function want() {
    console.log('这是你想要执行的代码');
}

function fn(want) {
    // console.log('这里表示执行了一大堆各种代码');

    // 返回Promise对象
    return new Promise(function (resolve, reject) {
        if (typeof want === 'function') {
            resolve(want);
        } else {
            reject('TypeError: ' + want + '不是一个函数')
        }
    }).then(function (want) {
        want();
    }).catch(function (err) {
        console.log(err);
    })
}

fn(want)

数据怎么传递

let fn = function (num) {
    return new Promise(function (resolve, reject) {
        if (typeof num === 'number') {
            resolve(num);
        } else {
            reject('typeError');
        }
    });
};

fn(2).then(num => {
    console.log('first:' + num);
    return num + 1;
}).then(num1 => {
    console.log('s:' + num1);
    return num1 + 1;
}).then(num2 => {
    console.log('t:' + num2);
    return num2 + 1;
});

封装一个ajax

const url = 'https://hq.tigerbrokers.com/fundamental/finance_calendar/getType/2017-02-26/2017-06-10'

function getJSON(url) {
    return new Promise(function (resolve, reject) {
        let XHR = new XMLHttpRequest();
        XHR = open('get', url, true);
        XHR = send();
        XHR.onreadystatechange = function () {
            if (XHR.readyState === 4) {
                if (XHR.status === 200) {
                    try {
                        let response = JSON.parse(XHR.responseText);
                        resolve(response);
                    } catch (e) {
                        reject(e);
                    }
                }
            } else {
                reject(new Error(XHR.statusText))
            }
        }
    })
}

getJSON(url).then(resp => {
    console.log(resp)
});