I saw such code in a book. The purpose of the code is to initiate two asynchronous ajax requests at the same time and push the results into an array in order

//假设request是基于primose的ajax工具
var arr = [];
function *reqData ( url ) {
    var data = yield request(url);
    yield;    //关于这一行书上是这样解释的,控制转移
    arr.push(data);
}
var it1 = reqData("http:....1"), it2 = reqData("http:....2");    //总之是2个不同的url地址
var p1 = it1.next(), p2 = it2.next();
p1.then( function(data) {
    it1.next(data);
} );
p2.then( function(data) {
    it2.next(data);
} );
Promise.all([p1, p2]).then( function () {
    it1.next();
    it2.next();
} );

I have never understood what control transfer is used for, and the book doesn’t say much about it

var arr = [];
function *reqData ( url ) {
    arr.push( yield  request(url) );
};
var it1 = reqData("http:....1"), it2 = reqData("http:....2");    //总之是2个不同的url地址
var p1 = it1.next(), p2 = it2.next();
Promise.all([p1, p2]).then( data => {
    var [data1, data2] = data;
    it1.next(data1);
    it2.next(data2);
});

This is my simplified code, removing the control transfer. What are the hidden dangers of doing this compared to the original one