Extract for-of iterator handling to a helper (#11262)

* Extract for-of iterator handling to a helper

Dis greatly recudes the code size when for-of is used multiple times across the
codebase. Also, makes it easier to read what's happening in a compiled loop.

* Unify spec and loose code

* Legacy implementation fallback

* Update tmp var name

* Updates from review and after rebase

packages/babel-helpers/src/helpers.js

@@ -1036,6 +1036,61 @@ helpers.nonIterableRest = helper("7.0.0-beta.0")`
   }
 `;
 
+helpers.createForOfIteratorHelper = helper("7.9.0")`
+  // s: start (create the iterator)
+  // n: next
+  // e: error (called whenever something throws)
+  // f: finish (always called at the end)
+
+  export default function _createForOfIteratorHelper(o) {
+    if (typeof Symbol === "undefined" || o[Symbol.iterator] == null)
+      throw new TypeError("Invalid attempt to iterate non-iterable instance.\\nIn order to be iterable, non-array objects must have a [Symbol.iterator]() method.");
+
+    var it, normalCompletion = true, didErr = false, err;
+
+    return {
+      s() {
+        it = o[Symbol.iterator]();
+      },
+      n() {
+        var step = it.next();
+        normalCompletion = step.done;
+        return step;
+      },
+      e(e) {
+        didErr = true;
+        err = e;
+      },
+      f() {
+        try {
+          if (!normalCompletion && it.return != null) it.return();
+        } finally {
+          if (didErr) throw err;
+        }
+      }
+    };
+  }
+`;
+
+helpers.createForOfIteratorHelperLoose = helper("7.9.0")`
+  export default function _createForOfIteratorHelperLoose(o) {
+    var i = 0;
+
+    if (typeof Symbol === "undefined" || o[Symbol.iterator] == null) {
+      if (Array.isArray(o))
+        return function() {
+          if (i >= o.length) return { done: true };
+          return { done: false, value: o[i++] };
+        }
+
+      throw new TypeError("Invalid attempt to iterate non-iterable instance.\\nIn order to be iterable, non-array objects must have a [Symbol.iterator]() method.");
+    }
+
+    i = o[Symbol.iterator]();
+    return i.next.bind(i);
+  }
+`;
+
 helpers.skipFirstGeneratorNext = helper("7.0.0-beta.0")`
   export default function _skipFirstGeneratorNext(fn) {
     return function () {

packages/babel-plugin-transform-block-scoping/test/fixtures/general/hoisting/options.json

@@ -4,6 +4,7 @@
     "syntax-jsx",
     "transform-react-jsx",
     "transform-block-scoped-functions",
-    "transform-for-of"
+    "transform-for-of",
+     ["external-helpers", { "helperVersion": "7.100.0" }]
   ]
 }

packages/babel-plugin-transform-block-scoping/test/fixtures/general/hoisting/output.js

@@ -10,29 +10,19 @@ var _loop = function (i) {
   });
 };
 
-var _iteratorNormalCompletion = true;
-var _didIteratorError = false;
-var _iteratorError = undefined;
+var _iterator = babelHelpers.createForOfIteratorHelper(nums),
+    _step;
 
 try {
-  for (var _iterator = nums[Symbol.iterator](), _step; !(_iteratorNormalCompletion = (_step = _iterator.next()).done); _iteratorNormalCompletion = true) {
+  for (_iterator.s(); !(_step = _iterator.n()).done;) {
     var i = _step.value;
     var x;
     var f;
 
     _loop(i);
   }
 } catch (err) {
-  _didIteratorError = true;
-  _iteratorError = err;
+  _iterator.e(err);
 } finally {
-  try {
-    if (!_iteratorNormalCompletion && _iterator.return != null) {
-      _iterator.return();
-    }
-  } finally {
-    if (_didIteratorError) {
-      throw _iteratorError;
-    }
-  }
+  _iterator.f();
 }