Ajax Promise封装

function fetchRequest (param) {
    const type = param.type || 'GET',
    const url = param.url;
    if (!url) {
        new TypeError('param url must be set...')
    }
    return new Promise( (resolve, reject) => {
        let xhr = new XMLHttpRequest();
        xhr.open(type, url, true);
        // 1. 监听状态
        xhr.onreadystatechange = function () {
            // 2. readyState = 4， status = 200 是请求成功的标识
            if (xhr.readyState === 4) {
                if (xhr.status === 200) {
                    resolve(xhr.responseText, xhr);
                } else {
                    reject({
                        code: xhr.status, 
                        message: xhr.response 
                    }, xhr);
                }
            }
        }
        xhr.send();
    })
}