Add unitests for basic functions.

package.json

@@ -65,6 +65,7 @@
     "eslint-config-prettier": "^8.5.0",
     "eslint-plugin-import": "^2.25.4",
     "eslint-plugin-prettier": "^4.2.1",
+    "jsdom": "^21.1.1",
     "prettier": "^2.6.0",
     "rollup": "^2.79.0",
     "rollup-plugin-git-info": "^1.0.0",