promise链式调用

//不了解promise可以先去看看，了解后再来相信大家读能看的懂
function Promise1(resolve, reject) {
    setTimeout(function() {
        resolve('1');
    },5000);
}

function Promise2(resolve, reject) {
    setTimeout(function() {
        resolve('2');
    },5000);
}

function Promise3(resolve) {
    setTimeout(function() {
        resolve('3');
    },5000);
}
new Promise(Promise1).then(function(val){
    console.log(val);
    return new Promise(Promise2);
}).then(function(val){
    console.log(val);
    return  new Promise(Promise3);
}).then(function(val){
    console.log(val);
});