使用promise封装ajax

// Promise 封装 ajax
function fetch(method, url, data){
    return new Promise((resolve, reject) => {
        var xhr = new XMLHttpRequest();
        var method = method || "GET";
        var data = data || null;
        xhr.open(method, url, true);
        xhr.onreadystatechange = function() {
            if(xhr.status === 200 && xhr.readyState === 4){
                resolve(xhr.responseText);
            } else {
                reject(xhr.responseText);
            }
        }
        xhr.send(data);
        })
}

// 使用
fetch("GET", "/some/url.json", null)
.then(result => {
    console.log(result);
})

// 封装 nodejs error first 风格回调
function readFile(url) {
    return new Promise((resolve, reject) => {
       fs.readFile(url,'utf8', (err, data) => {
        if(err) {
            reject(err);
            return;
        }
        resolve(data)
        }) 
    })
}