promise链式调用

var that = this;
that.hello().then(res => {

    return that.world(res);
}).then(res => {

    console.log(res);

})

function hello(){

    var that = this;

    var p = new Promise((resolve,reject) => {

        resolve('hello ');

    })

    return p;

}

function world(params){

    var that = this;

    var p = new Promise((resolve, reject) => {

        resolve(`${params}world!`);

    })

    return p;

}