问单击事件返回undefined，但当我在窗口上运行load事件时，该函数可以工作

EN

    //load an item from menu on window load
    window.addEventListener('load', init);

    const mealBtn = document.getElementById('mealBtn');
    const currentMeal = document.getElementById('current-meal');
    const message = document.getElementById('message');

    const menu = [
      'Macaroni',
      'Burgers',
      'Chili',
      'Breakfast',
      'Chicken',
      'Take Out?'
    ];

    function init(){
      showMeal(menu);
    }

    mealBtn.addEventListener('click', showMeal);

    //show a meal from menu array
    function showMeal(menu){
      const randIndex = Math.floor(Math.random() * menu.length);
      currentMeal.innerHTML = menu[randIndex];
      message.innerHTML = 'How about this?';
      message.style.color = '#003b6f'
    };