path.node;
 
+      // if it is a non-prefix update expression (x++ etc)
+      // then we must replace with the expression (_export('x', x + 1), x++)
+      // in order to ensure the same update expression value
+      let isPostUpdateExpression = path.isUpdateExpression() && !node.prefix;
+      if (isPostUpdateExpression) {
+        if (node.operator === "++")
+          node = t.binaryExpression("+", node.argument, t.numericLiteral(1));
+        else if (node.operator === "--")
+          node = t.binaryExpression("-", node.argument, t.numericLiteral(1));
+        else
+          isPostUpdateExpression = false;
+      }
+
       for (let exportedName of exportedNames) {
         node = this.buildCall(exportedName, node).expression;
       }
 
+      if (isPostUpdateExpression)
+        node = t.sequenceExpression([node, path.node]);
+
       path.replaceWith(node);
     }
   };

packages/babel-plugin-transform-es2015-modules-systemjs/test/fixtures/regression/T6973/actual.js → packages/babel-plugin-transform-es2015-modules-systemjs/test/fixtures/systemjs/hoisting-bindings/actual.js

@@ -1,7 +1,10 @@
 export function a() {
   alert("a");
+  c++;
 }
 
+export var c = 5;
+
 function b() {
   a();
 }

packages/babel-plugin-transform-es2015-modules-systemjs/test/fixtures/regression/T6973/expected.js → packages/babel-plugin-transform-es2015-modules-systemjs/test/fixtures/systemjs/hoisting-bindings/expected.js

@@ -1,8 +1,10 @@
 System.register([], function (_export, _context) {
   "use strict";
 
+  var c;
   function a() {
     alert("a");
+    _export("c", c + 1), c++;
   }
 
   _export("a", a);
@@ -14,6 +16,10 @@ System.register([], function (_export, _context) {
   return {
     setters: [],
     execute: function () {
+      _export("c", c = 5);
+
+      _export("c", c);
+
       b();
     }
   };

packages/babel-plugin-transform-es2015-modules-systemjs/test/fixtures/systemjs/remap/expected.js

@@ -10,7 +10,7 @@ System.register([], function (_export, _context) {
       _export("test", test);
 
       _export("test", test = 5);
-      _export("test", test++);
+      _export("test", test + 1), test++;
 
       (function () {
         var test = 2;