then()方法的链式调用；
    const p = new Promise((resolve,reject) =>{
    const data = ‘这是第一个获取到的’;
    resolve(data);
    });
    p.then(value =>{
    return new Promise((resolve,reject) =>{
    const data = ‘这是第二个获取到的’;
    resolve(value +’\n’+ data);
    });
    }).then(value =>{
    return new Promise((resolve,reject) =>{
    const data = ‘这是第三个获取到的’;
    resolve(value +’\n’+ data);
    })
    }).then(value =>{
    console.log(value);
    });