polish: skip creating extra reference for safely re-used node

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

@@ -16,14 +16,17 @@ export default declare((api, { loose = false }) => {
           return;
         }
 
-        const ref = scope.generateUidIdentifierBasedOnNode(node.left);
-        scope.push({ id: ref });
+        let ref, assignment;
+        // skip creating extra reference when `left` is semantically safe to re-use
+        if (t.isIdentifier(node.left)) {
+          ref = node.left;
+          assignment = t.cloneNode(node.left);
+        } else {
+          ref = scope.generateUidIdentifierBasedOnNode(node.left);
+          scope.push({ id: ref });
 
-        const assignment = t.assignmentExpression(
-          "=",
-          t.cloneNode(ref),
-          node.left,
-        );
+          assignment = t.assignmentExpression("=", t.cloneNode(ref), node.left);
+        }
 
         path.replaceWith(
           t.conditionalExpression(

packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-identifier-in-function/input.js

@@ -0,0 +1,3 @@
+function foo(opts) {
+  var foo = opts ?? {};
+}

packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-identifier-in-function/options.json

@@ -0,0 +1,3 @@
+{
+  "plugins": ["proposal-nullish-coalescing-operator"]
+}

packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-identifier-in-function/output.js

@@ -0,0 +1,3 @@
+function foo(opts) {
+  var foo = opts !== null && opts !== void 0 ? opts : {};
+}

packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default/input.js

@@ -1 +1 @@
-function foo(foo, bar = foo ?? "bar") {}
+function foo(foo, qux = foo.bar ?? "qux") {}

packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default/output.js

@@ -1,3 +1,3 @@
-function foo(foo, bar = (_foo = foo) !== null && _foo !== void 0 ? _foo : "bar") {
-  var _foo;
+function foo(foo, qux = (_foo$bar = foo.bar) !== null && _foo$bar !== void 0 ? _foo$bar : "qux") {
+  var _foo$bar;
 }

packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-indentifier-in-default/input.js

@@ -0,0 +1 @@
+function foo(foo, bar = foo ?? "bar") {}

packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-indentifier-in-default/options.json

@@ -0,0 +1,3 @@
+{
+  "plugins": ["proposal-nullish-coalescing-operator"]
+}

packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-indentifier-in-default/output.js

@@ -0,0 +1 @@
+function foo(foo, bar = foo !== null && foo !== void 0 ? foo : "bar") {}