js 多次调用只执行一次的函数

const once = fn => {
  let called = false
  return function () {
    if (!called) {
      called = true
      fn.apply(this, arguments)
    }
  }
};

// 例子
const startApp = function(event) {
  console.log(this, event); // document.body, MouseEvent
};
document.body.addEventListener('click', once(startApp)); // 在点击事件中传入工具函数包裹的回调函数