JS中如何处理多个ajax并发请求？

function async1(){
    //do sth...
    async2();
}
function async2(){
    //do sth...
}
async1();

function async1(){
    //do sth...
    callback();
}
function async2(){
    //do sth...
    callback();
}
function callback(){
    cnt++;
    if(2==cnt) console.log('都已执行完毕');
}

function async1(){
    //do sth...
    cnt++;
}
function async2(){
    //do sth...
    cnt++;
}
var interval = setInterval(function(){
    if(2===cnt){
        console.log('已执行完成');
        clearInterval(interval)
    }
};

var d1 = $.Deferred();
var d2 = $.Deferred();
function async1(){
    d1.resolve( "Fish" );
}
function async2(){
    d2.resolve( "Pizza" );
}
$.when( d1, d2 ).done(function ( v1, v2 ) {
    console.log( v1 + v2 + '已完成');
});