为什么我不能在javascript中使用函数进行冲突检查？

function checkProperty(obj, prop) {
  return obj.hasOwnProperty(prop);
}

const myObj = { name: 'John', age: 30 };
console.log(checkProperty(myObj, 'name')); // 输出 true
console.log(checkProperty(myObj, 'gender')); // 输出 false

function checkGlobal(name) {
  return typeof window[name] !== 'undefined';
}

if (checkGlobal('alert')) {
  console.log('alert 函数存在');
} else {
  console.log('alert 函数不存在');
}

(function() {
  var localVar = 'I am local';
  console.log(localVar); // 输出 "I am local"
})();

console.log(localVar); // 报错：ReferenceError: localVar is not defined

var myNamespace = {
  variable: 'I am in a namespace',
  function: function() {
    console.log('I am a function in a namespace');
  }
};

console.log(myNamespace.variable); // 输出 "I am in a namespace"
myNamespace.function(); // 输出 "I am a function in a namespace"