拥有事件侦听器后，变量不会更新

let data = null;

fetch('https://api.example.com/data')
  .then(response => response.json())
  .then(json => {
    data = json;
    console.log('Data updated:', data);
  });

console.log('Data before update:', data); // 输出: Data before update: null

function fetchData(callback) {
  fetch('https://api.example.com/data')
    .then(response => response.json())
    .then(json => {
      callback(json);
    });
}

let data = null;

fetchData((newData) => {
  data = newData;
  console.log('Data updated:', data);
});

let data = null;

fetch('https://api.example.com/data')
  .then(response => response.json())
  .then(json => {
    data = json;
    console.log('Data updated:', data);
  })
  .catch(error => {
    console.error('Error fetching data:', error);
  });

let data = null;

async function fetchData() {
  try {
    const response = await fetch('https://api.example.com/data');
    const json = await response.json();
    data = json;
    console.log('Data updated:', data);
  } catch (error) {
    console.error('Error fetching data:', error);
  }
}

fetchData();