js ajax请求同步请求吗

var xhr = new XMLHttpRequest();
xhr.open('GET', 'https://api.example.com/data', false); // 第三个参数设置为false表示同步请求
xhr.send(null);

if (xhr.status === 200) {
    console.log(xhr.responseText);
} else {
    console.error('请求失败');
}

function fetchData(url) {
    return new Promise((resolve, reject) => {
        var xhr = new XMLHttpRequest();
        xhr.open('GET', url, true); // 异步请求
        xhr.onload = function() {
            if (xhr.status === 200) {
                resolve(xhr.responseText);
            } else {
                reject(new Error('请求失败'));
            }
        };
        xhr.onerror = function() {
            reject(new Error('网络错误'));
        };
        xhr.send(null);
    });
}

// 使用Promise链来顺序执行多个请求
fetchData('https://api.example.com/data1')
    .then(response1 => {
        console.log(response1);
        return fetchData('https://api.example.com/data2');
    })
    .then(response2 => {
        console.log(response2);
    })
    .catch(error => {
        console.error(error);
    });

// 或者使用async/await
async function fetchSequentially() {
    try {
        const response1 = await fetchData('https://api.example.com/data1');
        console.log(response1);
        const response2 = await fetchData('https://api.example.com/data2');
        console.log(response2);
    } catch (error) {
        console.error(error);
    }
}

fetchSequentially();