fix: isIdentifierName should reject empty string (#11339)

packages/babel-helper-validator-identifier/src/identifier.js

@@ -98,5 +98,5 @@ export function isIdentifierName(name: string): boolean {
       return false;
     }
   }
-  return true;
+  return !isFirst;
 }

packages/babel-helper-validator-identifier/test/identifier.spec.js

@@ -0,0 +1,22 @@
+import { isIdentifierName } from "..";
+
+describe("isIdentifierName", function() {
+  it("returns false if provided string is empty", function() {
+    expect(isIdentifierName("")).toBe(false);
+  });
+  it.each(["hello", "$", "ゆゆ式", "$20", "hello20", "_", "if"])(
+    "returns true if provided string %p is an IdentifierName",
+    function(word) {
+      expect(isIdentifierName(word)).toBe(true);
+    },
+  );
+  it.each(["+hello", "0$", "-ゆゆ式", "#_", "_#"])(
+    "returns false if provided string %p is not an IdentifierName",
+    function(word) {
+      expect(isIdentifierName(word)).toBe(false);
+    },
+  );
+  it("supports astral symbols", function() {
+    expect(isIdentifierName("x\uDB40\uDDD5")).toBe(true);
+  });
+});