Implement transform for nullish-coalescing operator (#6483)

* Implement transform for nullish-coalescing operator

* Update example output

* Switch from BinaryExpression to LogicalExpression

* Address review comments

- Use generateUidIdentifierBasedOnNode
- Inline "??"
- Clone ref node
- Move "??" to LogicalExpression in babel-types

* Fix reference to @babel/helper-plugin-test-runner

* Fix reference to @babel/plugin-syntax-nullish-coalescing-operator

* Don't use parent scope

* Remove .vscode from .gitignore, change 'lib/index.js' to 'lib'

* Ensure `document.all ?? 0 === document.all`

* Fix note and copy to an inline comment

packages/babel-plugin-transform-nullish-coalescing-operator/src/index.js

@@ -0,0 +1,40 @@
+import syntaxNullishCoalescingOperator from "@babel/plugin-syntax-nullish-coalescing-operator";
+
+export default function({ types: t }) {
+  return {
+    inherits: syntaxNullishCoalescingOperator,
+
+    visitor: {
+      LogicalExpression(path) {
+        const { node, scope } = path;
+        if (node.operator !== "??") {
+          return;
+        }
+
+        const ref = scope.generateUidIdentifierBasedOnNode(node.left);
+        scope.push({ id: ref });
+
+        path.replaceWith(
+          t.sequenceExpression([
+            t.assignmentExpression("=", ref, node.left),
+            t.conditionalExpression(
+              // We cannot use `!= null` here because `document.all == null`
+              // and `document.all` has been deemed not "nullish".
+              t.logicalExpression(
+                "&&",
+                t.binaryExpression("!==", t.clone(ref), t.nullLiteral()),
+                t.binaryExpression(
+                  "!==",
+                  t.clone(ref),
+                  scope.buildUndefinedNode(),
+                ),
+              ),
+              t.clone(ref),
+              node.right,
+            ),
+          ]),
+        );
+      },
+    },
+  };
+}