fix(helpers): match Reflect.get behaviour

packages/babel-helpers/src/helpers.ts

@@ -651,10 +651,14 @@ helpers.superPropBase = helper("7.0.0-beta.0")`
   }
 `;
 
+// https://tc39.es/ecma262/multipage/reflection.html#sec-reflect.get
+//
+//  28.1.5 Reflect.get ( target, propertyKey [ , receiver ] )
+//
 helpers.get = helper("7.0.0-beta.0")`
   import superPropBase from "superPropBase";
 
-  export default function _get(target, property, receiver) {
+  export default function _get() {
     if (typeof Reflect !== "undefined" && Reflect.get) {
       _get = Reflect.get;
     } else {
@@ -665,13 +669,14 @@ helpers.get = helper("7.0.0-beta.0")`
 
         var desc = Object.getOwnPropertyDescriptor(base, property);
         if (desc.get) {
-          return desc.get.call(receiver);
+          // STEP 3. If receiver is not present, then set receiver to target.
+          return desc.get.call(arguments.length < 3 ? target : receiver);
         }
 
         return desc.value;
       };
     }
-    return _get(target, property, receiver || target);
+    return _get.apply(this, arguments);
   }
 `;
 

packages/babel-helpers/test/fixtures/behavior/get-with-falsy-receiver/exec.js

@@ -0,0 +1,23 @@
+// ensure we test the helper implementation,
+// not built-in Reflect.get to which it defers
+delete Reflect;
+
+class Target {
+  get typeOf() {
+    return this === null ? "null" : typeof this;
+  }
+};
+
+// check that the 1st argument (target) *is not* used
+// in place of present but undefined 3rd argument (receiver)
+expect(HELPER_GET(new Target, "typeOf", undefined)).toBe("undefined");
+
+// because the helper replaces itself upon invocation,
+// check it again with nullish arguments
+expect(HELPER_GET(new Target, "typeOf", undefined)).toBe("undefined");
+expect(HELPER_GET(new Target, "typeOf", null)).toBe("null");
+
+// check other falsy types
+expect(HELPER_GET(new Target, "typeOf", false)).toBe("boolean");
+expect(HELPER_GET(new Target, "typeOf", 0)).toBe("number");
+expect(HELPER_GET(new Target, "typeOf", "")).toBe("string");

packages/babel-helpers/test/fixtures/behavior/get-with-falsy-receiver/options.json

@@ -0,0 +1,3 @@
+{
+  "plugins": ["./plugin"]
+}

packages/babel-helpers/test/fixtures/behavior/get-with-falsy-receiver/plugin.js

@@ -0,0 +1,12 @@
+module.exports = function() {
+  return {
+    visitor: {
+      Identifier(path) {
+        if (path.node.name === "HELPER_GET") {
+          const helper = this.addHelper("get");
+          path.replaceWith(helper);
+        }
+      },
+    },
+  };
+};

packages/babel-helpers/test/fixtures/behavior/get-without-receiver/exec.js

@@ -0,0 +1,17 @@
+// ensure we test the helper implementation,
+// not built-in Reflect.get to which it defers
+delete Reflect;
+
+class Target {
+  get receiver() {
+    return this;
+  }
+};
+
+// check that the 1st argument (target) *is* used
+// in place of missing 3rd argument (receiver)
+expect(HELPER_GET(new Target, "receiver")).toBeInstanceOf(Target);
+
+// because the helper replaces itself upon invocation,
+// check it again with the same arguments
+expect(HELPER_GET(new Target, "receiver")).toBeInstanceOf(Target);

packages/babel-helpers/test/fixtures/behavior/get-without-receiver/options.json

@@ -0,0 +1,3 @@
+{
+  "plugins": ["./plugin"]
+}

packages/babel-helpers/test/fixtures/behavior/get-without-receiver/plugin.js

@@ -0,0 +1,12 @@
+module.exports = function() {
+  return {
+    visitor: {
+      Identifier(path) {
+        if (path.node.name === "HELPER_GET") {
+          const helper = this.addHelper("get");
+          path.replaceWith(helper);
+        }
+      },
+    },
+  };
+};