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;
}